Noncontrast Functional MRI of the Kidneys

A Review and Case Study of 3D Imaging Modalities for Female Amniote Reproductive Anatomy

Recent advances in non-invasive imaging methods have revitalised the field of comparative anatomy, and reproductive anatomy has been no exception. The reproductive systems of female amniotes present specific challenges, namely their often internal "hidden" anatomy. Quantifying female reproductive systems is crucial to recognising reproductive pathologies, monitoring menstrual cycles, and understanding copulatory mechanics. Here we conduct a review of the application of non-invasive imaging techniques to female amniote reproductive anatomy. We introduce the commonly used imaging modalities of computed tomography (CT) and magnetic resonance imaging (MRI), highlighting their advantages and limitations when applied to female reproductive tissues, and make suggestions for future advances. We also include a case study of micro CT and MRI, along with their associated staining protocols, applied to cadavers of female adult stoats (Mustela erminea). In doing so, we will progress the discussion surrounding the imaging of female reproductive anatomy, whilst also impacting the fields of sexual selection research and comparative anatomy more broadly.

Dynamic R2' Imaging can Be a Biomarker for Diagnosing and Staging Early Acute Kidney Injury in Animals

Background: Early diagnosis of acute kidney injury (AKI) is essential in clinical settings. None of the current biomarkers are widely applied. The combination of pulse-shifting multi-echo asymmetric spin-echo sequence (psMASE) and a modified hemodynamic response imaging (HRI) technique is promising. The purpose of this study was to evaluate the feasibility of psMASE combined with HRI in detecting early ischemic AKI in animal models of different severities.

Methods: Twenty rabbits were divided into four groups (mild, moderate, and severe AKI and control groups). Transarterial embolization with different doses of microspheres was performed to establish AKI animal models of different severities. The 3T psMASE and HRI scans of kidneys were conducted. The R2^*, R2, and R2' during room air and gas stimulation were acquired and the difference of R2' (dR2') was evaluated in different AKI groups.

Results: The values were not different in R2^* and R2 during room air and in R2^* and R2, and R2' during gas stimulation. The value of R2' was significantly different during room air (P = 0.014), but the difference was only found between control and moderate/severe AKI groups (P = 0.032 and 0.022). The values of dR2' were different among groups (P < 0.0001) and differences between every two groups except comparison of moderate and severe AKI groups were significant (P < 0.01).

Conclusion: The dR2' imaging acquired by a combination of renal psMASE and HRI technique can serve as a potential quantitative biomarker for early detection and staging of AKI.

Evaluation of renal oxygenation change under the influence of carbogen breathing using a dynamic R2 , R2 ' and R2 * quantification approach

The purpose of this study is to demonstrate the feasibility of dynamic renal R₂ /R₂ '/R₂ * measurements based on a method, denoted psMASE-ME, in which a periodic 180° pulse-shifting multi-echo asymmetric spin echo (psMASE) sequence, combined with a moving estimation (ME) strategy, is adopted. Following approval by the institutional animal care and use committee, a block design of respiratory challenge with interleaved air and carbogen (97% O₂ , 3% CO₂ ) breathing was employed in nine rabbits. Parametrical R₂ /R₂ '/R₂ * maps were computed and average R₂ /R₂ '/R₂ * values were measured in regions of interest in the renal medulla and cortex. Bland-Altman plots showed good agreement between the proposed method and reference standards of multi-echo spin echo and multi-echo gradient echo sequences. Renal R₂ , R₂ ' and R₂ * decreased significantly from 16.2 ± 4.4 s^-1 , 9.8 ± 5.2 s^-1 and 25.9 ± 5.0 s^-1 to 14.9 ± 4.4 s^-1 (p < 0.05), 8.5 ± 4.1 s^-1 (p < 0.05) and 23.4 ± 4.8 s^-1 (p < 0.05) in the cortex when switching the gas mixture from room air to carbogen. In the renal medulla, R₂ , R₂ ' and R₂ * also decreased significantly from 12.9 ± 4.7 s^-1 , 15.1 ± 5.8 s^-1 and 27.9 ± 5.3 s^-1 to 11.8 ± 4.5 s^-1 (p < 0.05), 14.2 ± 4.2 s^-1 (p < 0.05) and 25.8 ± 5.1 s^-1 (p < 0.05). No statistically significant differences in relative R₂ , R₂ ' and R₂ * changes were observed between the cortex and medulla (p = 0.72 for R₂ , p = 0.39 for R₂ ' and p = 0.61 for R₂ *). The psMASE-ME method for dynamic renal R₂ /R₂ '/R₂ * measurements, together with the respiratory challenge, has potential use in the evaluation of renal oxygenation in many renal diseases.

MRI tools for assessment of microstructure and nephron function of the kidney

MRI can provide excellent detail of renal structure and function. Recently, novel MR contrast mechanisms and imaging tools have been developed to evaluate microscopic kidney structures including the tubules and glomeruli. Quantitative MRI can assess local tubular function and is able to determine the concentrating mechanism of the kidney noninvasively in real time. Measuring single nephron function is now a near possibility. In parallel to advancing imaging techniques for kidney microstructure is a need to carefully understand the relationship between the local source of MRI contrast and the underlying physiological change. The development of these imaging markers can impact the accurate diagnosis and treatment of kidney disease. This study reviews the novel tools to examine kidney microstructure and local function and demonstrates the application of these methods in renal pathophysiology.

Nuclear medicine and the emergency department patient: an illustrative case-based approach

Radiologists are familiar with the use of radiographs, computed tomography, magnetic resonance imaging and ultrasound in the acute clinical setting. However, there are some specific clinical scenarios which may be found in nuclear medicine imaging problem-solving tools. These clinical scenarios and imaging techniques are less frequent, and the referring clinician from the emergency department may not consider these alternatives. It is important for the radiologist to be aware of these techniques to be able to guide the clinician to use those tools, which may result in optimal patient care. In this article, we will discuss those nuclear medicine studies which have application in the setting of an emergency radiology practice.

Image registration for triggered and non-triggered DTI of the human kidney: reduced variability of diffusion parameter estimation

BACKGROUND

To investigate if non-rigid image-registration reduces motion artifacts in triggered and non-triggered diffusion tensor imaging (DTI) of native kidneys. A secondary aim was to determine, if improvements through registration allow for omitting respiratory-triggering.

METHODS

Twenty volunteers underwent coronal DTI of the kidneys with nine b-values (10-700 s/mm(2) ) at 3 Tesla. Image-registration was performed using a multimodal nonrigid registration algorithm. Data processing yielded the apparent diffusion coefficient (ADC), the contribution of perfusion (FP ), and the fractional anisotropy (FA). For comparison of the data stability, the root mean square error (RMSE) of the fitting and the standard deviations within the regions of interest (SDROI ) were evaluated.

RESULTS

RMSEs decreased significantly after registration for triggered and also for non-triggered scans (P < 0.05). SDROI for ADC, FA, and FP were significantly lower after registration in both medulla and cortex of triggered scans (P < 0.01). Similarly the SDROI of FA and FP decreased significantly in non-triggered scans after registration (P < 0.05). RMSEs were significantly lower in triggered than in non-triggered scans, both with and without registration (P < 0.05).

CONCLUSION

Respiratory motion correction by registration of individual echo-planar images leads to clearly reduced signal variations in renal DTI for both triggered and particularly non-triggered scans. Secondarily, the results suggest that respiratory-triggering still seems advantageous.

Diffusion Tensor Imaging of the Kidneys: Influence of b-Value and Number of Encoding Directions on Image Quality and Diffusion Tensor Parameters

OBJECTIVES

The purpose of this study was to evaluate to which degree investment of acquisition time in more encoding directions leads to better image quality (IQ) and what influence the number of encoding directions and the choice of b-values have on renal diffusion tensor imaging (DTI) parameters.

MATERIAL AND METHODS

Eight healthy volunteers (32.3 y ± 5.1 y) consented to an examination in a 1.5T whole-body MR scanner. Coronal DTI data sets of the kidneys were acquired with systematic variation of b-values (50, 150, 300, 500, and 700 s/mm(2)) and number of diffusion-encoding directions (6, 15, and 32) using a respiratory-triggered echo-planar sequence (TR/TE 1500 ms/67 ms, matrix size 128 × 128). Additionally, two data sets with more than two b-values were acquired (0, 150, and 300 s/mm(2) and all six b-values). Parametrical maps were calculated on a pixel-by-pixel basis. Image quality was determined with a reader score.

RESULTS

Best IQ was visually assessed for images acquired with 15 and 32 encoding directions, whereas images acquired with six directions had significantly lower IQ ratings. Image quality, fractional anisotropy, and mean diffusivity only varied insignificantly for b-values between 300 and 500 s/mm(2). In the renal medulla fractional anisotropy (FA) values between 0.43 and 0.46 and mean diffusivity (MD) values between 1.8-2.1 × 10(-3) mm(2)/s were observed. In the renal cortex, the corresponding ranges were 0.24-0.25 (FA) and 2.2-2.8 × 10(-3) mm(2)/s (MD). Including b-values below 300 s/mm(2), notably higher MD values were observed, while FA remained constant. Susceptibility artifacts were more prominent in FA maps than in MD maps.

CONCLUSION

In DTI of the kidneys at 1.5T, the best compromise between acquisition time and resulting image quality seems the application of 15 encoding directions with b-values between 300 and 500 s/mm(2). Including lower b-values allows for assessment of fast diffusing spin components.