Liver imaging at 1.5 tesla: pulse sequence optimization based on improved measurement of tissue relaxation times

A novel moving phantom insert for image quality assessment in magnetic resonance imaging

Dedicated motion compensated Magnetic Resonance Imaging (MRI) for radiotherapy treatment planning promises to mitigate motion effects on imaging. We demonstrate a novel insert for an MRI safe motion phantom, which enables quality assurance of these image strategies. The capability to analyse apparent slice thickness, positional accuracy and motion blur is demonstrated for scenarios with and without motion. A respiratory-compensated scan with a 4 mm trigger-window and 16 mm peak-to-peak (p2p) motion showed a +5.0% deviation from the nominal 2 mm slice thickness. In contrast, a non-compensated scan with 4 mm p2p motion showed a +77.5% deviation, illustrating the effectiveness of motion compensation.

Survey of water proton longitudinal relaxation in liver in vivo

Objective

To determine the variability, and preferred values, for normal liver longitudinal water proton relaxation rate R₁ in the published literature.

Methods

Values of mean R₁ and between-subject variance were obtained from literature searching. Weighted means were fitted to a heuristic and to a model.

Results

After exclusions, 116 publications (143 studies) remained, representing apparently normal liver in 3392 humans, 99 mice and 249 rats. Seventeen field strengths were included between 0.04 T and 9.4 T. Older studies tended to report higher between-subject coefficients of variation (CoV), but for studies published since 1992, the median between-subject CoV was 7.4%, and in half of those studies, measured R₁ deviated from model by 8.0% or less.

Discussion

The within-study between-subject CoV incorporates repeatability error and true between-subject variation. Between-study variation also incorporates between-population variation, together with bias from interactions between methodology and physiology. While quantitative relaxometry ultimately requires validation with phantoms and analysis of propagation of errors, this survey allows investigators to compare their own R₁ and variability values with the range of existing literature.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10334-021-00928-x.

Gadoxetic acid-enhanced hepatobiliary phase MRI and high-b-value diffusion-weighted imaging to distinguish well-differentiated hepatocellular carcinomas from benign nodules in patients with chronic liver disease

OBJECTIVE

The purpose of this article is to evaluate the value of gadoxetic acid-enhanced hepatobiliary phase imaging and high-b-value diffusion-weighted imaging (DWI) for distinguishing well-differentiated hepatocellular carcinomas (HCCs) from benign hepatocellular nodules in patients with chronic liver disease using 3-T MRI.

MATERIALS AND METHODS

Forty-five patients with 46 well-differentiated HCCs (mean, 2.3 cm) and 21 patients with 24 benign hepatocellular nodules (five large regenerative nodules and 19 dysplastic nodules; mean, 1.8 cm) were included in this study. Diagnosis of well-differentiated HCCs and benign hepatocellular nodules was made histopathologically by percutaneous biopsy (n = 12 and n = 11, respectively) or surgical resection (n = 34 and n = 13, respectively). Gadoxetic acid-enhanced MRI was performed for all patients, and DWI (b values of 0 and 800 s/mm(2)) was performed for 31 well-differentiated HCCs and 11 benign hepatocellular nodules. Two radiologists performed a consensus review of the MRI scans for signal intensity compared with that of the surrounding liver parenchyma on hepatobiliary phase images and DWI (b value, 800 s/mm(2)) for qualitative analysis. The contrast-to-noise ratio (CNR) and relative contrast enhancement of lesions on hepatobiliary phase images and the apparent diffusion coefficient (ADC) values were assessed for quantitative analysis.

RESULTS

In the qualitative analysis, 39 well-differentiated HCCs (85%) and 14 benign hepatocellular nodules (58%) were hypointense on hepatobiliary phase images, and seven well-differentiated HCCs (15%) and 10 benign hepatocellular nodules (42%) were iso- or hyperintense (p = 0.04). Twenty-five well-differentiated HCCs (81%) and three benign hepatocellular nodules (27%) were hyperintense on DWI, with b value of 800 s/mm(2), and six well-differentiated HCCs (19%) and eight benign hepatocellular nodules (73%) were iso- or hypointense (p = 0.01). When lesion hypointensity on hepatobiliary phase images or hyperintensity on DWI were considered signs of HCC in cirrhotic liver, our results yielded sensitivities of 85% and 81% and specificities of 42% and 73%, respectively. In the quantitative analysis, the mean (± SD) relative contrast enhancement ratio of the well-differentiated HCCs (0.76 ± 2.30) was significantly higher than that of benign hepatocellular nodules (0.25 ± 0.97) (p = 0.02). The lesion-to-liver CNRs and the mean ADC values were not significantly different between the two groups (p > 0.05).

CONCLUSION

Hypointensity on gadoxetic acid-enhanced hepatobiliary phase images and hyperintensity on high-b-value DWI to surrounding liver parenchyma suggest well-differentiated HCCs rather than benign hepatocellular nodules in chronic liver disease.

Quantifying differences in hepatic uptake of the liver specific contrast agents Gd-EOB-DTPA and Gd-BOPTA: a pilot study

OBJECTIVES

To develop and evaluate a procedure for quantifying the hepatocyte-specific uptake of Gd-BOPTA and Gd-EOB-DTPA using dynamic contrast-enhanced (DCE) MRI.

METHODS

Ten healthy volunteers were prospectively recruited and 21 patients with suspected hepatobiliary disease were retrospectively evaluated. All subjects were examined with DCE-MRI using 0.025 mmol/kg of Gd-EOB-DTPA. The healthy volunteers underwent an additional examination using 0.05 mmol/kg of Gd-BOPTA. The signal intensities (SI) of liver and spleen parenchyma were obtained from unenhanced and enhanced acquisitions. Using pharmacokinetic models of the liver and spleen, and an SI rescaling procedure, a hepatic uptake rate, K (Hep), estimate was derived. The K (Hep) values for Gd-EOB-DTPA were then studied in relation to those for Gd-BOPTA and to a clinical classification of the patient's hepatobiliary dysfunction.

RESULTS

K (Hep) estimated using Gd-EOB-DTPA showed a significant Pearson correlation with K (Hep) estimated using Gd-BOPTA (r = 0.64; P < 0.05) in healthy subjects. Patients with impaired hepatobiliary function had significantly lower K (Hep) than patients with normal hepatobiliary function (K (Hep) = 0.09 ± 0.05 min(-1) versus K (Hep) = 0.24 ± 0.10 min(-1); P < 0.01).

CONCLUSIONS

A new procedure for quantifying the hepatocyte-specific uptake of T (1)-enhancing contrast agent was demonstrated and used to show that impaired hepatobiliary function severely influences the hepatic uptake of Gd-EOB-DTPA.

KEY POINTS

• The liver uptake of contrast agents may be measured with standard clinical MRI. • Calculation of liver contrast agent uptake is improved by considering splenic uptake. • Liver function affects the uptake of the liver-specific contrast agent Gd-EOB-DTPA. • Hepatic uptake of two contrast agents (Gd-EOB-DTPA, Gd-BOPTA) is correlated in healthy individuals. • This method can be useful for determining liver function, e.g. before hepatic surgery.

Imaging at higher magnetic fields: 3 T versus 1.5 T

Clinical hepatobiliary magnetic resonance (MR) imaging continues to evolve at a fast rate. However, three basic requirements must still be satisfied if novel high-field MR imaging techniques are to be included in the hepatobiliary imaging routine: improvement of parenchymal contrast, suppression of respiratory motion artifact, and anatomic coverage of the entire hepatobiliary system. This article outlines the various arenas involved in MR imaging of the hepatobiliary system at 3 Tesla (T) compared with 1.5 T by (1) highlighting magnetic field-dependent MR contrast phenomena that contribute to the overall appearance of high-field hepatobiliary imaging; (2) summarizing the biodistributions of different gadolinium chelates used as MR contrast agents and their effectiveness regarding the static magnetic field; (3) showing the implementation of advanced imaging techniques such as three-dimensional acquisition schemes and parallel acceleration techniques used in T1-, T2-, and diffusion-weighted hepatobiliary imaging; and (4) addressing artifact mechanisms exacerbated by, or originating from, increase of the static magnetic field.

[Modification of longitudinal relaxation time (T1) as a biomarker of patellar cartilage degeneration]

OBJECTIVES

To study the viability of longitudinal relaxation time (T1) of patellar cartilage as a biomarker of the degree of degeneration.

MATERIAL AND METHODS

We included 15 subjects classified into three groups according to clinical criteria (pain, functional limitation, and duration of symptoms) and imaging criteria as follows: (a) normal (3 men, 2 women; age 30+/-14 years), (b) with initial degeneration of the patellar cartilage (3 men, 2 women; age 30+/-6 years), or (c) with advanced degeneration (3 men, 2 women; age 57+/-10 years). All underwent MRI examination using special echo-gradient sequences to segment the cartilage and calculate the T1 maps. We selected the entire cartilage and the regions of interest classified according to clinical and imaging criteria as normal, initial degeneration, and advanced degeneration. The T1 values of the cartilage were obtained pixel by pixel and were calculated as the mean for the entire cartilage or by subregions (normal, initial, advanced). Differences between groups for the entire cartilage and the regions were analyzed using Student-Newman-Keuls post-hoc ANOVA. Reproducibility was evaluated using the coefficient of variance.

RESULTS

No significant differences in the overall analysis of the entire cartilage were found between the three groups (normal: 1003+/-172 ms, initial: 1064+/-124 ms, advanced: 1041+/-308 ms, p=0.665). However, the analysis by regions revealed significant differences (normal: 908+/-53 ms, initial degeneration: 1057+/-157 ms, advanced degeneration: 1133+/-116 ms, p=0.029). The reproducibility analysis found variations of 1.3% for the overall calculation, 3.7% for the regional calculation, and 8.2% for the acquisition.

CONCLUSION

In this preliminary study, calculating the T1 of the cartilage enabled regions with different degrees of degeneration to be differentiated.

Diffuse liver disease: strategies for hepatic CT and MR imaging

The liver plays several complex but essential roles in the metabolism of amino acids, carbohydrates, and lipids, as well as synthesis of proteins. The basic pathophysiology of diffuse parenchymal hepatic diseases usually represents a failure in one of these metabolic pathways. Specific parenchymal diseases can be categorized as storage, vascular, and inflammatory diseases. Cross-sectional hepatic imaging techniques, specifically multidetector computed tomography (CT) and magnetic resonance (MR) imaging, have roles in evaluation of diffuse liver disease. The prominent role of multidetector CT is primarily defined by its excellent morphologic visualization capabilities, in particular of diffuse or focal intrahepatic lesions as well as of anatomic relationships between the liver and adjacent organs. The variety of available multidetector CT scanners covers a huge spectrum of detector configurations ranging from equally sized and equally spaced detector arrays to asymmetric detector configurations, resulting in imaging protocols with unique parameters for almost each multidetector CT system. In addition to 64-detector row imaging, hepatic multidetector CT can be performed with emerging techniques such as dual-energy CT. Hepatic MR imaging has been proved to be a comprehensive modality for assessing the morphology and functional characteristics of the liver. Concurrent technical improvements as well as implementation of advanced imaging sequence designs permit high-quality examination of the liver with T1-, T2-, and diffusion-weighted pulse sequences. Three basic demands remain if MR imaging is chosen for hepatic imaging: to improve parenchymal contrast, to suppress respiratory motion, and to ensure complete anatomic coverage. Supplemental material available at http://radiographics.rsna.org/content/29/6/1591/suppl/DC1.

Diffusion-weighted MRI provides additional value to conventional dynamic contrast-enhanced MRI for detection of hepatocellular carcinoma

The purpose of this study was to evaluate the accuracy of diffusion-weighted magnetic resonance imaging (DW-MRI) in differentiating HCC from benign cirrhotic lesions compared with conventional dynamic contrast-enhanced MRI. Fifty-five patients with cirrhosis underwent conventional and DW-MRI at 1.5 Tesla. Signal intensity ratios (SI(ratio)) of solid liver lesions to adjacent hepatic parenchyma were measured for b0, b100, b600 and b1000, and the apparent diffusion coefficients (ADC) were calculated. In 27 patients, imaging results were compared to histopathology, and in 28 patients, to imaging follow-up. Based on predetermined thresholds, sensitivity and specificity of DW-MRI and conventional MRI were compared. SI(ratio) was significantly different between malignant and benign lesions at all b-values (P < 0.0001). No significant difference in ADC was seen (P = 0.47). For detection of malignant lesions, DW-MRI with b600-SI(ratio) yielded a sensitivity of 95.2% compared to 80.6% for conventional MRI (P = 0.023) and a specificity of 82.7% compared to 65.4% (P = 0.064). The improved accuracy was most beneficial for differentiating malignant lesions smaller than 2 cm. DW-MRI with b600-SI(ratio) improved the detection of small HCC and the differentiation of pseudotumoral lesions compared with conventional MRI.

Single breath-holdT1 measurement using low flip angle TrueFISP

A method for estimating T1 using a single breath-hold, segmented, inversion recovery prepared, true fast imaging with steady-state precession (sIR-TrueFISP) acquisition at low flip angle (FA) was implemented in this study. T1 values measured by sIR-TrueFISP technique in a Gd-DTPA-doped water phantom and the human brain and abdomen of healthy volunteers were compared with the results of the standard IR fast spin echo (FSE) technique. A good correlation between the two methods was observed (R2=0.999 in the phantom, and R2=0.943 in the brain and abdominal tissues). The T1 values of the tissues agreed well with published results. sIR-TrueFISP enables fast measurements of T1 to be obtained within a single breath-hold with good accuracy, which is particularly important for chest and abdominal imaging.

Liver and Spleen Volumetry with Quantitative MR Imaging and Dual-Space Clustering Segmentation

The purpose of this HIPAA-compliant, institutional review board-approved study was to assess the liver and spleen volumes calculated by using a semiautomated dual-space clustering segmentation technique, as compared with the volumes calculated by using the manual contour-tracing method. The quantitative magnetic resonance (MR) imaging data used as input were computed from images acquired by using a mixed fast spin-echo pulse sequence that was implemented with respiratory triggering. Linear regression analysis was used to assess agreement regarding the volumes calculated by using both segmentation techniques. There was strong agreement regarding the regression parameters for the liver (r = 0.98, P < .001) and the spleen (r = 0.99, P < .001) and the mean percentage volume differences for the liver (1.2%) and the spleen (0.9%). The mean segmentation time per patient was significantly shorter with use of the dual-space clustering method (P < .001).

Optimal design of k-space trajectories using a multi-objective genetic algorithm

Spiral, radial, and other nonrectilinear k-space trajectories are an area of active research in MRI due largely to their typically rapid acquisition times and benign artifact patterns. Trajectory design has commonly proceeded from a description of a simple shape to an investigation of its properties, because there is no general theory for the derivation of new trajectories with specific properties. Here such a generalized methodology is described. Specifically, a multi-objective genetic algorithm (GA) is used to design trajectories with beneficial flow and off-resonance properties. The algorithm converges to a well-defined optimal set with standard spiral trajectories on the rapid but low-quality end, and a new class of trajectories on the slower but high-quality end. The new trajectories all begin with nonzero gradient amplitude at the k-space origin, and curve gently outward relative to standard spirals. Improvements predicted in simulated imaging experiments were found to correlate well with improvements in actual experimental measures of image quality. The impact of deviations from the desired k-space trajectory is described, as is the impact of using different phantoms.

A prospective assessment of breath-hold fast spin echo and inversion recovery fast spin echo techniques for detection and characterization of focal hepatic lesions

The purpose of this study was to prospectively assess two breath-hold T(2)-weighted fast spin-echo sequences and two breath-hold inversion recovery fast spin-echo sequences to determine their relative ability to detect and characterize focal hepatic lesions. Fourteen patients with a total of nineteen proven focal hepatic lesions were imaged with two breath-hold T(2)-weighted (T2W) fast spin echo sequences (HASTE TE = 66 and HASTE TE = 120), two breath-hold inversion recovery fast spin echo sequences (IRFSE TE = 64 and IRFSE TE = 95), and a nonbreath-hold T(2)-weighted fast-spin echo sequence (FSE TE = 96-120). Contrast-to-noise ratios (CNRs) were measured for all proven lesions on all sequences. Both IRFSE sequences and the HASTE sequence with TE = 66 showed an improvement in lesion-liver and liver-spleen CNRs compared to the nonbreath-hold T2W sequence. The mean difference in CNR between benign and malignant lesions was largest for the HASTE TE = 120 sequence. These preliminary results suggest that a breath-hold IRFSE sequence (TE = 64 or 95) has an equal ability to detect focal hepatic lesions as a nonbreath-hold T2W FSE sequence (TE = 96-120). The HASTE TE = 120 showed the greatest ability to discriminate between benign and malignant lesions.

A compact respiratory-triggering device for routine microimaging of laboratory mice

A partial-body plethysmograph was developed for measuring the respiratory flow of anesthetized mice during routine microimaging experiments performed in the close confines of an 89-mm-diameter, vertical-bore magnet. Respiratory flow patterns were used for synchronizing conventional T2-weighted spin-echo imaging with the respiratory cycle, thereby, significantly reducing motion-induced artifacts and increasing observed liver lesion contrast.

MRI of the liver. A pictorial essay

Magnetic resonance imaging (MRI) is an extremely useful modality for evaluation of the complex pathophysiology of the liver. The high degree of soft tissue contrast afforded by MRI accurately detects and characterizes both focal and diffuse abnormalities of the liver. In this article we present a pictorial review of MRI of the liver.

Large angle spin-echo imaging

A study was undertaken to assess the use of excitation flip angles greater than 90 degrees for T1 weighted spin-echo (SE) imaging with a single 180 degrees refocusing pulse and short TR values. Theoretical predictions of signal intensity for SE images with excitation pulse angles of 90-180 degrees were calculated based on the Bloch equations and then measured experimentally from MR images of MnCl2 phantoms of various concentrations. Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110 degrees excitation flip angles. The theoretical predictions showed significant improvements in SNR with excitation flip angles > 90 degrees, which were more pronounced at small TR values. The phantom studies showed reasonably good agreement with the theoretical predictions in correlating the excitation pulse angle with signal intensity. In the human imaging studies, the 110 degrees excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CNR compared to the 90 degrees pulse angle at TR = 275 ms. Increased signal intensity resulting from the use of large flip angle excitation pulses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans.

Proton NMR relaxation times in the normal human liver at 0.08 T

The spin-lattice (T1) and spin-spin (T2) relaxation times of liver in 42 normal volunteers (21 male and 21 female) were measured using a calibrated 0.08 T resistive imager capable of accurate and reproducible relaxometry. T1 was determined using an interleaved gradient echo saturation recovery and inversion recovery technique and T2 using a four-echo Carr-Purcell-Meiboom-Gill sequence. The ranges obtained were T1 = 213 +/- 14 ms and T2 = 66 +/- 5 ms. More specific ranges were obtained for each sex and for younger and older subjects. A small variation in T1 was found between older (greater than 40 years) and younger (less than 40 years) subjects, but no such effect was observed in the case of T2. No significant variations were found when female volunteers were imaged at weekly intervals through the menstrual cycle, when a male volunteer was imaged repeatedly over the course of several months or when male volunteers consumed small quantities of alcohol.