A comparative study at 3 T of sequence dependence of T2 quantitation in the knee

Objective

T₂ mapping has been used widely in detecting cartilage degeneration in osteoarthritis. Several scanning sequences have been developed in the determination of T₂ relaxation times of tissues. However, the derivation of these times may vary from sequence to sequence. This study seeks to evaluate the sequence-dependent differences in T₂ quantitation of cartilage, muscle, fat and bone marrow in the knee joint at 3 T.

Methods

Three commercial phantoms and 10 healthy volunteers were studied using 3 T MR. T₂ relaxation times of the phantoms, cartilage, muscle, subcutaneous fat and marrow were derived using spin echo (SE), multiecho SE (MESE), fast SE (FSE) with varying echo train length (ETL), spiral and spoiler gradient (SPGR) sequences. The differences between these times were then evaluated using Student's t test. In addition, the signal-to-noise ratio (SNR) efficiency and coefficient of variation of T₂ from each sequence were calculated.

Results

The average T₂ relaxation time was 36.38±5.76 ms in cartilage and 34.08±6.55 ms in muscle, ranging from 27 to 45 ms in both tissues. The times for subcutaneous fat and marrow were longer and more varying, ranging from 41 to 143 ms and from 42 to 160 ms, respectively. In FSE acquisition, relaxation time significantly increases as ETL increases (P<.05). In cartilage, the SE acquisition yields the lowest T₂ values (27.52±3.10 ms), which is significantly lower than those obtained from other sequences (P<.002). T₂ values obtained from spiral acquisition (38.27±6.45 ms) were higher than those obtained from MESE (34.35±5.62 ms) and SPGR acquisition (31.64±4.53 ms). These differences, however, were not significant (P>.05).

Conclusion

T₂ quantification can be a valuable tool for the diagnosis of degenerative disease. Several different sequences exist to quantify the relaxation times of tissues. Sequences range in scan time, SNR efficiency, reproducibility and two- or three-dimensional mapping. However, when choosing a sequence for quantitation, it is important to realize that several factors affect the measured T₂ relaxation time.     

Measurement of regional cerebral glucose uptake by magnetic resonance spin-lock imaging

Purpose

The regional uptake of glucose in rat brain in vivo was measured at high resolution using spin-lock magnetic resonance imaging after infusion of the glucose analogue 2-deoxy-d-glucose (2DG). Previous studies of glucose metabolism have used ¹³C-labeled 2DG and NMR spectroscopy, ¹⁸F-labeled fluorodeoxyglucose (FDG) and PET, or chemical exchange saturation transfer (CEST) MRI, all of which have practical limitations. Our goal was to explore the ability of spin-lock sequences to detect specific chemically-exchanging species in vivo and to compare the effects of 2DG in brain tissue on CEST images.

Methods

Numerical simulations of R_1p and CEST contrasts for a variety of sample parameters were performed to evaluate the potential specificity of each method for detecting the exchange contributions of 2DG. Experimental measurements were made in tissue phantoms and in rat brain in vivo which demonstrated the ability of spin-lock sequences for detecting 2DG.

Results

R_1p contrast acquired with appropriate spin-lock sequences can isolate the contribution of exchanging protons in 2DG in vivo and appears to have better sensitivity and more specificity to 2DG–water exchange effects than CEST.

Conclusion

Spin-lock imaging provides a novel approach to the detection and measurement of glucose uptake in brain in vivo.     

Magnetic resonance T1ρ imaging of osteoarthritis: a rabbit ACL transection model

Objective

The objective of this study was to develop quantitative T_1ρ-weighted magnetic resonance imaging methodology for the detection and characterization of cartilage degeneration in a rabbit anterior cruciate ligament (ACL) transection model.

Methods

The right knee ACLs of 18 adult female New Zealand white rabbits were transected. The left knee joint served as a sham control. The rabbits were euthanized at 3 (Group 1), 6 (Group 2) and 12 (Group 3) weeks postoperatively. High-resolution 3D fat-saturated spoiled gradient echo images and T_1ρ-weighted images were obtained in both the sagittal and axial planes at 3 T using a quadrature wrist coil. Following MR analysis, histological slides from the lateral femoral condyle cartilage were graded using the Mankin grading system.

Results

For all three groups, the average overall T_1ρ values were significantly higher in the ACL-transected knee compared to control knee, and the percentage differences in T_1ρ values between ACL-transected and control increased with the duration of time after transection. The average Mankin score for ACL-transected knees was higher than that for control for each time point, but this difference was statistically significant only for all groups combined.

Conclusions

This study demonstrates the feasibility of using T_1ρ-weighted imaging as a useful tool in the detection and quantification of cartilage damage in all knee compartments in an ACL-transected rabbit model of cartilage degeneration.     

Examiner repeatability of patellar cartilage T2 values

Aim

The purpose of this study was to evaluate the intra- and interexaminer resegmentation precision of patellar cartilage T₂ mapping measurements in healthy subjects.

Materials and Methods

T₂-weighted images of patellar cartilage for 10 subjects were acquired. Two individuals manually segmented patellar cartilage at each slice location twice, once on each of two separate days. Bulk average and zonal T₂ values for the superficial, middle, and deep layers of cartilage were calculated. The root mean square (RMS) and coefficient of variation (COV) were calculated using the repeated measurements of each slice of each subject by each examiner.

Results

The intraexaminer bulk T₂ differences were 0.2±1.0 ms, with an RMS error of 0.7 ms and a COV of 1.9%. The differences of interexaminer bulk T₂ values was 1.0±1.4 ms, with an RMS error of 1.2 ms and a COV of 3.3%. The superficial zone of cartilage had the highest zonal variability of T₂ values. The average interexaminer T₂ values for the superficial, middle and deep zones were 42.2±5.6, 38.1±5.3 and 31.9±4.6 ms, respectively.

Conclusion

The interexaminer variability of calculated T₂ values highlights the difficulty of interpreting significant differences of T₂ values which are similar in magnitude. The repeatability measurements of patellar cartilage T₂ values were less than reported intersession T₂ repeatability.     

Ex vivo study of carotid endarterectomy specimens: quantitative relaxation times within atherosclerotic plaque tissues

Purpose

Previous studies reporting relaxation times within atherosclerotic plaque have typically used dedicated small-bore high-field systems and small sample sizes. This study reports quantitative T₁, T₂ and T₂^? relaxation times within plaque tissue at 1.5 T using spatially co-matched histology to determine tissue constituents.

Methods

Ten carotid endarterectomy specimens were removed from patients with advanced atherosclerosis. Imaging was performed on a 1.5-T whole-body scanner using a custom built 10-mm diameter receive-only solenoid coil. A protocol was defined to allow subsequent computation of T₁, T₂ and T₂^? relaxation times using multi-flip angle spoiled gradient echo, multi-echo fast spin echo and multi-echo gradient echo sequences, respectively. The specimens were subsequently processed for histology and individually sectioned into 2-mm blocks to allow subsequent co-registration. Each imaging sequence was imported into in-house software and displayed alongside the digitized histology sections. Regions of interest were defined to demarcate fibrous cap, connective tissue and lipid/necrotic core at matched slice-locations. Relaxation times were calculated using Levenberg-Marquardt's least squares curve fitting algorithm. A linear-mixed effect model was applied to account for multiple measurements from the same patient and establish if there was a statistically significant difference between the plaque tissue constituents.

Results

T₂ and T₂^? relaxation times were statistically different between all plaque tissues (P=.026 and P=.002 respectively) [T₂: lipid/necrotic core was lower 47±13.7 ms than connective tissue (67±22.5 ms) and fibrous cap (60±13.2 ms); T₂^?: fibrous cap was higher (48±15.5ms) than connective tissue (19±10.6 ms) and lipid/necrotic core (24±8.2 ms)]. T₁ relaxation times were not significantly different (P=.287) [T₁: Fibrous cap: 933±271.9 ms; connective tissue (1002±272.9 ms) and lipid/necrotic core (1044±304.0 ms)]. We were unable to demarcate hemorrhage and calcium following histology processing.

Conclusions

This study demonstrates that there is a significant difference between qT₂ and qT₂^? in plaque tissues types. Derivation of quantitative relaxation times shows promise for determining plaque tissue constituents.     

Consideration of slice profiles in inversion recovery Look–Locker relaxation parameter mapping

Purpose

To include the flip angle distribution caused by the slice profile into the model used for describing the relaxation curves observed in inversion recovery Look–Locker FLASH T₁ mapping for a more accurate determination of the relaxation parameters.

Materials and methods

For each inversion time, the flip angle dependent signal of the mono-exponential relaxation model is integrated across the slice profile. The resulting Consideration of Slice Profiles (CSP) relaxation curves are compared to the mono-exponential signal model in numerical simulations as well as in phantom and in-vivo experiments.

Results

All measured relaxation curves showed systematic deviations from a mono-exponential curve increasing with flip angle and T₁ but decreasing with repetition time. Additionally, the accuracy of T₁ was found to be largely dependent on the temporal coverage of the relaxation curve. All these systematic errors were largely reduced by the CSP model.

Conclusion

The proposed CSP model represents a useful extension of the conventionally used mono-exponential relaxation model. Despite inherent model inaccuracies, the mono-exponential model was found to be sufficient for many T₁ mapping situations. However, if only a poor temporal coverage of the relaxation process is achievable or a very precise modeling of the relaxation course is needed as in model-based techniques, the mono-exponential model leads to systematic errors and the CSP model should be used instead.     

Bias of cartilage T2 values related to method of calculation

Purpose

To determine how different methods for calculating T₂ affect the resulting T₂ values of patellar cartilage.

Materials and Methods

T₂-weighted images of patellar cartilage for 10 subjects were acquired using two MRI scanners. T₂ values of patellar cartilage were calculated using linear, weighted and nonlinear fitting algorithms for a monoexponential decay equation. T₂ values were also calculated for the superficial, middle and deep zones of the cartilage.

Results

All three methods of calculation resulted in significantly different T₂ values (P<.0001). The weighted calculation produced the highest T₂ values, and the nonlinear calculation produced the lowest T₂ values. The average difference of T₂ value between the methods was under 5 ms. Similar results were found in a zonal analysis of the tissue. The nonlinear calculation of T₂ consistently had the best fit to the acquired data.

Conclusion

The T₂ value of patellar cartilage depends on the method of calculation. It is unclear if larger T₂ value differences would be seen in subjects diagnosed with osteoarthritis. This study highlights the potential difficulty of comparing different studies with one another based on the method of T₂ calculation.     

MR T1ρ quantification of cartilage focal lesions in acutely injured knees: correlation with arthroscopic evaluation

Objective

Quantitative T₁ρ MRI has been suggested as a promising tool to detect changes in cartilage composition that are characteristic of cartilage damage and degeneration. The objective of this study was to evaluate the capability of MR T₁ρ to detect cartilage lesions as evaluated by arthroscopy in acutely ACL-injured knees and to compare with the Whole-Organ Magnetic Resonance Imaging Score (WORMS) using clinical standard MRI.

Method

Ten healthy controls (mean age 35) with no ACL injury or history of osteoarthritis (OA) and 10 patients with acute ACL injuries (mean age 39) were scanned at 3 Tesla (3 T). ACL patients underwent ACL reconstruction, where focal lesions were graded according to an Outerbridge grading system during arthroscopic evaluation. Normalized MR T₁ρ values (T₁ρ z-scores normalized to control values in matched regions) in full thickness, and superficial and deep layers of cartilage were compared between defined sub-compartments with and without focal lesions. Intraclass (ICC) correlation and the root mean square coefficient of variation (RMS-CV) were performed to evaluate the inter-observer reproducibility of T₁ρ quantification. Sub-compartments of cartilage were also evaluated using WORMS scoring and compared to their Outerbridge score respectively.

Results

The inter-observer ICC and the RMS-CV of the sub-compartment T₁ρ quantification were 0.961 and 3.9%, respectively. The average T₁ρ z-scores were significantly increased in sub-compartments with focal lesions compared to those without focal lesions and to the control cohort (p < 0.05).

Conclusion

Our results indicate that T₁ρ provided a better diagnostic capability than clinical standard MRI grading in detecting focal cartilage abnormalities after acute injuries. Quantitative MRI may have great potential in detecting cartilage abnormalities and degeneration non-invasively, which are occult with standard morphological MRI.     

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.     

Exchange-mediated contrast in CEST and spin-lock imaging

Purpose

Magnetic resonance images of biological media based on chemical exchange saturation transfer (CEST) show contrast that depends on chemical exchange between water and other protons. In addition, spin–lattice relaxation rates in the rotating frame (R_1ρ) are also affected by exchange, especially at high fields, and can be exploited to provide novel, exchange-dependent contrast. Here, we evaluate and compare the factors that modulate the exchange contrast for these methods using simulations and experiments on simple, biologically relevant samples.

Methods

Simulations and experimental measurements at 9.4 T of rotating frame relaxation rate dispersion and CEST contrast were performed on solutions of macromolecules containing amide and hydroxyl exchanging protons.

Results

The simulations and experimental measurements confirm that both CEST and R_1ρ measurements depend on similar exchange parameters, but they manifest themselves differently in their effects on contrast. CEST contrast may be larger in the slow and intermediate exchange regimes for protons with large resonant frequency offsets (e.g. > 2 ppm). Spin-locking techniques can produce larger contrast enhancement when resonant frequency offsets are small (< 2 ppm) and exchange is in the intermediate-to-fast regime. The image contrasts scale differently with field strength, exchange rate and concentration.

Conclusion

CEST and R_1ρ measurements provide different and somewhat complementary information about exchange in tissues. Whereas CEST can depict exchange of protons with specific chemical shifts, appropriate R_1ρ-dependent acquisitions can be employed to selectively portray protons of specific exchange rates.     

Variations in T(2)* and fat content of murine brown and white adipose tissues by chemical-shift MRI

Purpose

The purpose was to compare T₂* relaxation times and proton density fat-fraction (PDFF) values between brown (BAT) and white (WAT) adipose tissue in lean and ob/ob mice.

Materials and Methods

A group of lean male mice (n=6) and two groups of ob/ob male mice placed on similar 4-week (n=6) and 8-week (n=8) ad libitum diets were utilized. The animals were imaged at 3 T using a T₂*-corrected chemical-shift-based water–fat magnetic resonance imaging (MRI) method that provides simultaneous estimation of T₂* and PDFF on a voxel-wise basis. Regions of interest were drawn within the interscapular BAT and gonadal WAT depots on co-registered T₂* and PDFF maps. Measurements were assessed using analysis of variance, Bonferroni-adjusted t test for multigroup comparisons and the Tukey post hoc test.

Results

Significant differences (P<.01) in BAT T₂* and PDFF were observed between the lean and ob/ob groups. The ob/ob animals exhibited longer BAT T₂* and greater PDFF than lean animals. However, only BAT PDFF was significantly different (P<.01) between the two ob/ob groups. When comparing BAT to WAT within each group, T₂* and PDFF values were consistently lower in BAT than WAT (P<.01). The difference was most prominent in the lean animals. In both ob/ob groups, BAT exhibited very WAT-like appearances and properties on the MRI images.

Conclusion

T₂* and PDFF are lower in BAT than WAT. This is likely due to variations in tissue composition. The values were consistently lower in lean mice than in ob/ob mice, suggestive of the former's greater demand for BAT thermogenesis and reflective of leptin hormone deficiencies and diminished BAT metabolic activity in the latter.     

Occipital lobe's cortical thinning in ametropic amblyopia

Background and Purpose

The present study was designed to detect the abnormalities of the cortical thickness in children with ametropic amblyopia by a computer-aided MRI technique.

Methods

Nine children with ametropic amblyopia and eight age-matched normal controls underwent MRI brain scanning that was performed on a Siemens Avanto 1.5-T scanner, and standard T₁-weighted high-resolution anatomic scans of magnetization-prepared rapid gradient echo (MPRAGE) sequence were obtained. For the cortical thickness analysis, 3D MPRAGE images were processed with FreeSurfer software package (http://www.nmr.mgh.harvard.edu/freesurfer/), and the cortical thicknesses were compared between the patient group and the normal control group.

Results

The cortical thicknesses of the lingual and pericalcarine areas in the left hemisphere and of the cuneus, lateraloccipital and lingual areas in the right hemisphere in the amblyopic group were significantly thinner than those of the control group (P<.05).

Conclusion

The changes in cortical thickness of several occipital regions in amblyopic patients may be important in the diagnosis and treatment of this disease.     

Imaging longitudinal changes in articular cartilage and bone following doxycycline treatment in a rabbit anterior cruciate ligament transection model of osteoarthritis

Objective

The development of osteoarthritis following traumatic anterior cruciate ligament (ACL) injury is well established. However, few reliable indicators of early osteoarthritic changes have been established, which has limited the development of effective therapies. T_1ρ and T₂ mapping techniques have the ability to provide highly accurate and quantitative measurements of articular cartilage degeneration in vivo. Relating these cartilaginous changes to high-resolution bone-densitometric evaluations of the late-stage osteoarthritic bone is crucial in elucidating the mechanisms of development of traumatic osteoarthritis (OA) and potential therapies for early- or late-stage intervention.

Methods

Twelve rabbits were monitored with in vivo magnetic resonance imaging (MRI) scans following ACL transection surgery with a contralateral leg sham operation. Six of the rabbits were treated with oral doxycycline for the duration of the experiment. At 12 weeks, the excised knees from three animals from each group (n=6 overall) were subjected to micro-computed tomography (CT) analysis.

Results

Consistent with previous studies, initial elevations in T_1ρ and T₂ values in ACL-transected animals were observed with relative normalization towards values see in sham-operated legs over the 12-week study. This biphasic pattern could hold diagnostic potential to differentiate osteoarthritic cartilage by tracking the relative proportions of T_1ρ and T₂ values as they rise with inflammation then fall as collagen and proteoglycan loss leads to further dehydration. The addition of doxycycline resulted in inconclusive, yet potentially interesting, cartilaginous changes in several compartments of the rabbit legs. Micro-CT studies demonstrated decreased bone densitometrics in ACL-transected knees. Correlation studies suggest that the cartilaginous changes may be associated with some aspects of bony change and the development of OA.

Conclusion

We conclude that there are definite relationships between cartilaginous changes as seen on MRI and late-stage microstructural bony changes after traumatic ACL injury in rabbits. In addition, doxycycline may show promise in mitigating early-stage cartilage damage that may serve to lessen late-stage osteoarthritic changes. This study demonstrates the ability to track OA progression and therapeutic efficacy with imaging modalities in vivo.     

Quantitative liver iron measurement by magnetic resonance imaging: in vitro and in vivo assessment of the liver to muscle signal intensity and the R2* methods

Purpose

To evaluate the liver-to-muscle signal intensity and R2* methods to gain a transferable, clinical application for liver iron measurement.

Materials and Methods

Sixteen liver phantoms and 33 human subjects were examined using three 1.5-T MRI scanners from two different vendors. Phantom-to-muscle and liver-to-muscle signal intensity ratios were analyzed to determine MRI estimated phantom and hepatic iron concentration (M-PIC and M-HIC, respectively). R2* was calculated for the phantoms and the liver of human subjects. Seven patients' biochemical hepatic iron concentration was obtained.

Results

M-PIC and R2* results of three scanners correlated linearly to phantom iron concentrations (r=0.984 to 0.989 and r=0.972 to 0.981, respectively), and no significant difference between the scanners was found (P=.482 and P=.846, respectively) in vitro. The patients' R2* correlated linearly to M-HIC of the standard scanner (r=0.981). M-HIC values did not differ from those obtained from the biopsy specimens (P=.230). The difference in M-HIC was significant, but the difference in R2* was not significant between the scanners (P<.0001 and P=.505, respectively) in vivo.

Conclusion

Both methods, M-HIC and R2*, are reliable iron concentration indicators with linear dependence on iron concentration in vivo and in vitro. The R2* method was found to be comparable among different scanners. Transferability testing is needed for the use of the methods at various scanners.     

3D high-resolution contrast enhanced MRI of carotid atheroma — a technical update

Objective

Development of a fast 3D high-resolution magnetic resonance imaging (MRI) protocol for improved carotid artery plaque imaging.

Methods

Two patients with carotid atherosclerosis disease underwent 3D high-resolution MRI which included time-of-flight and T₁-weighted variable flip angle, fast-spin-echo (FSE) imaging, pre- and post-intravenous gadolinium-based contrast agent administration.

Results

Good quality images with intrinsic blood suppression were obtained pre- and post-contrast administration using a 3D FSE sequence. The plaque burden, lipid core volume, hemorrhage volume and fibrous cap thickness were well determined.

Conclusions

3D high-resolution MR imaging of carotid plaque using TOF and 3D FSE can achieve high isotropic resolution, large coverage, and excellent image quality within a short acquisition time.     

Fast volumetric spatial-spectral MR imaging of hyperpolarized C-labeled compounds using multiple echo 3D bSSFP

Purpose

The goal of this work was to develop a fast 3D chemical shift imaging technique for the noninvasive measurement of hyperpolarized ¹³C-labeled substrates and metabolic products at low concentration.

Materials and Methods

Multiple echo 3D balanced steady state magnetic resonance imaging (ME-3DbSSFP) was performed in vitro on a syringe containing hyperpolarized [1,3,3-2H3; 1-¹³C]2-hydroxyethylpropionate (HEP) adjacent to a ¹³C-enriched acetate phantom, and in vivo on a rat before and after intravenous injection of hyperpolarized HEP at 1.5 T. Chemical shift images of the hyperpolarized HEP were derived from the multiple echo data by Fourier transformation along the echoes on a voxel by voxel basis for each slice of the 3D data set.

Results

ME-3DbSSFP imaging was able to provide chemical shift images of hyperpolarized HEP in vitro, and in a rat with isotropic 7-mm spatial resolution, 93 Hz spectral resolution and 16-s temporal resolution for a period greater than 45 s.

Conclusion

Multiple echo 3D bSSFP imaging can provide chemical shift images of hyperpolarized ¹³C-labeled compounds in vivo with relatively high spatial resolution and moderate spectral resolution. The increased signal-to-noise ratio of this 3D technique will enable the detection of hyperpolarized ¹³C-labeled metabolites at lower concentrations as compared to a 2D technique.     

Prominent signal intensity of T1/T2 prolongation in subcortical white matter of the anterior temporal region on conventional screening MRI of late preterm infants with normal development

Purpose

This study discusses prominent signal intensity of T₁/T₂ prolongation of subcortical white matter within the anterior temporal region in premature infant brains that radiologists may encounter when interpreting conventional screening MRIs.

Materials and Methods

T₁- and T₂-weighted images of 69 preterm and term infants with no neurological abnormalities or developmental delays were evaluated retrospectively for areas of prominent signal intensity of T₁/T₂ prolongation in white matter. We measured signal intensities of anterior temporal white matter, deep temporal white matter, frontopolar white matter and subcortical white matter of the precentral gyrus. We accessed chronological changes in signal intensity in the anterior and deep temporal white matter. We also analyzed variance tests among the signal intensity ratios to the ipsilateral thalamus of white matter areas by gestational age.

Results

There was high frequency of prominent signal intensity of T₁/T₂ prolongation in the temporal tip, particularly at a gestational age of 36–38 weeks. Signal intensity ratio of the anterior temporal white matter was lower on T₁-weighted images and higher on T₂-weighted images, and the finding became less prominent with increasing gestational age. The signal intensity ratios of anterior temporal white matter at a gestational age of 36–37 weeks and 38–39 weeks were significantly different from other regions.

Conclusion

Prominent signal intensity of T₁/T₂ prolongation of subcortical white matter of the anterior temporal region is seen in normal premature infants, especially those at 36–39 gestational weeks. Although it is a prominent finding, radiologists should understand that these findings do not represent a pathological condition.     

Histopathologically confirmed focal nodular hyperplasia of the liver: Gadoxetic acid-enhanced MRI characteristics

Purpose

The purpose of this study was to evaluate enhancement characteristics of histopathologically confirmed focal nodular hyperplasia (FNHs) with gadoxetic acid-enhanced MRI.

Materials and Methods

Twenty-seven patients with all histopathologically proven FNHs were retrospectively identified. MRI consisted of T₁- and T₂-weighted (w) sequences with and without fat saturation (FS), multiphase dynamic T₁-w images, and FS T₁-w images during the hepatobiliary phase. Standard of reference was surgical resection (n = 24) or biopsy (n = 3). Images were analyzed for morphology and contrast behavior including signal intensity (SI) measurement on T₁-w images normalized to the pre-contrast base line.

Results

In total 36 FNHs were evaluated. All FNHs showed enhancement in the arterial phase, significant reduction contrast enhancement (“wash-out”) in the late dynamic phases was not present. In the hepatobiliary phase, all FNHs (100%) showed enhancement (overall SI increase, 118% (± 91%), P < 0.001) with at least partial hyperintensity to the liver. Upon visual comparison, 3 of 36 FNHs appeared with heterogeneous/partial enhancement (8%) and 7 (19%) showed rim-accentuated enhancement.

Conclusion

The typical enhancement pattern of FNH with gadoxetic acid consists of arterial hyperperfusion, no wash-out during the venous phase, and at least partial hyperintensity compared to the liver in the hepatobiliary phase. Partial hypointensity or rim-accentuated enhancement rarely occurs.     

Assessment of T1, T1ρ, and T2 values of the ulnocarpal disc in healthy subjects at 3 tesla

Objective

The purpose of this study was to implement clinically feasible imaging techniques for determination of T₁, T_1ρ, and T₂ values of the ulnocarpal disc and to assess those values in a cohort of asymptomatic subjects at 3 tesla. Resulting values were compared between different age groups, since former histological findings of the ulnocarpal disc indicated frequent early degenerative changes of this tissue starting in the third decade of life, even in asymptomatic subjects.

Materials and methods

Twenty-seven healthy subjects were included in this study. T₁ measurements were performed using 3D spoiled gradient-echo (GRE) sequence with variable flip angle. A series of T_1ρ and T₂-weighted images was acquired by a 3D GRE sequence after suitable magnetization preparation. T_1,T_1ρ, and T₂ maps of the ulnocarpal disc were calculated pixel-wise. Representative mean values from extended regions were analysed.

Results

Mean T₁ values of the ulnocarpal disc ranged from 722 ms in a 39 year-old subject to 1264 ms in a 65 year-old subject, T_1ρ ranged from 9.2 ms (26 year-old subject) to 25.9 ms (65 year-old subject). Calculated T₂ values showed a large range from 4.1 ms to 22.3 ms. T_1ρ and T₁ values tended to increase with age (p < 0.05), whereas T₂ did not.

Conclusions

MR relaxometry of the ulnocarpal disc is feasible, and T_1,T_1ρ, and T₂ values show modest variance in asymptomatic subjects. The potential of relaxation mapping to reveal relevant structural changes in patients has to be investigated in further studies.     

A semiautomatic postprocessing of liver R2* measurement for assessment of liver iron overload

Purpose

The purpose was to propose and evaluate a semiautomatic postprocessing method to measure liver R2^? values in patients with a broad range of liver iron content.

Materials and Methods

Multiecho gradient echo magnetic resonance images were acquired in patients diagnosed with thalassemia or other types of congenital anemias. Liver R2^? values were measured using a routine manually defined region-of-interest (mROI) method and a semiautomatic (SA) method. In the semiautomatic method, pixelwise (pSA) and averaged (aSA) signal fitting was performed on the segmented liver tissues after hepatic vessel extraction. The pixelwise fitting approach resulted in a liver R2^? map with an overlay of nonfitted pixels associated with noise performance. The following aSA approach derived overall R2^? by fitting the averaged signal intensities of all pixels within the liver ROI excluding vessels and nonfitted pixels. The measurement accuracy and interobserver agreement using mROI and the two semiautomatic approaches (pSA and aSA) were evaluated.

Results

In a total of 45 exams with R2^? ranging from 30 to 1500 s⁻¹, the R2^? measurements using all three methods were overall highly correlated and concordant with each other. R2^? values measured by aSA were consistently higher than those measured by mROI. At lower R2^? (<1000 s⁻¹), R2^? values measured by pSA were consistent with aSA but higher than mROI; with increasing R2^?, the pSA method became less stable and underestimated R2^? due to increased noise level. The interobserver agreement was higher for the aSA method compared to pSA and mROI.

Conclusion

The semiautomatic postprocessing method provides a promising tool for reliable liver R2^? measurement with additional information for overall evaluation of iron distribution and measurement confidence. This method may offer the potential of reducing interoperator variability and improving diagnostic confidence in patients with liver iron overload.