High-Resolution 3D Cartilage Imaging with IDEAL SPGR at 3 T

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1 Siepmann et al. Knee MRI with IDEL SPGR Musculoskeletal Imaging Technical Innovation David. Siepmann 1 Jeff McGovern 2 Jean H. rittain 3 Scott. Reeder 1,4 Siepmann D, McGovern J, rittain JH, Reeder S Keywords: cartilage, fat suppression, knee, MRI, spoiled gradient echo, water fat separation DOI: /JR Received July 22, 2006; accepted after revision June 10, The employment status of J. McGovern and J. H. rittain at GE Healthcare did not influence the data in this study. 1 Department of Radiology, University of Wisconsin School of Medicine, 600 Highland ve., CSC E1/374, Madison, WI ddress correspondence to S.. Reeder (sb.reeder@hosp.wisc.edu). 2 GE Healthcare, Waukesha, WI. 3 Global pplied Science Laboratory, GE Healthcare, Madison, WI. 4 Departments of Medical Physics, iomedical Engineering, and Medicine, University of Wisconsin, Madison, WI. JR 2007; 189: X/07/ merican Roentgen Ray Society High-Resolution 3D Cartilage Imaging with IDEL SPGR at 3 T OJECTIVE. 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 (IDEL) 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, IDEL SPGR imaging combined with parallel imaging at 3 T provides robust fat water separation and significant improvement in cartilage SNR. Use of IDEL SPGR also led to dramatic improvement in cartilage fluid contrast-to-noise ratio compared with fat-saturated SPGR imaging. Thus, use of IDEL 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. steoarthritis is one of the most O common human diseases. n estimated 20 million mericans had symptomatic disease in 1998 [1]. When it affects the major weight-bearing joints, particularly the hips and knees, osteoarthritis can substantially limit mobility and is the second leading cause of disability after heart disease [1]. Historically, management of osteoarthritis has been limited to symptomatic therapy for mild disease and joint replacement in advanced cases. Several treatment options are under investigation, including chondrocyte transplantation, microfracture, and osteochondral grafting [2]. Evaluation of these investigational therapies requires noninvasive monitoring, for which MRI is ideally suited. variety of MRI sequences have been developed for cartilage imaging. These sequences can be broadly divided into physiologic and morphologic techniques. Physiologic techniques, which are used to detect biochemical abnormalities before development of a morphologic abnormality, include T 2 shortening [3], 23 Na-MRI to detect regions of glycosaminoglycan depletion [4], diffusion-weighted imaging [5], T1ρ imaging [6], and delayed contrast enhancement [7]. Most physiologic methods are investigational. Clinical evaluation of cartilage has primarily focused on morphologic evaluation. Detection of the subtle early changes of osteoarthritis, that is, surface cartilage defects such as fissures, requires images of high resolution and high signal-to-noise ratio (SNR) with good fat suppression and good contrast between cartilage and synovial fluid. To be clinically practical, imaging must be completed in a reasonable time. ccurate quantification of cartilage volume is critical for clinical trials of drug therapies for osteoarthritis. Quantification enables detection of small changes in cartilage thickness and volume that would not be evident clinically or radiographically. ccurate volume measurements, which rely on computer segmentation [8], require good fat suppression, high SNR and, most important, excellent contrast between joint fluid and cartilage. This contrast is optimal when synovial fluid signal intensity is low compared with the relatively high signal intensity of cartilage. In clinical practice, the most commonly used sequences for cartilage evaluation are 2D fatsaturated fast spin-echo and 3D spoiled gradient echo (SPGR) with fat saturation [9, 10]. ecause of its high resolution, 3D coverage, and high cartilage signal intensity, fat-saturated SPGR has been considered the clinical reference standard for morphologic quantification and volumetric assessment of cartilage [9]. The 1510 JR:189, December 2007

2 Knee MRI with IDEL SPGR intermediate signal intensity of synovial fluid on images obtained with fat-saturated SPGR, however, can limit sensitivity to small lesions, especially those on the articular surface of the cartilage, and can make it difficult to perform accurate computer segmentation to measure cartilage thickness and volume. The major limitation of fat-saturated SPGR that has prevented widespread clinical acceptance is the prolonged imaging time, up to minutes [9]. Such long imaging times are needed for fat-saturated SPGR because the sequence is SNR-limited and requires multiple averages to improve SNR performance. The poor SNR performance is closely related to the SNR inefficiency of the SPGR pulse sequence and is worsened by the high overhead of playing a fat-saturation pulse every TR. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEL) [11] is used for robust separation of fat and water with very high SNR efficiency. Unlike conventional fat-saturation methods, IDEL is insensitive to magnetic field ( 0 and 1 ) inhomogeneity. lthough acquisition of three separate images with slightly different TEs is needed to decompose water from fat, IDEL is highly SNR-efficient, with an effective averaging value of 3.0 [12, 13]. This effective averaging is equivalent to the SNR performance that would be obtained if the object were to contain only water or fat and the three images were averaged together. In addition, IDEL does not require the overhead of a fat-saturation pulse, and in comparison with fat-saturated SPGR, it is highly efficient [10, 11]. For these reasons, IDEL SPGR is more SNR-efficient than fat-saturated SPGR and can lead to a substantially shorter TR [14, 15]. Finally, fat-saturation methods such as fat-saturated SPGR may suffer from reduced SNR as a result of partial water saturation from fat-saturation pulses. The need for three signal averages for IDEL SPGR can lead to prolonged imaging times. y taking advantage of the spatial information inherent in phased-array radiofrequency coils, parallel imaging can be used to reduce this acquisition time. The purposes of this study were to combine IDEL SPGR with parallel imaging at 3 T to acquire highresolution, high-snr images of cartilage across the entire knee within 5 minutes, an acceptable imaging time for a sequence added to routine clinical protocols, and to compare this new sequence with the traditional fat-saturated SPGR technique. Materials and Methods ll imaging was performed on a 3 T MRI system (TwinSpeed, GE Healthcare) with an eight-channel extremity coil (In Vivo). Eight knees in four adult volunteers (three men, one woman; average age, 33.5 years; range, years) were imaged. ll imaging was performed with the approval of our institutional review board after informed consent was obtained. In addition, all data were handled in compliance with the HIP. Only authors who were not employees of GE Healthcare had control of data and information that might have presented a conflict of interest for those who were employees of GE Healthcare. Parameters for both fat-saturated SPGR and IDEL SPGR imaging included a matrix (zero-filled to ), 16-cm 2 field of view, 1.0-mm slice thickness, 90 slices, and acquisition bandwidth of ± 41 khz. The resulting voxel dimensions were mm. Linear autoshimming was used for both sequences. Optimal flip angles to maximize cartilage signal intensity were calculated on the basis of the Ernst angle with use of the TR for each sequence and a presumed cartilage T1 signal time of 1,200 milliseconds at 3 T [16, 17]. IDEL SPGR imaging was performed with the minimum possible TR of 10.8 milliseconds. The TEs were selected on the basis of previous findings [13] that showed optimal SNR performance when the phase between water and fat for the center echo is π/2 + kπ (k is any integer), that is, when water and fat signals are in quadrature. The two remaining echoes are placed before and after the center echo by a water fat phase shift of 2π/3 relative to the center echo. This combination of water fat phase shifts optimizes the noise performance of IDEL for decomposition of water from fat signals. Several echo groups are possible, each with a different integer value of k. To maximize the pulse sequence efficiency and achieve the shortest TE (full echo), we chose the group in which the shortest echo equaled or exceeded the minimum TE determined with other sequence parameters. The TEs for this study corresponded to a k value of 4 based on a fat water chemical shift of 420 Hz at 3 T. This value corresponds to TE values of 4.6, 5.4, and 6.1 milliseconds. t 3 T, the spacing between echoes is 0.80 milliseconds, and the spacing between subsequent k groups is 1.2 milliseconds. To measure SNR when parallel imaging is used, IDEL SPGR acquisition was performed twice in succession on each knee (see later). We used a sensitivity encoding based [18] parallel acceleration method (SSET, GE Healthcare) with an acceleration factor of 2.22 for the IDEL SPGR acquisition. The optimized flip angle for imaging cartilage with IDEL SPGR was 8 based on a TR of 10.8 milliseconds, and the total imaging time was 4 minutes 59 seconds. Separate fat, water, and recombined images were reconstructed with an online reconstruction algorithm. Recombined images were corrected for chemical shift artifact in the readout direction by realigning the separated water and fat images by the chemical shift, which depends on imaging parameters, and is easily calculated (Yu H et al., presented at the 2004 annual meeting of the International Society for Magnetic Resonance in Medicine). lthough this implementation of IDEL involved three excitations, the absence of the fat-saturation pulse made the total imaging time less than three times greater than that of fat-saturated SPGR with a single excitation. Therefore, an acceleration factor of 2.22 resulted in equal imaging times for IDEL SPGR with three excitations and fat-saturated SPGR with a single excitation. Fat-saturated SPGR imaging was performed with the minimum possible TR of 14.8 milliseconds, TE of 4.6 milliseconds (full echo), and a 9 flip angle, which is the optimized flip angle for imaging cartilage with fat-saturated SPGR based on a TR of 14.8 milliseconds. We used one average and no parallel imaging for a total imaging time of 5 minutes. For SNR measurement, regions of interest (ROIs) were defined by a board-certified radiologist with 6 years of experience in the patellar cartilage, femoral condyle cartilage, muscle, and synovial fluid. The ROI shape was ovoid and optimized for best sampling of the tissue of interest. lthough ROI placement was customized for each knee, the same ROI was used for all imaging sequences in each knee. The ROI size averaged 24 mm 2 (114 pixels) for patellar cartilage and 8 mm 2 (38 pixels) for joint fluid. In conventional SNR measurement techniques, it is assumed that the distribution of noise across the imaged field of view is uniform [19, 20]. ecause noise is not uniform across images reconstructed with parallel imaging methods [18], conventional methods for measuring SNR cannot be used. Therefore, the SNR measurements for IDEL SPGR were made with the difference method [21] on the basis of two identical acquisitions. SNR for fat-saturated SPGR images was measured with conventional techniques with one ROI on the image and the noise ROI on a region outside the body. SNR was calculated as the quotient of the average signal from a small ROI of each studied tissue and the SD of the noise in an ROI placed on a region outside the body and free of artifact. ecause images were reconstructed with the square root of sum of squares technique [22] for multiple coils, a correction factor of 0.70 was used to adjust for the fact that noise in the magnitude image has a chi-square distribution that causes underestimation of noise [20]. To validate comparison between SNR measurements obtained with the two techniques, two iden- JR:189, December

3 Siepmann et al. tical fat-saturated SPGR acquisitions were obtained in four knees. SNR measurements were performed with both techniques with identical ROIs in patellar and femoral cartilage, synovial fluid, and muscle. These results were compared by use of a two-tailed Student s t test. For each knee, cartilage fluid contrast-to-noise ratio (CNR) was calculated as the difference between the SNR of the cartilage and the SNR of fluid. SNR and CNR values were compared by use of a paired Student s t test. Statistical comparisons were considered significant at p < ll statistical calculations for SNR measurements were made with Microsoft Excel 2004 version 11.0 for pple Macintosh. Results The imaged volume consisting of a 16-cm 2 field of view and 90 slices with 1.0-mm thickness provided sufficient coverage to include the entire articular cartilage in all eight imaged knees. Image quality was subjectively rated as very good by two board-certified radiologists for both techniques for all images in all studies with homogeneous fat suppression on both IDEL SPGR and fat-saturated SPGR images. Figure 1 shows a comparison of IDEL SPGR with fat-saturated SPGR images. Unlike fat-saturated SPGR, which yields fat-suppressed images only, IDEL SPGR yields a fat-only image and a water-only image. Water and fat images can be realigned to correct for chemical shift in the readout direction and can be recombined as shown in Figure 1. SNR calculated with both conventional and difference methods performed on four knees to validate SNR comparison by use of the two techniques was slightly lower with the difference method by a factor of (p < 0.05) when cartilage, muscle, and fluid ROIs were included. separate analysis in which two patellar and two femoral ROIs were evaluated in each of the four knees showed no significant difference between the techniques (ratio, 0.982; p = 0.18). SNR results for the eight knees are plotted in Figure 2. The SNR of patellar cartilage was 34% higher for IDEL SPGR (20.2 ± 4.0) than for fat-saturated SPGR (15.0 ± 2.8) (p < 0.001). The SNR of femoral trochlear cartilage was 31% higher with IDEL SPGR (22.5 ± 3.4) than with fat-saturated SPGR (17.2 ± 1.6) (p < 0.01). Muscle SNR was 26% higher for IDEL SPGR (16.2 ± 3.6) than for fat-saturated SPGR (12.9 ± 2.3) (p <0.001). For the SNR of synovial fluid, no significant difference was found between IDEL SPGR (12.5 ± 2.0) and fat-saturated SPGR (12.2 ± 1.8) (p < 0.001). The CNR between patellar cartilage and articular fluid was 2.7 times higher for IDEL SPGR (7.7 ± 3.4) than for fat-saturated SPGR (2.8 ± 1.3) (p < 0.01). The CNR between femoral cartilage and articular fluid was 2.0 times higher for IDEL SPGR (10.0 ± 3.2) than for fat-saturated SPGR (5.0 ± 1.3) (p < 0.01). Improved cartilage fluid contrast is shown in Figure 3, a magnified view of the patellofemoral joint of one volunteer. In addition to the water-only images analyzed in comparison with fat-saturated SPGR, the IDEL SPGR sequence yielded fat-only images and in-phase and out-of-phase recombined images, as shown in Figure 4. These recombined images were corrected for chemical shift artifact. Discussion The combination of IDEL SPGR at 3 T with parallel imaging yields high-resolution ( mm) images of cartilage in 5 minutes with an SNR of approximately 20. Use of IDEL leads to superior SNR performance owing to improved sequence efficiency due to elimination of the fat-saturation pulse and optimization of echo shifts that produce the best possible SNR performance of the IDEL water fat separation method. ecause of the absence of a fat-saturation pulse, an acceleration factor of 2.22 allows three effective signal averages of IDEL SPGR in the same acquisition time as a single average of fat-saturated SPGR. ecause both acceleration factor and number of signal averages are related to SNR by the square root of imaging time, we would expect an SNR advantage of the square root of 3/2.22, or 1.17, 17% higher for IDEL SPGR than for fatsaturated SPGR, assuming optimal coil geometry and ignoring the g-factor. This value compares to an observed SNR advantage of 34% for patellar cartilage in this study. dditional factors that may have led to this discrepancy between expected and observed SNRs include partial water saturation caused by the fat-saturation pulse and inadvertent miscalibration of the radiofrequency transmit pulse. The result would be a flip angle different from the intended value and optimized for the Ernst angle on the basis of published values of the T 1 signal of cartilage. Compared with fat-saturated SPGR, the IDEL SPGR technique showed a 2.7 times greater patellar cartilage fluid CNR. This improvement in CNR addressed a major limitation of fat-saturated SPGR for morphologic evaluation of cartilage. The absence of the fat-saturation pulse allows a substantially shorter TR for IDEL SPGR (10.8 milliseconds) compared with the minimum TR obtainable with fat-saturated SPGR (14.8 milliseconds). This decrease in TR likely explains the improved T1 contrast between fluid and cartilage. Synovial fluid is better suppressed with shorter TR because of the long T 1 signal of fluid. The receiver bandwidth was chosen to minimize echo and TR. lthough this relatively high bandwidth also limits the SNR, the same value was used for both sequences studied and therefore did not affect the comparison of SNRs of the two. The marked improvement in CNR may improve the sensitivity for detection of cartilage surface defects with IDEL SPGR compared with fat-saturated SPGR. It is also an important advantage for automated segmentation and volume measurements [8, 23, 24]. Such quantitative measurements are critical for assessment of promising drug therapies aimed at slowing, stopping, and reversing the cartilage loss of osteoarthritis. When it was compared with conventional SNR measurement in four knees imaged with two identical fat-saturated SPGR acquisitions, the difference method was shown to underestimate SNR by approximately 5% for all tissue types. No significant difference was seen when only cartilage was included in the comparison. The slight underestimation of SNR with the difference method can likely be attributed to misregistration between the two sequential acquisitions owing to minimal knee movement. ny misregistration increases the variance of the subtracted image and simulates noise, leading to underestimation of SNR. s might be expected, this effect seemed more evident in muscle, which is more heterogeneous, than in cartilage, which is more homogeneous. The small underestimation of SNR with the difference method suggests that the true SNR benefit of IDEL SPGR over fat-saturated SPGR may actually be slightly greater than these results indicate. With a change from 1.5 to 3 T comes a doubling in chemical shift, from 210 to 420 Hz. This effect can be a liability for many sequences at 3 T, necessitating doubling bandwidth to reduce the increased chemical shift artifact and sacrificing much of the SNR benefit of higher field strengths. Imaging at 3 T improves the performance of the IDEL method, resulting in closer spacing of echoes and of consecutive echo 1512 JR:189, December 2007

4 Knee MRI with IDEL SPGR SNR Patellar Cartilage C Femoral Cartilage D groups, further improving the SNR efficiency of the technique. The reduced spacing between echo groups greatly improves the timing flexibility of the pulse sequence [13]. In addition, the ability of IDEL to correct for chemical shift artifact obviates the need to increase bandwidth when increasing field strength to 3 T to reduce chemical shift artifact, allowing realization of the full SNR benefit of imaging at 3 T. lthough variants of IDEL have been developed that include all three echoes within a Fig year-old man with healthy knee. D, Representative lateral parasagittal MR images from IDEL SPGR ( C) and fat-saturated SPGR (D) acquisitions with equal acquired resolution and imaging times. IDEL SPGR acquisition provides fat only (C) and combined fat water images () in addition to water images (). This capability improves signal-to-noise ratio and markedly improves cartilage fluid contrast compared with that of equivalent fat-saturated SPGR image (D). Fig. 2 Comparison of calculated signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) between patellar cartilage and articular fluid values with IDEL SPGR (dark gray) and fat-saturated SPGR (light gray) techniques. Error bars represent standard error above and below mean. Other than SNR of synovial fluid, all results are statistically significant (p <0.05)., Graph shows IDEL SPGR provides significant SNR advantage for patellar and femoral cartilage and for muscle. SNR values for synovial fluid are approximately equivalent for the two techniques., Graph shows IDEL SPGR markedly improving CNR between patellar cartilage and articular fluid compared with fat-saturated SPGR for both patellar and femoral cartilage. CNR 12 Muscle Fluid Patellar Cartilage Fluid Femoral Cartilage Fluid single TR (Reeder S et al., presented at the 2006 annual meeting of the International Society for Magnetic Resonance in Medicine), most implementations of IDEL require three excitations with one echo per TR. This requirement can lead to prolonged imaging JR:189, December

5 Siepmann et al. times but provides excellent SNR. Parallel imaging methods are highly complementary to IDEL. Parallel imaging reduces the imaging-time penalty of IDEL, and the high SNR performance of IDEL offsets the SNR penalty of parallel imaging. s our findings show, combining IDEL with parallel imaging can provide an optimal combination of SNR and reasonable imaging time. In addition to fat-suppressed water-only imaging, IDEL yields separate fat, in-phase, and out-of-phase images with no additional imaging time penalty. s an adjunct to routine clinical sequences, the images may contribute C D to increased confidence in the diagnosis of noncartilaginous abnormalities, such as bone marrow edema, and of ligamentous and meniscal injuries. Recent work [25] has shown the value of in-phase and out-of-phase imaging for characterization of benign versus malignant disease in vertebral bone marrow. Such images, which can be acquired with the IDEL sequence, may aid in evaluation of incidental extremity marrow findings. The recombined images are corrected for chemical shift artifact in the readout direction, allowing use of lower receiver bandwidth with an associated increase in SNR. Fig year-old man with healthy knee. and, Magnified sagittal MR images allow better comparison of contrast-to-noise ratio (CNR) for patellar cartilage and synovial fluid. IDEL SPGR image () shows lower signal intensity in fluid, and therefore better CNR, than does fat-saturated SPGR image (). Fig year-old man with healthy knee. D, MR images show that in addition to signal-to-noise ratio and contrast-to-noise ratio advantages, IDEL SPGR sequence yields not only images with robust fat saturation () but also fat-only () and in-phase (C) and out-of phase (D) recombined images with no additional imaging time. This capability may lead to methods for evaluating bone marrow abnormalities such as edema, which is common in patients with osteoarthritis. Finally, the combination of high-resolution, high SNR, and improved fluid-cartilage CNR of IDEL offers potential for improved segmentation and volumetric analysis studies in addition to its benefits for clinical evaluation of cartilage morphology [8, 23, 24]. Future studies may prove the predicted increased accuracy of segmentation, as was the case in a study in which porcine knee specimens (with saline displacement techniques as a reference standard) were used to evaluate the advantage of 3 T over 1.5 T for fat-saturated SPGR [26] JR:189, December 2007

6 Knee MRI with IDEL SPGR In summary, this study showed that the combination of parallel imaging with IDEL for SPGR imaging of knee cartilage provides high-quality, high-snr images of the cartilage with better SNR performance than achieved with fat-saturated SPGR and a greatly improved cartilage fluid CNR. References 1. Lawrence RC, Helmick CG, rnett FC, et al. Estimates of the prevalence of arthritis and selected musculoskeletal disorders in the United States. rthritis Rheum 1998; 43: Garrett W Jr. Evaluation and treatment of the arthritic knee. J one Joint Surg m 2003; 85: Gold GE, Thedens DR, Pauly JM, et al. MR imaging of articular cartilage the knee: new methods using ultrashort TEs. JR 1998; 170: Reddy R, Insko EK, Noyszewski E, Dandora R, Kneeland J, Leigh JS. Sodium MRI of human articular cartilage in vivo. Magn Reson Med 1998; 39: Kneeland J. MRI probes biophysical structure of cartilage. Diagn Imaging (San Franc) 1996; 18: Duvvuri U, Reddy R, Patel SD, Kaufman JH, Kneeland J, Leigh JS. T1rho-relaxation in articular cartilage: effects of enzymatic degradation. Magn Reson Med 1997; 38: urstein D, Velyvis J, Scott KT, et al. Protocol issues for delayed Gd(DTP) enhanced MRI (dgemric) for clinical evaluation of articular cartilage. Magn Reson Med 2001; 45: Eckstein F, Westhoff J, Sittek H, et al. In vivo reproducibility of three-dimensional cartilage volume and thickness measurements with MR imaging. JR 1998; 170: Disler DG, Peters TL, Muscoreil SJ, et al. Fat-suppressed spoiled GRSS imaging of knee hyaline cartilage. JR 1994; 163: Hargreaves, Gold GE, eaulieu CF, Vasanawala SS, Nishimura DG, Pauly JM. Comparison of new sequences for high-resolution cartilage imaging. Magn Reson Med 2003; 49: Reeder S, Wen Z, Yu H, et al. Multicoil Dixon chemical species separation with an iterative leastsquares estimation method. Magn Reson Med 2004; 51: Reeder S, Pineda R, Wen Z, et al. Iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEL): application with fast spin-echo imaging. Magn Reson Med 2005; 54: Pineda R, Reeder S, Wen Z, Pelc NJ. Cramer- Rao bounds for three-point decomposition of water and fat. Magn Reson Med 2005; 54: Parker DL, Gullberg GT. Signal-to-noise efficiency in magnetic resonance imaging. Med Phys 1990; 17: Reeder S, McVeigh ER. The effect of high performance gradients on fast gradient echo imaging. Magn Reson Med 1994; 32: Gold GE, Han E, Stainsby J, Wright G, rittain JH, eaulieu C. Musculoskeletal MRI at 3.0 T: relaxation times and image contrast. JR 2004; 183: Stanisz GJ, Odrobina EE, Pun J, et al. T1, T2 relaxation and magnetization transfer in tissue at 3T. Magn Reson Med 2005; 54: Pruessmann KP, Weiger M, Scheidegger M, oesiger P. SENSE: sensitivity encoding for fast MRI. Magn Reson Med 1999; 42: Henkelman RM. Measurement of signal intensities in the presence of noise in MR images. Med Phys 1985; 12: Constantinides CD, talar E, McVeigh ER. Signalto-noise measurements in magnitude images from NMR phased arrays. Magn Reson Med; 1997; 38: Price R, xel L, Morgan T, et al. Quality assurance methods and phantoms for magnetic resonance imaging: report of the PM nuclear magnetic resonance Task Group No. 1. Med Phys 1990; 17: Roemer P, Edelstein W, Hayes CE, Souza SP, Mueller OM. The NMR phased array. Magn Reson Med; 1990; 16: Kornaat PR, Reeder S, Koo S, et al. MR imaging of articular cartilage at 1.5T and 3.0T: comparison of SPGR and SSFP sequences. Osteoarthritis Cartilage 2005; 13: Stammberger T, Eckstein F, Englmeier KH, Reiser M. Determination of 3D cartilage thickness data from MR imaging: computational method and reproducibility in the living. Magn Reson Med 1999; 41: Zajick DC, Morrison W, Schweitzer ME, Parellada J, Carrino J. enign and malignant processes: normal values and differentiation with chemical shift MR imaging in vertebral marrow. Radiology 2005; 237: auer JS, Krause SJ, Ross CJ, et al. Volumetric cartilage measurements of porcine knee at 1.5-T and 3.0-T MR imaging: evaluation of precision and accuracy. Radiology 2006; 241: JR:189, December