Spin‐echo micro‐MRI of trabecular bone using improved 3D fast large‐angle spin‐echo (FLASE)

Fast large‐angle spin echo (FLASE) is a common pulse sequence designed for quantitative imaging of trabecular bone (TB) microarchitecture. However, imperfections in the nonselective phase‐reversal pulse render it prone to stimulated echo artifacts. The problem is further exacerbated at isotropic resolution. Here, a substantially improved RF‐spoiled FLASE sequence (sp‐FLASE) is described and its performance is illustrated with data at 1.5T and 3T. Additional enhancements include navigator echoes for translational motion sensing applied in a slice parallel to the imaging slab. Whereas recent work suggests the use of fully‐balanced FLASE (b‐FLASE) to be advantageous from a signal‐to‐noise ratio (SNR) point of view, evidence is provided here that the greater robustness of sp‐FLASE may outweigh the benefits of the minor SNR gain of b‐FLASE for the target application of TB imaging in the distal extremities, sites of exclusively fatty marrow. Results are supported by a theoretical Bloch equation analysis and the pulse sequence dependence of the effective T₂ of triglyceride protons. Last, sp‐FLASE images are shown to provide detailed and reproducible visual depiction of trabecular networks in three dimensions at both anisotropic (137 × 137 × 410 μm³) and isotropic (160 × 160 × 160 μm³) resolutions in the human distal tibia in vivo. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Application of refocused steady-state free-precession methods at 1.5 and 3 T to in vivo high-resolution MRI of trabecular bone: simulations and experiments

PURPOSE: To evaluate the potential of fully-balanced steady-state free-precession (SSFP) sequences in in vivo high-resolution (HR) MRI of trabecular bone at field strengths of 1.5 and 3 T by simulation and experimental methods. MATERIALS AND METHODS: Using simulation studies, refocused SSFP acquisition was optimized for our imaging purposes with a focus on signal-to-noise ratio (SNR) and SNR efficiency. The signal behavior in trabecular bone was estimated using a magnetostatic model of the trabecular bone and marrow. Eight normal volunteers were imaged at the proximal femur, calcaneus, and the distal tibia on a GE Signa scanner at 1.5 and at 3 T with an optimized single-acquisition SSFP sequence (three-dimensional FIESTA) and an optimized multiple-acquisition SSFP sequence (three-dimensional FIESTA-c). Images were also acquired with a fast gradient echo (FGRE) sequence for evaluation of the SNR performance of SSFP methods. RESULTS: Refocused SSFP images outperformed FGRE acquisitions in both SNR and SNR efficiency at both field strengths. At 3 T, susceptibility effects were visible in FIESTA and FGRE images and much reduced in FIESTA-c images. The magnitude of SNR boost at 3 T was closely predicted by simulations. CONCLUSION: Single-acquisition SSFP (at 1.5 T) and multiple-acquisition SSFP (at 3 T) hold great potential for HR-MRI of trabecular bone.     

Fast low-angle dual spin-echo (FLADE): a new robust pulse sequence for structural imaging of trabecular bone.

Mechanical strength and fracture resistance of trabecular bone (TB) are largely determined by the structural arrangement of individual trabeculae. Fast 3D spin-echo approaches are preferable to gradient echoes in that they are less sensitive to local induced gradients at the bone/marrow interface caused by magnetic susceptibility difference between the two tissues. FLASE is a 3D pulse sequence that serves this purpose. Here, we present a new pulse sequence dubbed FLADE (fast low-angle dual spin-echo) that overcomes some of the limitations inherent to FLASE, such as sensitivity to artifactual stimulated echoes. The double-echo sequence features a flip angle <90 degrees allowing for TR < T(1). The second phase-reversal pulse has the dual function of creating a second echo and restoring inverted longitudinal magnetization. The prolonged TR, made possible by sampling only half of k(z)-space, is used to collect navigator echoes in adjacent slabs for sensing subpixel translational displacements. FLADE is shown to provide SNR comparable to FLASE while having narrower point-spread function and being more robust to imperfections in the nonselective 180 degree pulses. Structural parameters derived from the in vivo images with the two pulse sequences are highly correlated, therefore suggesting that clinical data obtained with either pulse sequence can be merged.     

Cartilage morphology at 3.0T: Assessment of three‐dimensional magnetic resonance imaging techniques

Purpose:

To compare six new three‐dimensional (3D) magnetic resonance (MR) methods for evaluating knee cartilage at 3.0T.

Materials and Methods:

We compared: fast‐spin‐echo cube (FSE‐Cube), vastly undersampled isotropic projection reconstruction balanced steady‐state free precession (VIPR‐bSSFP), iterative decomposition of water and fat with echo asymmetry and least‐squares estimation combined with spoiled gradient echo (IDEAL‐SPGR) and gradient echo (IDEAL‐GRASS), multiecho in steady‐state acquisition (MENSA), and coherent oscillatory state acquisition for manipulation of image contrast (COSMIC). Five‐minute sequences were performed twice on 10 healthy volunteers and once on five osteoarthritis (OA) patients. Signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) were measured from the volunteers. Images of the five volunteers and the five OA patients were ranked on tissue contrast, articular surface clarity, reformat quality, and lesion conspicuity. FSE‐Cube and VIPR‐bSSFP were compared to IDEAL‐SPGR for cartilage volume measurements.

Results:

FSE‐Cube had top rankings for lesion conspicuity, overall SNR, and CNR (P < 0.02). VIPR‐bSSFP had top rankings in tissue contrast and articular surface clarity. VIPR and FSE‐Cube tied for best in reformatting ability. FSE‐Cube and VIPR‐bSSFP compared favorably to IDEAL‐SPGR in accuracy and precision of cartilage volume measurements.

Conclusion:

FSE‐Cube and VIPR‐bSSFP produce high image quality with accurate volume measurement of knee cartilage. J. Magn. Reson. Imaging 2010;32:173–183. © 2010 Wiley‐Liss, Inc.     

High-resolution 3D cartilage imaging with IDEAL SPGR at 3 T

OBJECTIVE: The purpose of this study was to perform imaging of cartilage at high resolution with a high signal-to-noise ratio (SNR) with a combination of iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) with parallel imaging at 3 T and spoiled gradient echo (SPGR) imaging. The findings with the combined technique were compared with those obtained with conventional fat-saturated SPGR imaging. CONCLUSION: Compared with fat-saturated SPGR, IDEAL-SPGR imaging combined with parallel imaging at 3 T provides robust fat-water separation and significant improvement in cartilage SNR. Use of IDEAL-SPGR also led to dramatic improvement in cartilage-fluid contrast-to-noise ratio compared with fat-saturated SPGR imaging. Thus, use of IDEAL-SPGR may improve the accuracy of cartilage volume measurements and detection of cartilage surface defects. Excellent evaluation of the morphologic features of the knee cartilage with high-resolution, high-SNR images can be performed in 5 minutes.     

Steady-state free precession MR imaging: improved myocardial tag persistence and signal-to-noise ratio for analysis of myocardial motion

Tagging with balanced steady-state free-precession (SSFP) magnetic resonance (MR) imaging by using a steady-state storage scheme for myocardial motion analysis was evaluated. Signal-to-noise ratio (SNR), blood-tissue contrast, and tag persistence in volunteers and phantoms showed improved performance of SSFP imaging with tagging compared with that of radiofrequency spoiled gradient-echo (SPGR) MR imaging with tagging. Choice of flip angle with SSFP imaging involved a trade-off among SNR, blood-tissue contrast, and tag persistence. Increased SNR and tag persistence can be achieved simultaneously with SSFP imaging compared with SPGR tagging methods. As a result, the proposed technique may be useful for analysis of diastolic ventricular function.     

Pulmonary MR angiography utilizing phased-array surface coils.

Magnetic resonance angiography of the pulmonary vasculature was evaluated in 12 subjects using breath-hold gradient echo scans and surface coils at 1.5 T. Flow-compensated GRASS, spoiled GRASS (SPGR), and WARP-SPGR sequences were utilized. Comparisons were made among flip angles of 10-60 degrees, slice thicknesses of 3-10 mm, and body coil as well as Helmholtz pair and phased-array multiple coils. With 30-40 contiguous slices encompassing the lung, intrathoracic vasculature was segmented using a UNIX/X-windows based package dubbed VIDA. Three-dimensional anatomy was visualized by a brightest voxel projection algorithm, following reduction of chest wall pixel intensities by an operator-interactive module. Both SPGR (30 degrees flip angle, 4 mm slice thickness) and WARPSPGR (15 degrees flip angle, 5 mm slice thickness) in combination with phased-array multiple coils provided the most satisfactory images, based upon observations by three radiologists and signal-to-noise ratio measurements. The MR angiograms visualized vessels as distal as sixth to seventh order branches. The technique was successfully applied to three patients with pulmonary embolism. The results of this study demonstrate that the pulmonary vascular tree can be imaged by MR angiography combining a high resolution technique utilizing phased-array multiple coils, fast gradient echo sequences with breath-holding, and postprocessing of the volumetric image data. The technique is attractive since it is noninvasive and provides a full three-dimensional portrayal of the pulmonary vasculature.     

Quantitative high-resolution magnetic resonance imaging reveals structural implications of renal osteodystrophy on trabecular and cortical bone

PURPOSE: To explore the potential role of micro-magnetic resonance imaging (micro-MRI) for quantifying trabecular and cortical bone structural parameters in renal osteodystrophy (ROD), a multifactorial disorder of bone metabolism, traditionally evaluated by bone biopsy. MATERIALS AND METHODS: Seventeen hemodialysis patients (average PTH level = 502 +/- 415 microg/liter) were compared with 17 age-, gender-, and body mass index (BMI)-matched control subjects. The average dialysis duration for the patients was 5.5 years (range = 0.96-18.2 years). Three-dimensional (3D) fast large-angle spin-echo (FLASE) MR images of the distal tibia (voxel size = 137 x 137 x 410 microm(3)) were processed to yield bone volume fraction (BV/TV). From a skeletonized representation of the trabecular bone network, the topology of each bone voxel was determined providing surface and curve voxel densities (SURF and CURV) and the topological erosion index (EI). Further, high-resolution two-dimensional (2D) spin-echo images were collected at the tibial midshaft for measurement of cortical bone cross-sectional area (CCA), relative CCA expressed as a percentage of total bone area (RCA), and mean cortical thickness (MCT). RESULTS: The data show both RCA and MCT to be lower in the patients (61.2 vs. 69.1%, P = 0.008, and 4.53 vs. 5.19 mm, P = 0.01). BV/TV and SURF were lower, while EI was increased in the patients, although these differences were not quite significant (P = 0.06-0.09). All of the cortical and trabecular findings are consistent with increased bone fragility. CONCLUSION: The data suggest that micro-MRI may have potential to characterize the structural implications of metabolic bone disease, potentially providing a noninvasive tool for the evaluation of therapies for ROD.     

MR angiography with an ultrasmall superparamagnetic iron oxide blood pool agent

The purpose of the study was to investigate the use of a dextran-coated ultrasmall superparamagnetic iron oxide (USPIO) as a blood pool contrast agent for thoracic and abdominal MR angiography. Abdominal and thoracic MR angiography was performed in six healthy volunteers using two-dimensional and three-dimensional spoiled gradient echo (SPGR) sequences before and after intravenous administration of USPIO. Doses ranged from 1.1 to 2.6 mg Fe/kg. Flip angle was varied from 20 to 60°. Subjective image quality, analysis of signal-to-noise ratio (SNR), and blood T1 relaxation times were measured. USPIO significantly lowered the T1 of blood (from 1,210 ms precontrast to 159 ms postcontrast at a dose of 2.6 mg Fe/kg) (P < .01). Image quality on coronal fast three-dimensional breath-hold SPGR images of the abdomen increased with increasing dose and was maximum at the highest dose, producing an aortic SNR of 9.6 compared to 1.8 precontrast. Axial two-dimensional time-of-flight (TOF) aortic SNR was reduced significantly from 13 on precontrast to 6 on the postcontrast images at the highest dose (P < .05) due to T2* shortening effects. There was little flip angle dependence on image quality. Due to the T1 shortening effect and long intravascular half-life, USPIO improved visualization of vascular anatomy using three-dimensional fast SPGR imaging. The echo time must be minimized to minimize signal loss from T2* shortening effects. The blood pool distribution of USPIO is useful for equilibriumphase MR angiography.     

In vivo single scan detection of both iron-labeled cells and breast cancer metastases in the mouse brain using balanced steady-state free precession imaging at 1.5 T

Purpose:

To simultaneously detect iron‐labeled cancer cells and brain tumors in vivo in one scan, the balanced steady‐state free precession (b‐SSFP) imaging sequence was optimized at 1.5 T on mice developing brain metastases subsequent to the injection of micron‐sized iron oxide particle‐labeled human breast cancer cells.

Materials and Methods:

b‐SSFP sequence parameters (repetition time, flip angle, and receiver bandwidth) were varied and the signal‐to‐noise ratio, contrast between the brain and tumors, and the number of detected iron‐labeled cells were evaluated.

Results:

Optimal b‐SSFP images were acquired with a 26 msec repetition time, 35° flip angle, and bandwidth of ±21 kHz. b‐SSFP images were compared with T₂‐weighted 2D fast spin echo (FSE) and 3D spoiled gradient recalled echo (SPGR) images. The mean tumor‐brain contrast‐to‐noise ratio and the ability to detect iron‐labeled cells were the highest in the b‐SSFP images.

Conclusion:

A single b‐SSFP scan can be used to visualize both iron‐labeled cells and brain metastases. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Rapid MR imaging of articular cartilage with steady-state free precession and multipoint fat-water separation

OBJECTIVE: To obtain high-quality high-resolution images of articular cartilage with reduced imaging time, we combined a novel technique of generalized multipoint fat-water separation with three-dimensional (3D) steady-state free precession (SSFP) imaging. SUBJECTS AND METHODS: The cartilage of 10 knees in five healthy volunteers was imaged with 3D SSFP imaging and a multipoint fat-water separation method capable of separating fat and water with short TE increments. Fat-saturated 3D spoiled gradient-echo (SPGR) images were obtained for comparison. RESULTS: High-quality images of the knee with excellent fat-water separation were obtained with 3D SSFP imaging. Total imaging time required was 58% less than that required for 3D SPGR imaging with a comparable cartilage signal-to-noise ratio and spatial resolution. Unlike 3D SPGR images, 3D SSFP images exhibited bright synovial fluid, providing a potential arthrographic effect. CONCLUSION: High-quality high-resolution images of articular cartilage with improved fat-water separation, bright synovial fluid, and markedly reduced acquisition times can be obtained with 3D SSFP imaging combined with a fat-water separation technique.     

Cartilaginous endplates of the spine: MRI with anatomic correlation in cadavers

PURPOSE: The purpose of this work was to describe the MR appearance of cartilaginous endplates (CEPs) with close anatomic correlation in cadavers derived from elderly subjects. METHOD: High-resolution MRI was performed on five cadaveric lumbar spines, and a total of 48 CEPs were studied with T1-weighted spin echo, T2-weighted fast spin echo, and fat-suppressed 3D spoiled GRASS gradient echo (SPGR) MR images. All specimens underwent anatomic sectioning, and gross anatomic findings were correlated with those of MRI. RESULTS: Conventional MR images allowed gross morphologic evaluation of the integrity of the CEPs and demonstrated cartilaginous nodes. In all specimens, fat-suppressed 3D-SPGR images invariably improved visualization of the fine anatomic structures at the diskovertebral junction (p < 0.01). Various morphologic abnormalities of the CEPs demonstrated on MR images, including thinning, irregularity, erosions, cartilaginous defects, and Schmorl nodes, were confirmed on anatomic inspection. CONCLUSION: Results in our study indicate that MRI may delineate the normal anatomy of CEPs and demonstrate morphologic changes occurring at the diskovertebral junction. Dedicated high-resolution technique and fat-suppressed 3D-SPGR images may significantly improve the diagnostic capabilities of MRI of this particular anatomic region.     

Multicoil Dixon chemical species separation with an iterative least-squares estimation method.

This work describes a new approach to multipoint Dixon fat-water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady-state free precession (SSFP) and fast spin-echo (FSE) imaging. Using an iterative linear least-squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal-to-noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single- and multicoil fat-water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient-echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water-fat-silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed.     

Cervical spine: MR imaging with a partial flip angle, gradient- refocused pulse sequence. Part II. Spinal cord disease

A magnetic resonance imaging pulse sequence (GRASS) with a short repetition time (TR), short echo time (TE), partial flip angle, and gradient refocused echo was prospectively evaluated for the detection of cervical cord disease that caused minimal or no cord enlargement in eight patients. Sagittal T2-weighted, cerebrospinal fluid (CSF)-gated images and sagittal and axial GRASS images were obtained in all patients. The following GRASS parameters were manipulated to determine their effect on signal-to-noise ratio (S/N) and contrast: flip angle (4 degrees-18 degrees), TR (22-50 msec), and TE (12.5-25 msec). Flip angle had the greatest effect on S/N and contrast. There were no differences between axial and sagittal imaging for the spinal cord or lesion. However, because the signal intensity of CSF did differ on sagittal and axial images and because this influenced the conspicuity of lesions, there was a difference in the useful flip angle range for axial and sagittal imaging. No one set of imaging parameters was clearly superior, and in all patients, the gated image was superior to the sagittal GRASS image in lesion detection. GRASS images should be used in the axial plane primarily to confirm spinal cord disease detected on sagittal CSF-gated images. For this, a balanced approach is suggested (TR = 40 msec, TE = 20 msec, with flip angles of 4 degrees-6 degrees for sagittal and 6 degrees-8 degrees for axial imaging).     

Analysis of cardiac function--comparison between 1.5 Tesla and 3.0 Tesla cardiac cine magnetic resonance imaging: preliminary experience

PURPOSE: We sought to assess the feasibility of magnetic resonance imaging to evaluate cardiac function at 3.0 T compared with 1.5 T. MATERIAL AND METHODS: In a prospective intraindividual comparative study, 12 volunteers (range, 18-54 years), and 2 patients (range, 43-53 years) underwent cardiac cine magnetic resonance at both 3.0 T and 1.5 T. Data were acquired both with a steady-state free precession sequence (SSFP) and a spoiled gradient echo (SGE) sequence. If necessary, a frequency scout was used to correct for off-resonance artifacts. For both SSFP and SGE imaging, 6-mm thick retrospectively EKG-gated short axis views were acquired with equal matrix size (192 x 163) and comparable repetition time (TR). Cardiac function parameters were determined manually by a single investigator. Cardiac function parameters, signal to noise ratio (SNR), contrast to noise ratio (CNR), and the presence of artifacts were compared between the 2 magnetic field strengths. For statistical analysis, a Pearson's correlation coefficient was calculated, and a paired Student t test was used to test statistical significance. RESULTS: Very good correlations between cardiac function parameters at 1.5 T and 3.0 T (r > 0.84, P < 0.0011) were obtained. Compared with SGE, SSFP more frequently was prone to artifacts. With SSFP/SGE at 3.0 T, a SNR gain of 9.4/16% was achieved compared with 1.5 T. CONCLUSION: Functional cardiac cine magnetic resonance imaging can be regarded as equally accurate at 3.0 T compared with 1.5 T. Compared with SSFP imaging, the SGE sequence benefits more from higher field strengths and is less affected by artifacts.     

Articular cartilage of the knee: evaluation with fluctuating equilibrium MR imaging--initial experience in healthy volunteers

Institutional review board approval and informed consent were obtained for this HIPAA-compliant study, whose purpose was to prospectively compare three magnetic resonance (MR) imaging techniques-fluctuating equilibrium, three-dimensional (3D) spoiled gradient-recalled acquisition in the steady state (SPGR), and two-dimensional (2D) fast spin echo (SE)-for evaluating articular cartilage in the knee. The study cohort consisted of 10 healthy volunteers (four men, six women; age range, 26-42 years). Cartilage signal-to-noise ratio (SNR), SNR efficiency, cartilage-fluid contrast-to-noise ratio (CNR), CNR efficiency, image quality, cartilage visibility, and fat suppression were compared. Cartilage volume was compared for the fluctuating equilibrium and 3D SPGR techniques. Compared with 3D SPGR and 2D fast SE, fluctuating equilibrium yielded the highest cartilage SNR efficiency and cartilage-fluid CNR efficiency (P < .01 for both). Image quality was similar with all sequences. Fluctuating equilibrium imaging yielded higher cartilage visibility than did 2D fast SE imaging (P <. 01) but worse fat suppression than did 3D SPGR and 2D fast SE imaging (P < .04). Cartilage volume measurements with fluctuating equilibrium and 3D SPGR were similar. Fluctuating equilibrium MR imaging is a promising method for evaluating articular cartilage in the knee.     

Free-breathing, three-dimensional coronary artery magnetic resonance angiography: comparison of sequences

PURPOSE: To compare six free-breathing, three-dimensional, magnetization-prepared coronary magnetic resonance angiography (MRA) sequences. MATERIALS AND METHODS: Six bright-blood sequences were evaluated: Cartesian segmented gradient echo (C-SGE), radial SGE (R-SGE), spiral SGE (S-SGE), spiral gradient echo (S-GE), Cartesian steady-state free precession (C-SSFP), and radial SSFP (R-SSFP). The right coronary artery (RCA) was imaged in 10 healthy volunteers using all six sequences in randomized order. Images were evaluated by two observers with respect to signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), visible vessel length, vessel edge sharpness, and vessel diameter. RESULTS: C-SSFP depicted RCA over the longest distance with high vessel sharpness, good SNR, and excellent background suppression. S-GE provided best SNR and CNR in proximal segments, but more vessel blurring and poorer background suppression, resulting in poor visualization of distal segments. R-SSFP images showed good background suppression and best vessel sharpness, but only moderate SNR. C-SGE provided good SNR and reasonable CNR, but lowest vessel sharpness. S-SGE and R-SGE visualized the RCA over the smallest distance, mostly due to vessel blurring and low SNR, respectively. CONCLUSION: Overall, Cartesian SSFP provided the best image quality with excellent vessel sharpness, visualization of long vessel segments, and good SNR and CNR.     

MRI of the wrist at 7 tesla using an eight‐channel array coil combined with parallel imaging: Preliminary results

Purpose:

To determine the feasibility of performing MRI of the wrist at 7 Tesla (T) with parallel imaging and to evaluate how acceleration factors (AF) affect signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), and image quality.

Materials and Methods:

This study had institutional review board approval. A four‐transmit eight‐receive channel array coil was constructed in‐house. Nine healthy subjects were scanned on a 7T whole‐body MR scanner. Coronal and axial images of cartilage and trabecular bone micro‐architecture (3D‐Fast Low Angle Shot (FLASH) with and without fat suppression, repetition time/echo time = 20 ms/4.5 ms, flip angle = 10°, 0.169–0.195 × 0.169–0.195 mm, 0.5–1 mm slice thickness) were obtained with AF 1, 2, 3, 4. T1‐weighted fast spin‐echo (FSE), proton density‐weighted FSE, and multiple‐echo data image combination (MEDIC) sequences were also performed. SNR and CNR were measured. Three musculoskeletal radiologists rated image quality. Linear correlation analysis and paired t‐tests were performed.

Results:

At higher AF, SNR and CNR decreased linearly for cartilage, muscle, and trabecular bone (r < ?0.98). At AF 4, reductions in SNR/CNR were:52%/60% (cartilage), 72%/63% (muscle), 45%/50% (trabecular bone). Radiologists scored images with AF 1 and 2 as near‐excellent, AF 3 as good‐to‐excellent (P = 0.075), and AF 4 as average‐to‐good (P = 0.11).

Conclusion:

It is feasible to perform high resolution 7T MRI of the wrist with parallel imaging. SNR and CNR decrease with higher AF, but image quality remains above‐average. J. Magn. Reson. Imaging 2010;31:740–746. © 2010 Wiley‐Liss, Inc.

     

Magnetic resonance imaging of articular cartilage of the knee: comparison between fat-suppressed three-dimensional SPGR imaging, fat-suppressed FSE imaging, and fat-suppressed three-dimensional DEFT imaging, and correlation with arthroscopy

PURPOSE: To compare signal-to-noise ratios (S/N) and contrast-to-noise ratios (C/N) in various MR sequences, including fat-suppressed three-dimensional spoiled gradient-echo (SPGR) imaging, fat-suppressed fast spin echo (FSE) imaging, and fat-suppressed three-dimensional driven equilibrium Fourier transform (DEFT) imaging, and to determine the diagnostic accuracy of these imaging sequences for detecting cartilage lesions in osteoarthritic knees, as compared with arthroscopy. MATERIALS AND METHODS: Two sagittal fat-suppressed FSE images (repetition time [TR] / echo time [TE], 4000/13 [FSE short TE] and 4000/39 [FSE long TE]), sagittal fat-suppressed three-dimensional SPGR images (60/5, 40 degrees flip angle), and sagittal fat-suppressed echo-planar three-dimensional DEFT images (400/21.2) were acquired in 35 knees from 28 patients with osteoarthritis of the knee. The S/N efficiencies (S/Neffs) of cartilage, synovial fluid, muscle, meniscus, bone marrow, and fat tissue, and the C/N efficiencies (C/Neffs) of these structures were calculated. Kappa values, exact agreement, sensitivity, specificity, positive predictive value, and negative predictive value were determined by comparison of MR grading with arthroscopic results. RESULTS: The synovial fluid S/Neff on fat-suppressed FSE short TE images, fat-suppressed FSE long TE images, and fat-suppressed three-dimensional DEFT images showed similar values. Fat-suppressed three-dimensional DEFT images showed the highest fluid-cartilage C/Neff of all sequences. All images showed fair to good agreement with arthroscopy (kappa, 0.615 in FSE short TE, 0.601 in FSE long TE, 0.583 in three-dimensional SPGR, and 0.561 in three-dimensional DEFT). Although the sensitivity of all sequences was high (100% in FSE short TE, FSE long TE, and DEFT; 96.7% in SPGR), specificity was relatively low (67.6% in FSE short TE and FSE long TE; 85.3% in SPGR; 58.3% in DEFT). The peripheral area of bone marrow edema or whole area of bone marrow edema on fat-suppressed FSE images was demonstrated as low or iso-signal intensity on fat-suppressed three-dimensional DEFT images. CONCLUSION: Fat-suppressed three-dimensional SPGR imaging and fat-suppressed FSE imaging showed high sensitivity and high negative predictive values, but relatively low specificity. The Kappa value and exact agreement was the highest on fat-suppressed FSE short TE images. Fat-suppressed three-dimensional DEFT images showed results similar to the conventional sequences.     

In vivo ultra-high-field magnetic resonance imaging of trabecular bone microarchitecture at 7 T

Purpose

To investigate the feasibility of 7T magnetic resonance imaging (MRI) to visualize and quantify trabecular bone structure in vivo by comparison with 3T MRI and in vivo three‐dimensional (3D) high‐resolution peripheral quantitative computed tomography (HR‐pQCT).

Materials and Methods

The distal tibiae of 10 healthy volunteers were imaged. Therefore, fully balanced steady state free precession (bSSFP) and spin‐echo (bSSSE) pulse sequences were implemented and optimized for 7T. Structural bone parameters, such as apparent bone‐volume over total‐volume fraction (app.BV/TV), apparent trabecular plate separation (app.TbSp), apparent trabecular plate thickness (app.TbTh), and apparent trabecular plate number (app.TbN), were derived.

Results

All structural trabecular bone parameters correlated well (r > 0.6) between 7T and 3T, and between 7T and HR‐pQCT (r > 0.69), with the exception of app.TbTh, which correlated modestly (r = 0.41) between field strengths and very low with HR‐pQCT (r < 0.16). Regarding absolute values, app.TbN varied only 4% between field strengths, and only 0.6% between 7T and HR‐pQCT. App.TbSp correlated best between 7T and HR‐pQCT (r = 0.89). Using bSSSE, significant smaller trabecular thickness and significant higher trabecular number were found compared to bSSFP.

Conclusion

We concluded that imaging and quantification of the trabecular bone architecture at 7T is feasible and preferably done using bSSSE. There exists great potential for ultra‐high‐field (UHF) MRI applied to trabecular bone measurements. J. Magn. Reson. Imaging 2008;27:854–859. © 2008 Wiley‐Liss, Inc.