Breath-hold R2* mapping with a multiple gradient-recalled echo sequence: application to the evaluation of intrarenal oxygenation

Ultrasound and magnetic resonance imaging-based investigation of the role of perfusion and oxygen availability in menstrual pain

BACKGROUND

The mechanisms responsible for menstrual pain are poorly understood. However, dynamic, noninvasive pelvic imaging of menstrual pain sufferers could aid in identifying therapeutic targets and testing novel treatments.

OBJECTIVE

To study the mechanisms responsible for menstrual pain, we analyzed ultrasonographic and complementary functional magnetic resonance imaging parameters in dysmenorrhea sufferers and pain-free controls under multiple conditions.

STUDY DESIGN

We performed functional magnetic resonance imaging on participants with and those without dysmenorrhea during menses and outside menses. To clarify whether regional changes in oxygen availability and perfusion occur, functional magnetic resonance imaging R2∗ measurements of the endometrium and myometrium were obtained. R2∗ measurements are calculated nuclear magnetic resonance relaxation rates sensitive to the paramagnetic properties of oxygenated and deoxygenated hemoglobin. We also compared parameters before and after an analgesic dose of naproxen sodium. In addition, we performed similar measurements with Doppler ultrasonography to identify if changes in uterine arterial velocity occurred during menstrual cramping in real time. Mixed model statistics were performed to account for within-subject effects across conditions. Corrections for multiple comparisons were made with a false discovery rate adjustment.

RESULTS

During menstruation, a notable increase in R2∗ values, indicative of tissue ischemia, was observed in both the myometrium (beta ± standard error of the mean, 15.74±2.29 s-1; P=.001; q=.002) and the endometrium (26.37±9.33 s-1; P=.005; q=.008) of participants who experienced dysmenorrhea. A similar increase was noted in the myometrium (28.89±2.85 s-1; P=.001; q=.002) and endometrium (75.50±2.57 s-1; P=.001; q=.003) of pain-free controls. Post hoc analyses revealed that the R2∗ values during menstruation were significantly higher among the pain-free controls (myometrium, P=.008; endometrium, P=.043). Although naproxen sodium increased the endometrial R2∗ values among participants with dysmenorrhea (48.29±15.78 s-1; P=.005; q=.008), it decreased myometrial R2∗ values among pain-free controls. The Doppler findings were consistent with the functional magnetic resonance imaging (-8.62±3.25 s-1; P=.008; q=.011). The pulsatility index (-0.42±0.14; P=.004; q=.004) and resistance index (-0.042±0.012; P=.001; q=.001) decreased during menses when compared with the measurements outside of menses, and the effects were significantly reversed by naproxen sodium. Naproxen sodium had the opposite effect in pain-free controls. There were no significant real-time changes in the pulsatility index, resistance index, peak systolic velocity, or minimum diastolic velocity during episodes of symptomatic menstrual cramping.

CONCLUSION

Functional magnetic resonance imaging and Doppler metrics suggest that participants with dysmenorrhea have better perfusion and oxygen availability than pain-free controls. Naproxen sodium's therapeutic mechanism is associated with relative reductions in uterine perfusion and oxygen availability. An opposite pharmacologic effect was observed in pain-free controls. During menstrual cramping, there is insufficient evidence of episodic impaired uterine perfusion. Thus, prostaglandins may have protective vasoconstrictive effects in pain-free controls and opposite effects in participants with dysmenorrhea.

Accurate exclusion of kidney regions affected by susceptibility artifact in blood oxygenation level-dependent (BOLD) images using deep-learning-based segmentation

Susceptibility artifact (SA) is common in renal blood oxygenation level-dependent (BOLD) images, and including the SA-affected region could induce much error in renal oxygenation quantification. In this paper, we propose to exclude kidney regions affected by SA in gradient echo images with different echo times (TE), based on a deep-learning segmentation approach. For kidney segmentation, a ResUNet was trained with 4000 CT images and then tuned with 60 BOLD images. Verified by a Monte Carlo simulation, the presence of SA leads to a bilinear pattern for the segmented area of kidney as function of TE, and the segmented kidney in the image of turning point's TE would exclude SA-affected regions. To evaluate the accuracy of excluding SA-affected regions, we compared the SA-free segmentations by the proposed method against manual segmentation by an experienced user for BOLD images of 35 subjects, and found DICE of 93.9% ± 3.4%. For 10 kidneys with severe SA, the DICE was 94.5% ± 1.7%, for 14 with moderate SA, 92.8% ± 4.7%, and for 46 with mild or no SA, 94.3% ± 3.8%. For the three sub-groups of kidneys, correction of SA led to a decrease of R2* of 8.5 ± 2.8, 4.7 ± 1.8, and 1.6 ± 0.9 s-1, respectively. In conclusion, the proposed method is capable of segmenting kidneys in BOLD images and at the same time excluding SA-affected region in a fully automatic way, therefore can potentially improve both speed and accuracy of the quantification procedure of renal BOLD data.

Medullary Blood Oxygen Level-Dependent MRI Index (R2*) is Associated with Annual Loss of Kidney Function in Moderate CKD

BACKGROUND

The estimated glomerular filtration rate (eGFR) is frequently used to monitor progression of kidney disease. Multiple values have to be obtained, sometimes over years to determine the rate of decline in kidney function. Recent data suggest that functional MRI (fMRI) methods may be able to predict loss of eGFR. In a prior study, baseline data with multi-parametric MRI in individuals with diabetes and moderate CKD was reported. This report extends our prior observations in order to evaluate the temporal variability of the fMRI measurements over 36 months and their association with annual change in eGFR.

METHODS

Twenty-four subjects with moderate CKD completed 3 sets of MRI scans over a 36-month period. Blood oxygenation level-dependent (BOLD), arterial spin labeling perfusion, and diffusion MRI images were acquired using a 3 T scanner. Coefficients of variation was used to evaluate variability between subjects at each time point and temporal variability within each subject. We have conducted mixed effects models to examine the trajectory change in GFR over time using time and MRI variables as fixed effects and baseline intercept as random effect. Associations of MRI image markers with annual change in eGFR were evaluated.

RESULTS

Multi-parametric functional renal MRI techniques in individuals with moderate CKD showed higher temporal variability in R2* of medulla compared to healthy individuals. This was consistent with the significant lower R2* in medulla observed at 36 months compared to baseline values. The results of linear mixed model showing that R2*_Medulla was the only predictor associated with change in eGFR over time. Furthermore, a significant association of medullary R2* with annual loss of eGFR was observed at all the 3 time points.

CONCLUSIONS

The lower R2* values and the higher temporal variability in the renal medulla over time suggest the ability to monitor progressive CKD. These were confirmed by the fact that reduced medullary R2* was associated with higher annual loss in eGFR. These data collectively emphasize the need for inclusion of medulla in the analysis of renal BOLD MRI studies.

MRI Mapping of the Blood Oxygenation Sensitive Parameter T2* in the Kidney: Basic Concept

The role of hypoxia in renal disease and injury has long been suggested but much work still remains, especially as it relates to human translation. Invasive pO₂ probes are feasible in animal models but not for human use. In addition, they only provide localized measurements. Histological methods can identify hypoxic tissue and provide a spatial distribution, but are invasive and allow only one-time point. Blood oxygenation level dependent (BOLD) MRI is a noninvasive method that can monitor relative oxygen availability across the kidney. It is based on the inherent differences in magnetic properties of oxygenated vs. deoxygenated hemoglobin. Presence of deoxyhemoglobin enhances the spin-spin relaxation rate measured using a gradient echo sequence, known as R₂* (= 1/T₂*). While the key interest of BOLD MRI is in the application to humans, use in preclinical models is necessary primarily to validate the measurement against invasive methods, to better understand physiology and pathophysiology, and to evaluate novel interventions. Application of MRI acquisitions in preclinical settings involves several challenges both in terms of logistics and data acquisition. This section will introduce the concept of BOLD MRI and provide some illustrative applications. The following sections will discuss the technical issues associated with data acquisition and analysis.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Consensus-based technical recommendations for clinical translation of renal BOLD MRI

Harmonization of acquisition and analysis protocols is an important step in the validation of BOLD MRI as a renal biomarker. This harmonization initiative provides technical recommendations based on a consensus report with the aim to move towards standardized protocols that facilitate clinical translation and comparison of data across sites. We used a recently published systematic review paper, which included a detailed summary of renal BOLD MRI technical parameters and areas of investigation in its supplementary material, as the starting point in developing the survey questionnaires for seeking consensus. Survey data were collected via the Delphi consensus process from 24 researchers on renal BOLD MRI exam preparation, data acquisition, data analysis, and interpretation. Consensus was defined as ≥ 75% unanimity in response. Among 31 survey questions, 14 achieved consensus resolution, 12 showed clear respondent preference (65-74% agreement), and 5 showed equal (50/50%) split in opinion among respondents. Recommendations for subject preparation, data acquisition, processing and reporting are given based on the survey results and review of the literature. These technical recommendations are aimed towards increased inter-site harmonization, a first step towards standardization of renal BOLD MRI protocols across sites. We expect this to be an iterative process updated dynamically based on progress in the field.

Consensus-based technical recommendations for clinical translation of renal BOLD MRI

Harmonization of acquisition and analysis protocols is an important step in the validation of BOLD MRI as a renal biomarker. This harmonization initiative provides technical recommendations based on a consensus report with the aim to move towards standardized protocols that facilitate clinical translation and comparison of data across sites. We used a recently published systematic review paper, which included a detailed summary of renal BOLD MRI technical parameters and areas of investigation in its supplementary material, as the starting point in developing the survey questionnaires for seeking consensus. Survey data were collected via the Delphi consensus process from 24 researchers on renal BOLD MRI exam preparation, data acquisition, data analysis, and interpretation. Consensus was defined as ≥ 75% unanimity in response. Among 31 survey questions, 14 achieved consensus resolution, 12 showed clear respondent preference (65-74% agreement), and 5 showed equal (50/50%) split in opinion among respondents. Recommendations for subject preparation, data acquisition, processing and reporting are given based on the survey results and review of the literature. These technical recommendations are aimed towards increased inter-site harmonization, a first step towards standardization of renal BOLD MRI protocols across sites. We expect this to be an iterative process updated dynamically based on progress in the field.

Why Have Detection, Understanding and Management of Kidney Hypoxic Injury Lagged Behind those for the Heart?

The outcome of patients with acute myocardial infarction (AMI) has dramatically improved over recent decades, thanks to early detection and prompt interventions to restore coronary blood flow. In contrast, the prognosis of patients with hypoxic acute kidney injury (AKI) remained unchanged over the years. Delayed diagnosis of AKI is a major reason for this discrepancy, reflecting the lack of symptoms and diagnostic tools indicating at real time altered renal microcirculation, oxygenation, functional derangement and tissue injury. New tools addressing these deficiencies, such as biomarkers of tissue damage are yet far less distinctive than myocardial biomarkers and advanced functional renal imaging technologies are non-available in the clinical practice. Moreover, our understanding of pathogenic mechanisms likely suffers from conceptual errors, generated by the extensive use of the wrong animal model, namely warm ischemia and reperfusion. This model parallels mechanistically type I AMI, which properly represents the rare conditions leading to renal infarcts, whereas common scenarios leading to hypoxic AKI parallel physiologically type II AMI, with tissue hypoxic damage generated by altered oxygen supply/demand equilibrium. Better understanding the pathogenesis of hypoxic AKI and its management requires a more extensive use of models of type II-rather than type I hypoxic AKI.

Cortical Perfusion and Tubular Function as Evaluated by Magnetic Resonance Imaging Correlates with Annual Loss in Renal Function in Moderate Chronic Kidney Disease

BACKGROUND

Chronic hypoxia is a well-recognized factor in the pathogenesis of chronic kidney disease (CKD). Loss of microcirculation is thought to lead to enhanced renal hypoxia, which in turn results in the development of fibrosis, a hallmark of progressive CKD. To evaluate the role of functional magnetic resonance imaging (MRI), we performed perfusion, oxygenation, and diffusion MRI measurements in individuals with diabetes and stage 3 CKD.

METHODS

Fifty-four subjects (41 individuals with diabetes and stage 3 CKD and 13 healthy controls) participated in this study. Data with blood oxygenation level dependent (BOLD), arterial spin labeling perfusion and diffusion MRI were acquired using a 3T scanner.

RESULTS

Renal cortical perfusion was reduced in CKD compared to the controls (109.54 ± 25.38 vs. 203.17 ± 27.47 mL/min/100 g; p < 0.001). Cortical apparent diffusion coefficient showed no significant reduction in CKD compared to controls (1,596.10 ± 196.64 vs. 1,668.72 ± 77.29 × 10-6 mm2/s; p = 0.45) but was significantly associated with perfusion. Cortical R2* values were modestly increased in CKD (20.76 ± 4.08 vs. 18.74 ± 2.37 s-1; p = 0.12). Within the CKD group, R2*_Medulla and R2*_Kidney were moderately and negatively associated with estimated glomerular filtration rate. There was a significant association between cortical perfusion and medullary response to furosemide with annual loss of renal function, used as an estimate of CKD progression.

CONCLUSIONS

Subjects with a moderate degree of CKD had significantly lower renal perfusion. Diffusion and BOLD MRI showed more modest differences between the groups. Individuals with progressive CKD had lower perfusion and response to furosemide.

Role of Blood Oxygen Level-dependent MRI in Differentiation of Acute Renal Allograft Dysfunction

Early graft dysfunction after renal transplantation manifests as acute rejection (AR) or acute tubular necrosis (ATN). Blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging is a noninvasive method of assessing tissue oxygenation, which may be useful for predicting acute allograft dysfunction. This was a prospective study involving 40 patients scheduled for renal transplantation from August 2012 to August 2014. In addition, 15 healthy donors were also enrolled in this study. All recipients underwent BOLD MR imaging (MRI) and R2* mapping 10–20 days after transplant, and additionally within 48 h of biopsy if there was any evidence of graft dysfunction. The healthy donors underwent BOLD MRI 1–2 days before surgery. The biopsies were grouped into AR, ATN, and no evidence of AR or ATN. The mean medullary R2*, cortical R2*, corticomedullary gradient, and medullary: cortical R2* ratio were compared between groups using one-way analysis of variance. Spearman's correlation and multinomial linear regression were applied to determine the influence factors of R2* value. Overall, nine patients had graft dysfunction. Six were reported as AR, two as ATN, and one as no evidence of ATN or rejection. The mean medullary and cortical R2* were significantly higher in ATN group compared with AR and normal group, whereas the mean medullary and cortical R2* of AR group were significantly lower than normal group. The corticomedullary gradient of AR group was significantly lower compared with ATN and normal group. Medullary R2*:cortical R2* ratio was significantly lower in AR group compared with normal group. No significant difference was noted between the 15 donors and patients with normal graft function. R2* values on BOLD MRI are significantly decreased in AR allografts and increased in an early stage of ATN allografts, suggesting that BOLD MRI can become a valuable tool for discriminating between AR and ATN.

Imaging of intrarenal haemodynamics and oxygen metabolism

The interruption of blood flow results in impaired oxygenation and metabolism. This can lead to electrophysiological changes, functional impairment and symptoms in quick succession. Quantitative measures of organ perfusion, perfusion reserve and tissue oxygenation are crucial to assess normal tissue metabolism and function. Magnetic resonance imaging (MRI) provides a number of quantitative methods to assess physiology in the kidney. Blood oxygenation level-dependent (BOLD) MRI provides a method for the assessment of oxygenation. Blood flow to the kidney can be assessed using phase contrast MRI. Dynamic contrast-enhanced MRI and arterial spin labelling (ASL) provide methods to assess tissue perfusion, ASL using the magnetization of endogenous water protons and thus providing a non-invasive method to assess perfusion. The application of diffusion-weighted MRI allows molecular motion in the kidney to be measured. Novel techniques can also be used to assess oxygenation in the renal arteries and veins and, combined with flow measures, provide an estimation of oxygen metabolism. Magnetic resonance imaging provides a synergy of non-invasive techniques to study renal function and the demand for these techniques is likely to be driven by the incentive to avoid the use of contrast media, to avoid radiation and to avoid complications with intervention procedures.

GESFIDE-PROPELLER approach for simultaneous R2 and R2* measurements in the abdomen

PURPOSE

To investigate the feasibility of combining GESFIDE with PROPELLER sampling approaches for simultaneous abdominal R2 and R2* mapping.

MATERIALS AND METHODS

R2 and R2* measurements were performed in 9 healthy volunteers and phantoms using the GESFIDE-PROPELLER and the conventional Cartesian-sampling GESFIDE approaches.

RESULTS

Images acquired with the GESFIDE-PROPELLER sequence effectively mitigated the respiratory motion artifacts, which were clearly evident in the images acquired using the conventional GESFIDE approach. There was no significant difference between GESFIDE-PROPELLER and reference MGRE R2* measurements (p=0.162) whereas the Cartesian-sampling based GESFIDE methods significantly overestimated R2* values compared to MGRE measurements (p<0.001).

CONCLUSION

The GESFIDE-PROPELLER sequence provided high quality images and accurate abdominal R2 and R2* maps while avoiding the motion artifacts common to the conventional Cartesian-sampling GESFIDE approaches.

Physiological biomarkers of acute kidney injury: a conceptual approach to improving outcomes

The past 5-10 years have brought significant advances in the identification and validation of novel biochemical biomarkers in the prevention and treatment of acute kidney injury (AKI). These biochemical biomarkers remain research tools but we anticipate that soon they will be employed in clinical practice. A Consensus Conference held by the Acute Dialysis Quality Initiative (ADQI) recently reviewed the evidence, and identified gaps and a research agenda. Furthermore, at this meeting was the birth of an initiative to comprehensively identify new opportunities to characterize the physiological changes during the course of AKI based upon a conceptual framework for the detection and monitoring of renal ischemia-reperfusion injury. This framework includes a transition from monitoring physiological biomarkers of adequate renal perfusion, to pathophysiologic biomarkers of renal hypoperfusion, and finally biomarkers of kidney cell structural injury/damage. Techniques to measure physiological changes in AKI include several physiological variables that might be used in an interactive way to supplement clinical information and biochemical damage biomarkers in the diagnosis and management of AKI. This review summarizes the spectrum of physiological parameters and potential new physiological methods that enable identification of high-risk patients for AKI, facilitate early diagnosis, and differential diagnosis to guide therapeutic management and prognostication. Finally, we propose a research agenda for the next 5 years to facilitate the development and validation of physiological biomarkers in AKI.

Feasibility of noninvasive quantitative measurements of intrarenal R(2) ' in humans using an asymmetric spin echo echo planar imaging sequence

The purpose of this study was to demonstrate the feasibility of an asymmetric spin echo (ASE) single-shot echo planar imaging (EPI) sequence for the noninvasive quantitative measurement of intrarenal R(2) ' in humans within 20 s. The reproducibility of R(2) ' measurements with the ASE-EPI sequence was assessed in nine healthy young subjects in repeated studies conducted over three consecutive days. Moreover, we also evaluated whether the ASE-EPI sequence-measured R(2) ' reflected the intrarenal oxygenation changes induced by furosemide in another group of normal human subjects (n = 10). Different flow attenuation gradients (b = 0, 40 and 80 s/mm(2) ) were utilized to examine the impact of the intravascular signal contribution on the estimation of intrarenal R(2) '. In the absence of flow dephasing gradients (b = 0 s/mm(2) ), the computed coefficient of variation (CV) of R(2) ' was 21.31 ± 4.52%, and the estimated R(2) ' value decreased slightly, but not statistically significantly (p > 0.05), after the administration of furosemide in the medullary region. However, CV of R(2) ' was much smaller in the presence of flow dephasing gradients (9.68 ± 3.58% with b = 40 s/mm(2) and 10.50 ± 3.62% with b = 80 s/mm(2) ). Moreover, a significant reduction in R(2) ' in the renal medulla was obtained (p < 0.05 for both b = 40 s/mm(2) and b = 80 s/mm(2) ) after the administration of furosemide, reflecting an increase in oxygen tension in the medullary region. In addition, R(2) ' measurements did not differ between the b = 40 s/mm(2) and b = 80 s/mm(2) scans, suggesting that small diffusion gradients were sufficient to minimize the intravascular signal contribution. In summary, we have demonstrated that renal R(2) ' can be obtained rapidly using an ASE-EPI sequence. The measurement was highly reproducible and reflected the expected intrarenal oxygenation changes induced by furosemide.

R1 and R2 * changes in the human placenta in response to maternal oxygen challenge

PURPOSE

Pregnancy complications such as preeclampsia and fetal growth restriction are sometimes thought to be caused by placental abnormalities associated with reduced oxygenation. Oxygen-enhanced MRI (R1 contrast) and BOLD MRI (R2 * contrast) have the potential to noninvasively investigate this oxygen environment at a range of gestational ages.

METHODS

Scanning was carried out at 1.5 T under maternal air and oxygen breathing in a single placental slice in 14 healthy pregnant subjects of gestational ages 21-37 weeks. We report R1 changes using a respiratory-triggered inversion recovery-turbo spin-echo sequence, which is sensitive to changes in PO2 , and R2 * changes using a breathhold multiple gradient-recalled echo sequence sensitive to changes in oxygen saturation.

RESULTS

Significant R1 increases (P < 0.005, paired t-test) and R2 * decreases (P < 0.0001, paired t-test) between air and oxygen breathing were demonstrated. ΔR1 decreased with gestational age (P < 0.0005, r = -0.835, Pearson correlation test). No significant effect of gestational age on R2 * change was observed.

CONCLUSION

The results demonstrate the feasibility of non-invasive investigation of placental oxygenation using MRI and the sensitivity of R1 oxygen-enhanced MRI to gestational age. The techniques have the potential to provide unique noninvasive biomarkers in compromised pregnancies.

Evaluation of transplanted kidneys using blood oxygenation level-dependent MRI at 3 T: a preliminary study. AJR Am J Roentgenol. 2012;198:1108-1114. [PMID: 22528900 DOI: 10.2214/ajr.11.7253]

OBJECTIVE

The objective of our study was to investigate the feasibility and reproducibility of blood oxygenation level-dependent (BOLD) MRI using different gradient echoes at 3 T in patients with renal allografts and healthy volunteers and to evaluate whether BOLD MRI can be used to distinguish between cases of acute allograft rejection and normally functioning allografts.

SUBJECTS AND METHODS

BOLD MRI at 3 T was performed of eight patients with normal allografts, four patients with acute allograft rejection, and 10 healthy volunteers. Multiple fast-field echo sequences were performed at gradient echoes of 8, 16, and 20 to obtain T2(*)-weighted images. The reproducibility of BOLD MRI was evaluated in patients with normal allografts.

RESULTS

Cortical and medullary R2(*) values were not significantly different between healthy volunteers and patients with normal allografts, but medullary R2(*) values were significantly greater than cortical R2(*) values in both groups for all three protocols (p < 0.01). Medullary R2(*) values were significantly lower in cases of acute allograft rejection than in normal allografts for all three protocols (p < 0.001). The mean difference in cortical or medullary R2(*) values was 3.8% or less in all protocols.

CONCLUSION

BOLD MRI performed using different gradient echoes at 3 T is feasible and reproducible in patients with renal allografts and can show significant changes in medullary oxygenation in patients with acute rejection.

Renal oxygen content is increased in healthy subjects after angiotensin-converting enzyme inhibition

OBJECTIVE

The association between renal hypoxia and the development of renal injury is well established. However, no adequate method currently exists to non-invasively measure functional changes in renal oxygenation in normal and injured patients.

METHOD

R2* quantification was performed using renal blood oxygen level-dependent properties. Five healthy normotensive women (50 ± 5.3 years) underwent magnetic resonance imaging in a 1.5T Signa Excite HDx scanner (GE Healthcare, Waukesha, WI). A multiple fast gradient-echo sequence was used to acquire R2*/T2* images (sixteen echoes from 2.1 ms/slice to 49.6 ms/slice in a single breath hold per location). The images were post-processed to generate R2* maps for quantification. Data were recorded before and at 30 minutes after the oral administration of an angiotensin II-converting enzyme inhibitor (captopril, 25 mg). The results were compared using an ANOVA for repeated measurements (mean + standard deviation) followed by the Tukey test. ClinicalTrials.gov: NCT01545479.

RESULTS

A significant difference (p<0.001) in renal oxygenation (R2*) was observed in the cortex and medulla before and after captopril administration: right kidney, cortex = 11.08 ± 0.56 ms, medulla = 17.21 ± 1.47 ms and cortex = 10.30 ± 0.44 ms, medulla = 16.06 ± 1.74 ms, respectively; and left kidney, cortex= 11.79 ± 1.85 ms, medulla = 17.03 ± 0.88 ms and cortex = 10.89 ± 0.91 ms, medulla = 16.43 ± 1.49 ms, respectively.

CONCLUSIONS

This result suggests that the technique efficiently measured alterations in renal blood oxygenation after angiotensin II-converting enzyme inhibition and that it may provide a new strategy for identifying the early stages of renal disease and perhaps new therapeutic targets.

Non-invasive investigation of kidney disease in type 1 diabetes by magnetic resonance imaging

AIMS/HYPOTHESIS

Pathophysiological abnormalities in early diabetic nephropathy are poorly understood. We employed MRI to characterise renal perfusion, tissue oxygenation and kidney size in non-diabetic volunteers and type 1 diabetic patients without and with early renal disease.

METHODS

We studied ten control participants (C; age 40.0 [range 31-54] years), nine longstanding normotensive type 1 diabetic patients (T1Normo; age 40.1 [31-50] years, estimated glomerular filtration rate [eGFR] 83.4 ± 10.6 ml min(-1) 1.73 m(-2)) and eight microalbuminuric type 1 diabetic patients (T1Micro; age 42.4 [33-52] years, eGFR 71.6 ± 13.7 ml min(-1) 1.73 m(-2)). Six microalbuminuric patients were restudied after 4 weeks without renin-angiotensin-aldosterone system inhibitors. Phase contrast angiography and kidney blood oxygen level dependent (BOLD) (R(2)(*)) MRI were performed, before and during water diuresis. Contrast-enhanced MRI was performed at baseline urine flow rate. Renal artery flow, renal vascular resistance (RVR), cortical and medullary volumes, and R(2)(*) were determined.

RESULTS

Renal cortical and medullary volumes were similar in all groups (cortex: C 108 ± 16, T1Normo 112 ± 21, T1Micro 111 ± 10 cm(3)/1.73 m(2); medulla: C 35 ± 14, T1Normo 29 ± 10, 33 ± 6 cm(3)/1.73 m(2)). RVR increased from control to normoalbuminuric to microalbuminuric type 1 diabetic patients (C 0.061 ± 0.018, T1Normo 0.077 ± 0.014, T1Micro 0.093 ± 0.024 mmHg ml(-1) min(-1) 1.73 m(-2), ANOVA p = 0.012). RVR correlated inversely with eGFR in normoalbuminuric, but not in microalbuminuric diabetic patients. Renal artery flow was lower in the whole diabetes cohort (control 740 ± 205 vs diabetes 591 ± 128 ml min(-1) 1.73 m(-2), p = 0.035).

CONCLUSIONS/INTERPRETATION

Cortical and medullary volumes remain normal in early diabetic nephropathy. Decreased renal flow in longstanding normoalbuminuric type 1 diabetic patients may reflect intrarenal vascular stiffening, whereas in the microalbuminuric patients it may also reflect increased intraglomerular pressure.

BOLD imaging: a potential predictive biomarker of renal functional outcome following revascularization in atheromatous renovascular disease

BACKGROUND

Stenting of the stenosed renal artery is commonly employed in atheromatous renovascular disease (ARVD) in order to revascularize the affected kidney. However, it is still far from clear which patient subgroups should be revascularized as stenting carries small but significant risks. We have previously demonstrated that the ratio of magnetic resonance-measured renal volume to isotopic single kidney glomerular filtration rate (isoSK-GFR) is higher in kidneys which show functional improvement after revascularization. Blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) does not require contrast administration and is sensitive to changes in tissue concentration of deoxyhaemoglobin.

METHODS

In this study, we test the hypothesis that baseline BOLD R2* map signal and R2*:isoSK-GFR ratio will provide an additional independent predictive biomarker of response to revascularization.

RESULTS

Studies were performed in 28 subjects (16 ARVD and 12 controls). All subjects had R2* mapping and isoSK-GFR measured at baseline and at 4-month follow-up. MRI data were collected on a 3 T whole-body MRI scanner using a coronal dual-echo, 2D gradient-echo breath-hold acquisition. Parenchymal regions of interest (ROIs) were drawn on a representative slice through the middle of the kidney. Parametric maps of R2* were generated and mean values of R2* were calculated for every ROI. The ratio of R2*:isoSK-GFR at baseline was significantly greater in kidneys where renal function improved (5.91 ± 6.51) versus stable (1.78 ± 1.11), deteriorated (2.15 ± 1.79) or controls (1.5 ± 0.91), P = 0.003. R2*:isoSK-GFR ratio that was greater than 95% confidence interval of the control kidneys was 66.7% sensitive, but 85.7% specific in predicting a positive renal functional outcome.

CONCLUSIONS

These pilot data show that BOLD R2* imaging, presumably by detecting intra-renal deoxyhaemoglobin in still viable 'hibernating' parenchyma, coupled with isoSK-GFR may provide an effective predictive biomarker for positive renal functional response to revascularization. R2* imaging is non-invasive, quick to perform and could provide further insight into reversible parenchymal changes in ARVD kidneys.

Functional evaluation of transplanted kidneys in normal function and acute rejection using BOLD MR imaging. Eur J Radiol. 2012;81:838-845. [PMID: 21392910 DOI: 10.1016/j.ejrad.2011.02.041]

In this study, we evaluated a large number of subjects using BOLD MRI to provide more information about oxygen metabolism in the normal function of transplanted kidneys and to distinguish acute graft rejection from normal function kidneys. This study included 122 subjects (20 volunteers, 72 patients with normal functioning transplants, and 21 patients with acute rejection), and 9 patients had normal function grafts received examination while grafts dysfunction occurred within 6 months during the follow-up. The R2 (1/s) values in the cortex and medulla as well as the R2 ratio of the medulla to cortex (R2 ratio of M/C) were recorded. The R2 values of the medulla were higher than those of the cortex in the normal function group and the volunteers which have a steep R2 ratio of M/C. All the R2 values in the acute rejection group were lower than those in the normal function grafts group (P<0.001). Moreover, in the 9 patients as normal function, the R2 values of the cortex and medulla were different from the normal function grafts, which was lower in 5 patients and was higher in the 4 remaining patients. Conversely, the R2 ratios of M/C of the 9 patients were similar to those in the normal function group. BOLD MRI shows that decreased R2 values of the cortex and medulla and R2 ratio of M/C suggest acute renal graft rejection; furthermore, a steep R2 ratio of M/C (>1.1) is an important reason for keeping clinical normal function.

Respiratory self-gated multiple gradient recalled echo sequence for free-breathing abdominal R2* mapping

Abdominal effective transverse relaxation rate (R(2)*) mapping is critical for a wide range of applications. However, respiratory motion can lead to significant image quality deterioration and R(2)* overestimation. For this work, we explored the feasibility of combining respiratory self-gating techniques with a multiple gradient-recalled echo sequence for free-breathing abdominal R(2)* measurements. In a series of eight normal volunteers, respiratory self-gated-multiple gradient-recalled echo methods effectively avoided motion artifacts to produce quantitative R(2)* measurements in liver, spleen, and kidneys that were comparable to R(2)* measurements produced while breath-holding. Respiratory self-gated-multiple gradient-recalled echo methods demonstrated the potential to avoid the need for breath-holding during abdominal R(2)* mapping. For clinical application, respiratory self-gated-multiple gradient-recalled echo approaches could be particularly useful for R(2)* measurements in those patients unable or unwilling to sustain sufficiently long breath-holds to avoid motion artifacts.

Functional MRI of liver using BOLD MRI: effect of glucose

PURPOSE

To demonstrate feasibility of functional MRI of liver using glucose as a stimulus to monitor metabolic changes using blood oxygenation level dependent (BOLD) contrast. We hypothesized that during hyperglycemia, liver stores the glucose and consequently there is a reduction in oxygen consumption, which can be detected using BOLD MRI.

MATERIALS AND METHODS

In four mini pigs, measurements were made before and after 54 g of glucose administered intravenously. In six healthy young human subjects, measurements were made before and after oral ingestion of 75 g of glucose. T(2)* weighted images of the liver were obtained on a Siemens 3 Tesla Verio MRI scanner using multiple gradient recalled echo (mGRE) sequence.

RESULTS

A statistically significant decrease (P < 0.05) in R2* (1/T(2)*) was observed postglucose both in swine (110.41 ± 14.1 s(-1) to 72.22 ± 5.7 s(-1)) and human (55.84 ±3.8 s(-1) to 50.6 ±0.5 s(-1)), suggesting improved liver oxygenation during hyperglycemia.

CONCLUSION

Our preliminary data presented here demonstrate the feasibility of obtaining functional liver images that illustrate the changes in oxygen consumption. Further studies are necessary to fully validate the technique. J. Magn. Reson. Imaging 2010;32:988-991. © 2010 Wiley-Liss, Inc.

Blood oxygenation level dependent (BOLD)--renal imaging: concepts and applications. Z Med Phys. 2010;20:88-100. [PMID: 20807689 DOI: 10.1016/j.zemedi.2010.01.003]

Many renal diseases as well as several pharmacons cause a change in renal blood flow and/or renal oxygenation. The blood oxygenation level dependent (BOLD) imaging takes advantage of local field inhomogeneities and is based on a T2*-weighted sequence. BOLD is a non-invasive method allowing an estimation of the renal, particularly the medullary oxygenation, and an indirect measurement of blood flow without administration of contrast agents. Thus, effects of different drugs on the kidney and various renal diseases can be controlled and observed. This work will provide an overview of the studies carried out so far and identify ways how BOLD can be used in clinical studies.

Comparison of 1.5 and 3 T BOLD MR to study oxygenation of kidney cortex and medulla in human renovascular disease

OBJECTIVES

Imaging of the kidney using blood oxygen level dependent MR presents a major opportunity to examine differences in tissue oxygenation within the cortex and medulla applicable to human disease. We sought to define the differences between regions within kidneys and to optimize selection of regions of interest for study with 1.5 and 3 Tesla systems.

MATERIALS AND METHODS

Studies in 38 subjects were performed under baseline conditions and after administration of furosemide intravenously to examine changes in R2* as a result of suppressing oxygen consumption related to medullary tubular solute transport. These studies were carried out in patients with atherosclerotic renal artery stenosis (n = 24 kidneys) or essential hypertension or nonstenotic kidneys (n = 39). All patients but one were treated with agents to block the renin angiotensin system (ACE inhibitors or angiotensin receptor blockers). For each kidney, 3 levels (upper pole, hilum, and lower pole) were examined, including 3 individual segments (anterior, lateral, and posterior).

RESULTS

Low basal R2* levels in kidney cortex (12.06 +/- 0.84 s(-1)) at 1.5 Tesla reflected robust blood flow and oxygenation and agreed closely with values obtained at 3.0 Tesla (13.62 +/- 0.56 s(-1), NS). Coefficients of variation ranged between 15% and 20% between segments and levels at both field strengths. By contrast, inner medullary R2* levels were higher at 3 T (31.66 +/- 0.74 s(-1)) as compared with 1.5 T (22.19 +/- 1.52 s(-1), P < 0.01). Medullary R2* values fell after furosemide administration reflecting reduced deoxyhemoglobin levels associated with blocked energy-dependent transport. The fall in medullary R2* at 3.0 Tesla (-12.61 +/- 0.97 s(-1)) was greater than observed at 1.5 T (-6.07 +/- 1.38 s(-1), P < 0.05). Cortical R2* levels remained low after furosemide and did not vary with field strength. Correlations between measurements of defined cortical and medullary regions of interest within kidneys were greater at each sampling level and segment at 3.0 T as compared to 1.5 T. For patients studied with 3.0 T, furosemide administration induced a lesser fall in R2* in poststenotic kidneys at 3.0 T (-10.61 +/- 1.61 s(-1)) versus nonstenotic kidneys (-13.21 +/- 0.72 s(-1), P < 0.05). This difference was not evident in comparisons made at 1.5 T. The magnitude of furosemide-suppressible oxygen consumption at 3.0 T (-43%) corresponded more closely with reported experimental differences observed during direct measurement with tissue electrodes (45%-50%) than changes measured at 1.5 T.

CONCLUSION

These results indicate that blood oxygen level dependent MR measurements at high field strength can better distinguish discrete cortical and inner medullary regions of the kidney and approximate measured differences in oxygen tension. Maneuvers that reduce oxygen consumption related to tubular solute transport allow functional evaluation of the interstitial compartment as a function of tissue oxygenation. Impaired response to alterations in oxygen consumption can be detected at 3 T more effectively than at 1.5 T and may provide real-time tools to examine developing parenchymal injury associated with impaired oxygenation.

Renal functional MRI: Are we ready for clinical application?

OBJECTIVE

We review the basics of functional renal imaging and highlight a few clinical applications.

CONCLUSION

Techniques such as contrast-enhanced MR renography, diffusion-weighted imaging, and blood oxygen level-dependent MRI have been investigated in animal models and in a few human studies. Functional renal imaging is a rapidly growing field that has the potential to provide new insight into the pathophysiology of renal disease.

Fat-water separated delayed hyperenhanced myocardial infarct imaging

Fat deposition associated with myocardial infarction (MI) has been reported as a commonly occurring phenomenon. Magnetic resonance imaging (MRI) has the ability to efficiently detect MI using T(1)-sensitive contrast-enhanced sequences and fat via its resonant frequency shift. In this work, the feasibility of fat-water separation applied to the conventional delayed hyperenhanced (DHE) MI imaging technique is demonstrated. A three-point Dixon acquisition and reconstruction was combined with an inversion recovery gradient-echo pulse sequence. This allowed fat-water separation along with T(1) sensitive imaging after injection of a gadolinium contrast agent. The technique is demonstrated in phantom experiments and three subjects with chronic MI. Areas of infarction were well defined as conventional hyperenhancement in water images. In two cases, fatty deposition was detected in fat images and confirmed by precontrast opposed-phase imaging.

Blood oxygen level-dependent MR imaging of the kidneys

Oxygenation status plays a major role in renal physiology and pathophysiology, and thus has attracted considerable attention in recent years. While much of the early work and a significant amount of present work is based on invasive methods or ex vivo analysis, and is therefore restricted to animal models, blood oxygen level-dependent (BOLD) MR imaging has been shown to extend these findings to human beings. BOLD MR imaging is most useful in monitoring effects of physiologic or pharmacologic maneuvers. Several teams around the world have demonstrated reproducible data and have illustrated several useful applications. Studies supporting the use of renal BOLD MR imaging in characterizing disease with prognostic value have also been reported. This article provides an overview of current state-of-the art of renal BOLD MR imaging.

Evaluation of renal oxygenation in rat by using R2' at 3-T magnetic resonance: initial observation

RATIONALE AND OBJECTIVES

We sought to initially evaluate the feasibility of R2' on a 3-T magnetic resonance (MR) scanner for assessment of renal oxygenation changes following administration of furosemide in rats.

MATERIALS AND METHODS

Eight intact male Wistar rats were involved in experimental group. The experiment was performed at a 3-T MR scanner using a multiple gradient-echo (mGRE) sequence for R2* map and a multiecho fast spin-echo (FSE) sequence for R2 map. R2' values of cortex and medulla were calculated using the equation R2* = R2 + R2'. The values of R2 and R2* were measured and R2' was calculated before and after administration of furosemide, and the changes (delta values) were calculated.

RESULTS

Both R2* and R2 values decreased significantly after administration of furosemide (P < .001) in both the cortex and medulla. DeltaR2* in the medulla was significantly higher than in the cortex (P < .05). DeltaR2 was not significantly different between the cortex and medulla (P > .05). The baseline R2' value was 12.13 +/- 0.59 1/s in the cortex and 19.52 +/- 3.44 1/s in the medulla. R2' value decreased significantly in the medulla after administration of furosemide (P < .05), but there was no significant difference in the cortex before and after administration of furosemide (P > .05).

CONCLUSION

R2' may be more appropriate than R2* to indicate the change of oxygenation after administration of furosemide in intact rats at 3-T MR. Further studies are needed for both intact animals and experimental models in comparison with non-MR imaging methods to validate this initial observation.

Early changes with diabetes in renal medullary hemodynamics as evaluated by fiberoptic probes and BOLD magnetic resonance imaging

OBJECTIVE

We sought to evaluate the influence of streptozotocin (STZ)-induced diabetes on renal outer medullary pO2 and blood flow by invasive microprobes and to demonstrate feasibility that blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) can monitor these changes.

MATERIALS AND METHODS

A total of 60 Wistar-Furth rats were used. Diabetes was induced by STZ in 48. Animals were divided into OxyLite group (n=30) and BOLD MRI groups (n=30) each with a 5 subgroups of 6 animals: control and 2, 5, 14, and 28 days after induction of diabetes. Outer renal medullary oxygen tension and blood flow were measured by the combined OxyLite/OxyFlo probes.

RESULTS

Both OxyLite and BOLD MRI showed a significant increase in the renal hypoxia levels after STZ at all time points. However, no changes were observed in the outer renal medullary oxygen tension and blood flow between diabetic and control groups.

CONCLUSIONS

These preliminary results suggest that hypoxic changes can be detected as early as 2 days in rat kidneys with diabetes by BOLD MRI and that these early changes are not dependent on blood flow.

Functional Evaluation of Transplanted Kidneys with Diffusion-weighted and BOLD MR Imaging: Initial Experience

PURPOSE

To prospectively evaluate feasibility and reproducibility of diffusion-weighted (DW) and blood oxygenation level-dependent (BOLD) magnetic resonance (MR) imaging in patients with renal allografts, as compared with these features in healthy volunteers with native kidneys.

MATERIALS AND METHODS

The local ethics committee approved the study protocol; patients provided written informed consent. Fifteen patients with a renal allograft and in stable condition (nine men, six women; age range, 20-67 years) and 15 age- and sex-matched healthy volunteers underwent DW and BOLD MR imaging. Seven patients with renal allografts were examined twice to assess reproducibility of results. DW MR imaging yielded a total apparent diffusion coefficient including diffusion and microperfusion (ADC(tot)), as well as an ADC reflecting predominantly pure diffusion (ADC(D)) and the perfusion fraction. R2* of BOLD MR imaging enabled the estimation of renal oxygenation. Statistical analysis was performed, and analysis of variance was used for repeated measurements. Coefficients of variation between and within subjects were calculated to assess reproducibility.

RESULTS

In patients, ADC(tot), ADC(D), and perfusion fraction were similar in the cortex and medulla. In volunteers, values in the medulla were similar to those in the cortex and medulla of patients; however, values in the cortex were higher than those in the medulla (P < .05). Medullary R2* was higher than cortical R2* in patients (12.9 sec(-1) +/- 2.1 [standard deviation] vs 11.0 sec(-1) +/- 0.6, P < .007) and volunteers (15.3 sec(-1) +/- 1.1 vs 11.5 sec(-1) +/- 0.5, P < .0001). However, medullary R2* was lower in patients than in volunteers (P < .004). Increased medullary R2* was paralleled by decreased diffusion in patients with allografts. A low coefficient of variation in the cortex and medulla within subjects was obtained for ADC(tot), ADC(D), and R2* (<5.2%), while coefficient of variation within subjects was higher for perfusion fraction (medulla, 15.1%; cortex, 8.6%). Diffusion and perfusion indexes correlated significantly with serum creatinine concentrations.

CONCLUSION

DW and BOLD MR imaging are feasible and reproducible in patients with renal allografts.

MR urography in children

MR urography represents the next step in the evolution of uroradiology in children by combining superb anatomic imaging with quantitative functional evaluation in a single examination that does not use ionizing radiation. MR imaging has inherently greater soft-tissue contrast than other imaging techniques. When used in conjunction with dynamic scanning after administration of a contrast agent, it provides non-invasive analysis of the perfusion, concentration and excretion of each kidney. The purpose of this review is to outline our experience with more than 500 MR urograms in children. We outline our technique in detail, showing how we calculate differential renal function and how we assess concentration and excretion in the different regions of the kidney. We show that the dynamic contrast-enhanced data can be processed to yield quantitative measures of individual kidney GFR. In the clinical section we show how MR urography adds unique aspects to the anatomic evaluation of the urinary tract, and by combining the anatomic information with functional information, how we assess hydronephrosis and obstructive uropathy, congenital malformations, pyelonephritis and renal scarring.

BOLD-MRI assessment of intrarenal oxygenation and oxidative stress in patients with chronic kidney allograft dysfunction

Blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) uses deoxyhemoglobin as an endogenous contrast agent for the noninvasive assessment of tissue oxygen bioavailability. We hypothesized that intrarenal oxygenation was impaired in patients with chronic allograft nephropathy (CAN). Ten kidney-transplant recipients with CAN and nine healthy volunteers underwent BOLD-MRI. Medullary R2* (MR2*) and cortical R2* (CR2*) levels (measures directly proportional to tissue deoxyhemoglobin levels) were determined alongside urine and serum markers of oxidative stress (OS): hydrogen peroxide (H(2)O(2)), F(2)-isoprostanes, total nitric oxide (NO), heat shock protein 27 (HSP27), and total antioxidant property (TAOP). Mean MR2* and CR2* levels were significantly decreased in CAN (increased local oxyhemoglobin concentration) compared with healthy volunteers (20.7 +/- 1.6 vs. 23.1 +/- 1.8/s, P = 0.03 and 15.9 +/- 1.9 vs. 13.6 +/- 2.3/s, P = 0.05, respectively). There was a significant increase in serum and urine levels of H(2)O(2) and serum HSP27 levels in patients with CAN. Conversely, urine NO levels and TAOP were significantly increased in healthy volunteers. Multiple linear regression analyses showed a significant association between MR2* and CR2* levels and serum/urine biomarkers of OS. BOLD-MRI demonstrated significant changes in medullary and cortical oxygen bioavailability in allografts with CAN. These correlated with serum/urine biomarkers of OS, suggesting an association between intrarenal oxygenation and OS.

Bilder in der Urologie: Faszination und Perspektiven

In contrast to other countries (e.g., USA) the German urologist routinely utilizes imaging in order to evaluate urological disorders. Ultrasound as a basic tool has acquired importance similar to the physical examination or the patient history. Because of its minimal invasiveness and low cost, it is increasingly utilized as a first-line exam.In correlation with the patient history and laboratory data more invasive imaging studies are performed and in unclear cases or in the preoperative work-up more extensive imaging procedures like computed tomography (CT) or magnetic resonance imaging (MRI) are utilized. Even in emergency situations the urologist is able to guide interventions under ultrasound or conventional X-ray guidance (e.g., percutaneous drainage of dilated kidney), which resulted in a much lower complication rate of the various procedures. In those cases in which ultrasound is technically infeasible or in unclear cases CT and MRI are used as problem-solving procedures and are able to give the correct diagnosis in a large percentage of cases.After a brief historical overview, newer modalities and innovative techniques are explored and presented. Assuming that these innovative approaches lead to more accurate diagnosis and staging of various neoplastic and nonneoplastic conditions, treatment can be performed in earlier stages of diseases and better stage-adapted treatment can be offered to the patients.

Noninvasive Assessment of Early Kidney Allograft Dysfunction by Blood Oxygen Level-Dependent Magnetic Resonance Imaging

BACKGROUND

Blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) is a noninvasive method to assess tissue oxygen bioavailability, using deoxyhemoglobin as an endogenous contrast agent. We hypothesized that BOLD-MRI could accurately discriminate different types of rejection early after kidney transplantation.

METHODS

Twenty-three patients underwent imaging in the first four months posttransplant. Five had normal functioning transplants and 18 had biopsy-proven acute allograft dysfunction (acute tubular necrosis [ATN, n=5] and acute rejection [n=13] including borderline rejection: n=3; IA rejection: n=4; IIA rejection: n=6: C4d(+) rejection: n=9).

RESULTS

Mean medullary R2* (MR2*) levels (a measure directly proportional to tissue deoxyhemoglobin levels) were significantly higher in normal functioning allografts (R2*=24.3/s+/-2.3) versus acute rejection (R2*=16.6/s+/-2.1) and ATN (R2*=20.9/s+/-1.8) (P<0.05). The lowest MR2* levels were observed in acute rejection episodes with vascular injury i.e. IIA and C4d (+). Similarly, the lowest medullary to cortical R2* ratios (MCR2*) were present in allografts with IIA (1.24+/-0.05) and C4d(+) rejection (1.26+/-0.06). ROC curve analyses suggested that MR2* and MCR2* values could accurately discriminate acute rejection in the early posttransplant period.

CONCLUSIONS

BOLD-MRI demonstrated significant changes in medullary oxygen bioavailability in allografts with biopsy-proven ATN and acute rejection, suggesting that there may be a role for this noninvasive tool to evaluate kidney function early after transplantation.

Functional MRI of the kidney: tools for translational studies of pathophysiology of renal disease

Magnetic resonance imaging (MRI) provides exquisite anatomic detail of various organs and is capable of providing additional functional information. This combination allows for comprehensive diagnostic evaluation of pathologies such as ischemic renal disease. Noninvasive MRI techniques could facilitate translation of many studies performed in controlled animal models using technologies that are invasive to humans. Such a translation is being recognized as essential because many proposed interventions and drugs that prove efficacious in animal models fail to do so in humans. In this article, we review the state-of-the-art functional MRI technique as applied to the kidneys.

BOLD-MRI for the assessment of renal oxygenation in humans: acute effect of nephrotoxic xenobiotics

Hypoxia of renal medulla is a key factor implicated in the development of drug-induced renal failure. Drugs are known to influence renal hemodynamics and, subsequently, affect renal tissue oxygenation. Changes in renal oxygenation can be assessed non-invasively in humans using blood oxygenation level-dependent magnetic resonance imaging (BOLD-MRI). This study was designed to test the acute effects of administration of specific drugs in healthy human kidney oxygenation using BOLD-MRI. Acute changes in renal tissue oxygenation induced by the non-steroidal anti-inflammatory drug indomethacin, the iodinated radio-contrast media (RCM) iopromidum, and the calcineurin inhibitors cyclosporine micro-emulsion (CsA-ME) and tracrolimus were studied in 30 healthy volunteers. A modified Multi Echo Data Image Combination sequence was used to acquire 12 T(2)(*)-weighted images. Four coronal slices were selected to cover both kidneys. The mean R(2)(*) (1/T(2)(*)) values determined in medulla and cortex showed no significant changes induced by indomethacin and tacrolimus administration. CsA-ME decreased medullary (P=0.008) and cortical (P=0.004) R(2)(*) values 2 h after ingestion. Iopromidum caused a significant increase in medullary R(2)(*) within the first 20 min after injection (P<0.001), whereas no relevant changes were observed in renal cortex. None of the measurements showed left-right kidney differences. Significant differences in renal medullary oxygenation were evidenced between female and male subjects (P=0.013). BOLD-MRI was efficient to show effects of specific drugs in healthy renal tissue. Cyclosporine increased renal medullary oxygenation 2 h after ingestion of a single dose, whereas indomethacin and tacrolimus showed no effect on renal oxygenation. Injection of iodinated RCM decreased renal medullary oxygenation.

Evaluation of intra-renal oxygenation during water diuresis: a time-resolved study using BOLD MRI

Hypoxia of the renal medulla is a possible precursor to the onset of acute renal failure in humans and therefore an understanding of the factors influencing the oxygenation status within the renal medulla is very important. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) has been shown to non-invasively evaluate intra-renal oxygenation levels of the renal medulla in humans. A newly implemented three-dimensional (3-D) multiple gradient-recalled echo sequence, which permits examination of temporal responses to physiological or pharmacological stimuli, was used to monitor changes in intra-renal oxygenation status during water diuresis. Five healthy, young subjects (22+/-1.2 years) took part in the study. BOLD MRI data were acquired before and after water loading. Studies were repeated on a separate day after the subjects were pretreated with naproxen. Water diuresis significantly improved renal medullary oxygenation levels in all subjects (pre-waterload=30.3 1/s vs post-waterload 22.8 1/s); however, the temporal response was found to be subject dependent. In the presence of cyclooxygenase (COX) inhibition by naproxen, the improvement in oxygenation during water diuresis was completely abolished (pre-waterload=27.5 1/s vs post-waterload 28.5 1/s). Monitoring of temporal responses for the first time during water loading allowed for an appreciation of subject dependence. Comparison of the temporal response in terms of slopes demonstrated a significant difference between the waterload studies with and without naproxen (with naproxen=0.056 1/(s min) vs without naproxen=0.25 1/(s min)). The observed effects of naproxen were consistent with previous findings with COX inhibition.

Evaluation of Intra-Renal Oxygenation by BOLD MRI

This is a review of blood oxygenation level-dependent (BOLD) MRI as applied to the kidney. It has been shown that BOLD MRI measurements reflect changes in renal oxygenation, especially in the medulla. Renal medulla functions in a hypoxic milieu and is extremely sensitive to further decrease in blood flow or increase in oxygen consumption. Availability of a non-invasive technique such as BOLD MRI should allow for better understanding of the factors involved in the maintenance of renal oxygenation status, not only in animal models, but also in humans.

Update of renal imaging

Significant technical improvements have allowed the use of radiological techniques to play a growing role in the imaging of renal diseases. Noninvasive ultrasound methods (ie, sonography and Doppler) are now positioned as first-line methods for the evaluation of renovascular diseases. Multidetector computed tomography is able to provide high spatial resolution images of the kidneys and renal arterial vessels. Magnetic resonance imaging, which provides higher signal-to-noise ratio and higher spatial and/or temporal resolution, can display both morphological information about renal parenchyma and vessels and functional data, including perfusion, filtration, diffusion, or oxygenation. In renovascular diseases, these techniques have the potential to drive new strategies, including Doppler sonography as a first-line method, followed by computed tomography angiography or magnetic resonance angiography, depending mainly on renal function. Imaging of parenchymal renal diseases is developing toward more quantitative (volumetric and functional measurements) and more specific (through in vivo cell targeting) acquisitions for obtaining the adequate information on tissue characteristics relevant either for diagnosis or for prognosis or treatment follow-up.

Evaluation of Intrarenal Oxygenation at 3.0 T Using 3-Dimensional Multiple Gradient-Recalled Echo Sequence

OBJECTIVE

The objective of this study was to validate quantitation of R2* and DeltaR2* measurements obtained with a 3-dimensional (3-D) multiple gradient-recalled echo (mGRE) sequence for evaluating intrarenal oxygenation in humans.

MATERIALS AND METHODS

Validation was accomplished (1) by comparing R2* values with previously established 2-D techniques (n = 5, mean age = 33.6 years) and (2) by measuring change in DeltaR2* after furosemide (20 mg intravenously) administration (n = 5, mean age = 22 years). Additional pre- and postfurosemide scans were done at 1.5 T for comparison purposes.

RESULTS

R2* measurements with the 3-D technique showed good agreement with the 2-D techniques. The baseline medullary R2* at 3.0 T was about twice the value found at 1.5 T. Furosemide-induced change in R2* was observed within 5 minutes after administration.

CONCLUSIONS

R2* measurements with 3-D mGRE were comparable with those reported using 2-D techniques. The 3-D implementation facilitates observation of temporal changes in the medullary oxygenation without compromising spatial coverage.

Non-invasive monitoring of renal oxygenation using BOLD-MRI: a reproducibility study

Blood oxygenation level-dependent (BOLD) MRI was shown to allow non-invasive observation of renal oxygenation in humans. However, clinical applications of this type of functional MRI of the kidney are still limited, most likely because of difficulties in obtaining reproducible and reliable information. The aim of this study was to evaluate the reproducibility and robustness of a BOLD method applied to the kidneys and to identify systematic physiological changes potentially influencing the renal oxygenation of healthy volunteers. To measure the BOLD effect, a modified multi-echo data image combination (MEDIC) sequence was used to acquire 12 T2*-weighted images within a single breath-hold. Three identical measurements were performed on three axial and three coronal slices of right and left kidneys in 18 volunteers. The mean R2* (1/T2*) values determined in medulla and cortex showed no significant differences over three repetitions and low intra-subject coefficients of variation (CV) (3 and 4% in medulla and cortex, respectively). The average R2* values were higher in the medulla (16.15 +/- 0.11) than in the cortex (11.69 +/- 0.18) (P < 0.001). Only a minor influence of slice orientation was observed. Mean R2* values were slightly higher (3%) in the left than in the right kidney (P < 0.001). Differences between volunteers were identified (P < 0.001). Part of these differences was attributable to age-dependent R2* values, since these values increased with age when medulla (P < 0.001, r = 0.67) or cortex (P < 0.020, r = 0.42) were considered. Thus, BOLD measurements in the kidney are highly reproducible and robust. The results allow one to identify the known cortico-medullary gradient of oxygenation evidenced by the gradient of R2* values and suggest that medulla is more hypoxic in older than younger individuals. BOLD-MRI is therefore a useful tool to study sequentially and non-invasively regional oxygenation of human kidneys.

MR imaging of nephropathies

Acute and chronic nephropathies are responsible for morphologic and functional renal changes. However, radiologic techniques currently play a minor role in imaging of parenchymal nephropathies in native or transplanted kidneys. From a morphologic point of view, three-dimensional magnetic resonance (MR) volumetric biomarkers of kidney function, such as renal and cortical volumes or cystic volume, in polycystic kidney diseases play a growing role in nephrologic practice. From a functional point of view, if scintigraphic techniques remain the major sources of renal performance assessment, new MR imaging systems and specific MR contrast agents may soon provide significant developments in the evaluation of renal performance (glomerular filtration rate measurement), in the search for prognostic factors (hypoxia, inflammation, cell viability, degree of tubular function, and interstitial fibrosis), and for monitoring new cell therapies. New developments that have provided higher signal-to-noise ratio and higher spatial and/or temporal resolutions have the potential to direct new opportunities for obtaining morphologic and functional information on tissue characteristics that are relevant for various renal diseases with respect to diagnosis, prognosis, and treatment follow-up.

Assessment of acute renal transplant rejection with blood oxygen level-dependent MR imaging: initial experience

PURPOSE

To prospectively assess the oxygenation state of renal transplants and determine the feasibility of using blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging to differentiate between acute tubular necrosis (ATN), acute rejection, and normal function.

MATERIALS AND METHODS

This HIPAA-compliant study had institutional human subjects review committee approval, and written informed consent was obtained from all patients. BOLD MR imaging was performed in 20 patients (age range, 21-70 years) who had recently received renal transplants. Six patients had clinically normal functioning transplants, eight had biopsy-proved rejection, and six had biopsy-proved ATN. R2* (1/sec) measurements were obtained in the medulla and cortex of transplanted kidneys. R2* is a measure of the rate of signal loss in a specific region and is related to the amount of deoxyhemoglobin present. Statistical analysis was performed by using a two-sample t test. Threshold R2* values were identified to discriminate between transplanted kidneys with ATN, those with acute rejection, and those with normal function.

RESULTS

R2* values for the medulla were significantly lower in the acute rejection group (R2* = 15.8/sec +/- 1.5) than in normally functioning transplants (R2* = 23.9/sec +/- 3.2) and transplants with ATN (R2* = 21.3/sec +/- 1.9). The differences between the acute rejection and normal function groups (P = .001), as well as between the acute rejection and ATN groups (P < .001), were significant. Acute rejection could be differentiated from normal function and ATN in all cases by using a threshold R2* value of 18/sec. R2* values for the cortex were higher in ATN (R2* = 14.2/sec +/- 1.4) than for normally functioning transplants (R2* = 12.7/sec +/- 1.6) and transplants with rejection (R2* = 12.4/sec +/- 1.2). The difference in R2* values in the cortex between ATN and rejection was statistically significant (P = .034), although there was no threshold value that enabled differentiation of all cases of ATN from cases of normal function or acute rejection.

CONCLUSION

R2* measurements in the medullary regions of transplanted kidneys with acute rejection were significantly lower than those in normally functioning transplants or transplants with ATN. These results suggest that marked changes in intrarenal oxygenation occur during acute transplant rejection.