MRI of Soft-Tissue Tumors: Fast STIR Sequence as Substitute for T1-Weighted Fat-Suppressed Contrast-Enhanced Spin-Echo Sequence

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1 Musculoskeletal Imaging Original Research Tokuda et al. MRI of Soft-Tissue Tumors Musculoskeletal Imaging Original Research Osamu Tokuda 1 Yuko Harada Naofumi Matsunaga Tokuda O, Harada Y, Matsunaga N Keywords: contrast-enhanced, contrast-to-noise ratio, MRI, signal-to-noise ratio, soft-tissue tumor, STIR DOI: /AJR Received March 2, 2009; accepted after revision May 13, All authors: Department of Radiology, Yamaguchi University Graduate School of Medicine, Minamikogushi, Ube, Yamaguchi , Japan. Address correspondence to O. Tokuda (chinupapa@goo.jp). AJR 2009; 193: X/09/ American Roentgen Ray Society MRI of Soft-Tissue Tumors: Fast STIR Sequence as Substitute for T1-Weighted Fat-Suppressed Contrast-Enhanced Spin-Echo Sequence OBJECTIVE. The purpose of this study was to assess the value of the fast STIR sequence in comparison with the T1- sequence in the evaluation of soft-tissue tumors. MATERIALS AND METHODS. Sixty-seven soft-tissue tumors imaged with both STIR and T1- sequences were evaluated. The signal-to-noise and contrast-to-noise ratios of the tumors in comparison with normal muscle, bone marrow, and fat were measured. Subjective image contrast between soft-tissue tumors and the nearest normal tissue was evaluated by two observers. The observers classified the soft-tissue tumors as benign or malignant using a 5-point scale, and sensitivity, specificity, and accuracy were calculated. The results of the two readings were assessed with receiver operating characteristic analysis. RESULTS. The contrast-to-noise ratios of all tumors in comparison with muscle (p < 0.01), bone marrow (p < 0.05), and fat (p < 0.05) were significantly higher on the fast STIR images than on the T1- images. Both observers mean ratings of benign, malignant, and all tumors in comparison with muscle on fast STIR images were significantly higher than those on T1- images. For both observers, the mean sensitivity, specificity, accuracy, and area under the receiver operating characteristic curve in evaluation of the fast STIR images did not differ significantly from those in evaluation of the T1- images. CONCLUSION. The fast STIR sequence is excellent for evaluation of soft-tissue tumors, and contrast-enhancement is not always needed. T he most promising and important sequences for evaluation of soft-tissue tumors are those that entail fat suppression. Suppression of the relatively high signal intensity of fat leads to more efficient use of the dynamic range for display of tissue contrast on MR images. Fat suppression also may be helpful for reducing the severity of artifacts. The conspicuousness of phase-encoding errors caused by motion is proportional to the signal intensity of the moving structure. Because fat is a major contributor to many motion artifacts, fat suppression can be effective in reducing their prominence [1, 2]. The conspicuousness of abnormal enhancement after injection of paramagnetic contrast material can increase on T1-weighted images with the use of fat suppression [3 6]. T1-weighted fat-suppressed contrast-enhanced imaging, which is the combination of gadopentetate dimeglumine contrast enhancement and fat-suppression technique, is a superior method for evaluating adrenal masses [7] and neoplastic and inflammatory diseases of the spine [5, 8], kidney [9], and head and neck [10 12]. T1- imaging also is useful for evaluation of softtissue tumors because of the greater conspicuity of these lesions after enhancement. Use of T1-weighted fat-suppressed contrastenhanced imaging can improve lesion detection, tissue characterization, and determination of tumor extent. STIR technique entails an alternative MRI sequence that suppresses the signal intensity of fat and the additive effects of T1 and T2 mechanisms on tissue signal intensity [4, 13 19]. STIR imaging is commonly used to detect bone marrow lesions because it is sensitive in the detection of tumor, edema, and infection in bone marrow [3, 4, 17, 18]. Fast STIR imaging is superior to T1-weighted fat- AJR:193, December

2 Tokuda et al. suppressed contrast-enhanced imaging in the evaluation of all bone marrow components, and the two techniques have comparable results in the evaluation of surrounding softtissue components [17]. To our knowledge, however, the value of fast STIR imaging in comparison with T1-weighted fat-suppressed contrast-enhanced imaging in the evaluation of soft-tissue tumors has not been reported. The purpose of this retrospective study was to assess the value of the fast STIR sequence in comparison with the T1-weighted fat-suppressed contrast-enhanced sequence in MRI evaluation of soft-tissue tumors. Materials and Methods A retrospective review was conducted of 85 MRI examinations performed from 2001 to 2007 to identify imaging findings of soft-tissue tumors with histopathologic confirmation in a biopsy or surgical specimen. The inclusion criterion was availability of both fast STIR and T1-weighted fatsuppressed contrast-enhanced images. Six patients were excluded because of severe motion artifact, and 12 patients were excluded because of inhomogeneous fat suppression on T1-weighted fatsuppressed contrast-enhanced images. The study group included 67 patients (39 men and boys, 28 women and girls; mean age, 55.1 years; range, 1 91 years). The histologic types and locations of the soft-tissue tumors are shown in Table 1. MR images were obtained with a 1.5-T superconducting MRI system (Signa Horizon, GE Healthcare). T1-weighted spin-echo (TR, milliseconds; effective TE, milliseconds) and T2-weighted fast spin-echo (TR/TE, 4,000 4,500/96) imaging was performed in two planes. A fast spin-echo STIR sequence (4,000 6,000/60 100; inversion time, 150 milliseconds; echo-train length, 10; field of view, mm 2 ; matrix size, ; section thickness, 3 5 mm; and intersection gap, mm) also was performed in two planes (transaxial and coronal or sagittal). The number of signals acquired was four, and the acquisition time for the fast STIR sequence varied between 5 minutes 12 seconds and 5 minutes 42 seconds. All patients agreed to receive an additional IV injection of contrast material. A T1-weighted spin-echo sequence ( /10 12) with fat suppression was performed in two planes (transaxial and coronal or sagittal) after administration of gadopentetate dimeglumine (0.1 mmol/kg body weight IV, Magnevist, Bayer Schering Pharma). The section thickness was 4 5 mm; intersection gap, mm; field of view, mm; image matrix size, , identical to the parameters used for the fast STIR sequence. The number TABLE 1: Histologic Types and Location of Tumors (n = 67) Benign (n = 34) Histologic Type of signals acquired was two, and the acquisition time for the fat-suppressed T1-weighted sequence varied between 3 minutes 30 seconds and 5 minutes 12 seconds. Semiquantitative imaging analysis was performed. The contrast-to-noise ratios (CNRs) of the soft-tissue tumors were measured. One slice position showing a prominent tumorous component was chosen, and a region of abnormal signals in the tumor was identified for analysis. A region of interest was placed within the zones of abnormal signal intensity of soft-tissue tumors, the nearest normal tissue (muscle, bone marrow, and fat), and air near the lesion. The mean size of each region of interest within the zones of abnormal signal intensity of soft-tissue tumors was cm 2 (range, ,343.2 cm 2 ), within normal muscle was cm 2 (range, cm 2 ), within normal bone marrow was cm 2 (range, cm 2 ), within normal fat was 76.9 cm 2 (range, cm 2 ), and within air near the Location No. of Lesions Schwannoma Shoulder, forearm, thigh, lower thigh 9 Neurofibroma Forearm, thigh 2 Glomus tumor Finger, toe 5 Desmoid Shoulder 1 Hemangioma Thigh, lower thigh, palm, finger, upper arm 6 Solitary fibrous tumor Groin, thigh 3 Fibromatosis Palm 1 Morton s neuroma Third toe 1 Lipoma Thigh, neck 4 Myxoma Thigh 2 Malignant (n = 33) Myxoid liposarcoma Thigh 1 Well-differentiated liposarcoma Thigh 2 Pleomorphic liposarcoma Buttocks, forearm 3 Epithelioid sarcoma Axilla 1 Malignant fibrous histiocytoma Groin, thigh, lower thigh, forearm 12 Malignant peripheral nerve sheath tumor Pelvic wall, buttocks, forearm 2 Malignant lymphoma Groin 1 Extraskeletal osteosarcoma Knee 1 Malignant rhabdoid tumor Thigh 1 Malignant melanoma Upper arm, foot 2 Multiple myeloma Sternoclavicular joint 1 Myxoid chondrosarcoma Axilla, groin, shoulder, lower thigh 4 Rhabdomyosarcoma Lower thigh 1 Synovial sarcoma Thigh 1 lesion was 95.3 cm 2 (range, cm 2 ). The average signal intensity of the region was measured on both fast STIR and T1-weighted fat-suppressed contrast-enhanced images. The signal intensity of normal muscle, bone marrow, fat, and air near the tumors was measured. All measurements were performed twice, and the mean values were adopted. All measurements were performed manually by one musculoskeletal radiologist (18 years of experience) using a standard MRI console (Image VINS Pro, Yokogawa Electric Corporation). The CNR of a tumor was calculated as the mean signal intensity of the lesion minus the mean signal intensity of the normal muscle, bone marrow, and subcutaneous fat near the lesion divided by the SD of air near the lesion. The signal-to-noise (SNR) of the tumors was calculated as the mean signal intensity of the lesion divided by the SD of air near the lesion. An additional qualitative analysis was performed by two musculoskeletal radiologists (18 and AJR:193, December 2009

3 MRI of Soft-Tissue Tumors TABLE 2: Contrast-to-Noise Ratio Comparison of Fast STIR and T1-Weighted Fat-Suppressed Contrast-Enhanced Images of Soft-Tissue Tumors Muscle Soft-Tissue Tumor Fast STIR T1-Weighted Fat-Suppressed Contrast-Enhanced 95% CI p Benign tumors ± ± to < 0.01 Malignant tumors ± ± to < 0.01 All tumors ± ± to < Bone marrow Fat Benign tumors ± ± to Malignant tumors ± ± to < 0.05 All tumors ± ± to < 0.05 Benign tumors ± ± to Malignant tumors ± ± to < 0.05 All tumors ± ± to < 0.05 Note Values are mean ± SD. years of experience) blinded to the clinical data. They evaluated 59 of the soft-tissue tumors on coronal images, five tumors on axial images, and three tumors on sagittal images. Both observers separately reviewed the fast STIR images and 2 weeks later separately reviewed the T1-weighted fat-suppressed contrast-enhanced images. The images were presented randomly to each of the readers at each session. Subjective image contrast, which differentiates soft-tissue tumors and normal tissue (muscle, bone marrow, and fat) near the tumors, was graded (1, poor; 2, acceptable; 3, good; 4, excellent). The observers then classified the soft-tissue tumors as benign or malignant using a 5-point scale to assign a confidence level (1, definitely benign; 2, probably benign; 3, equivocal; 4, probably malignant; 5, definitely malignant). The confidence level ratings of the images were used to calculate sensitivity, specificity, and accuracy for each observer in the diagnosis of malignancy using MR images. Ratings of 1 and 2 indicated a benign tumor; 4 or 5, a malignant tumor; and 3, an incorrect reading. In addition, MRI of seven tumors (two myxomas, one myxoid liposarcoma, four myxoid chondrosarcomas) showed a cystic mass. Sensitivity, specificity, and accuracy for each observer in differentiating benign from malignant tumors were calculated for these seven cystic masses. The paired Student s t test was used to compare the CNR and SNR of soft-tissue tumors on fast STIR images with those on T1-weighted fat-suppressed contrast-enhanced images; the observer ratings of the subjective image contrast of the tumors on the 4-point scale on both types of images; the observer ratings of confidence level regarding benign or malignant; and the sensitivity, specificity, and accuracy of fast STIR imaging with those of T1- imaging. Software (Statcel version 7.0, OMS) was used to calculate p; p < 0.05 was used to indicate a statistically significant difference. To assess interobserver and intraobserver variability in assignment of a confidence level to lesion status, the weighted kappa value [20] was calculated. The level of agreement was defined as follows: a kappa value less than 0 indicated no agreement; , poor agreement; , good agreement; , excellent agreement. Kappa values were calculated with statistical software (Excel Statistics 2008 for Mirosoft Windows, SSRI). Receiver operating characteristic curves were calculated to compare the fast STIR reading with the T1- readings. True-positive cases were defined as malignant tumors correctly designated as such. False-positive cases were defined as benign tumors incorrectly designated as such. Diagnostic capability was determined by calculation of the area under each reader-specific receiver operating characteristics curve (A z ). The results were expressed as mean ± SD. Calculation of A z was performed with software (Rockit beta version 0.9.1, C. E. Metz and B. A. Herman, University of Chicago). The A z values for fast STIR readings were compared with those for T1-weighted fat-suppressed contrast-enhanced readings by use of onefactor analysis of variance for both observers. Results CNR comparisons for the fast STIR versus T1- images of soft-tissue tumors (benign, malignant, and all tumors) are shown in Table 2. In comparison with the CNR of muscle, the mean CNRs of benign, malignant, and all tumors were significantly higher on fast STIR images than on T1-weighted fat-suppressed contrast-enhanced images (p < 0.01) (Figs. 1 and 2). In comparison with bone marrow, the mean CNRs of malignant and all tumors were significantly higher on fast STIR than on T1-weighted fat-suppressed contrast-enhanced images (p < 0.05). There was no significant difference, however, between the mean CNR of benign tumors on the fast STIR images and that on T1-weighted fat-suppressed contrast-enhanced images (p = 0.16) (Fig. 3). In comparison with the CNR of fat, the mean CNRs of malignant and all tumors were significantly higher on fast STIR images than on T1-weighted fatsuppressed contrast-enhanced images (p < 0.05). However, there was no significant difference between the mean CNR of benign tumors on fast STIR images and that on T1- images (p = 0.32). There were no significant differences between the mean SNRs of benign, malignant, and all tumors in the muscle, bone marrow, and fat on fast STIR images and those on T1- images. The observer ratings of subjective image contrast of the tumors in comparison with normal muscle, bone marrow, and fat on fast STIR and T1- images are shown in Table 3. In the comparison between tumor and muscle, AJR:193, December

4 Tokuda et al. both observers rated the fast STIR images significantly higher than the T1-weighted fat-suppressed contrast-enhanced images of benign tumors (observer 1, p < ; observer 2, p < 0.05), malignant tumors (observer 1, p < 0.001; observer 2, p < 0.05), and all tumors (observer 1, p < ; observer 2, p < 0.01). In the comparison between tumor and bone marrow, observer 1 rated the fast STIR images significantly higher than the T1-weighted fatsuppressed contrast-enhanced images of benign tumors (p < 0.001) and all tumors (p < 0.01). There were no significant differences, however, between the mean ratings of fast STIR images and those of T1-weighted-fatsuppressed contrast-enhanced images of malignant tumors. For observer 2, there were no significant differences between the mean ratings of the fast STIR images and T1-weighted fat-suppressed contrast-enhanced images of benign, malignant, and all tumors. In the comparison of fat with benign and all tumors, observer 1 gave the fast STIR images significantly higher mean ratings than the T1- images (p < 0.05). In the comparison of fat with malignant tumors by observer 1, no significant difference (p = 0.54) was found between the mean rating of the fast STIR images and that of the T1-weighted fat-suppressed contrast-enhanced images. For observer 2, there were no significant differences between the mean ratings of the fast STIR images of benign, malignant, and all tumors and those of the T1-weighted fat-suppressed contrast-enhanced images. The sensitivity, specificity, and accuracy for differentiation of benign from malignant tumors on fast STIR and T1-weighted fat-suppressed contrast-enhanced images by both observers are shown in Table 4. The sensitivity, specificity, and accuracy of observer 1 reading the fast STIR images were 93.9%, 73.5%, and 80.6% and reading the T1-weighted fat-suppressed contrast-enhanced images were 97.0%, 58.8%, and 77.6%. The sensitivity, specificity, and accuracy of observer 2 reading the STIR images were 81.8%, 52.9%, and 67.2%, and reading the T1-weighted fatsuppressed contrast-enhanced images were 84.8%, 47.1%, and 65.7%. For both observers, there were no significant differences between the sensitivity, specificity, and accuracy of reading fast STIR images and those of reading T1-weighted fat-suppressed contrastenhanced images. In the analysis of the seven cystic masses, the sensitivity, specificity, and accuracy of observer 1 reading fast STIR images were 60.0% (3/5), 0.0% (0/2), and 42.9% (3/7) and for reading T1- images were 100% (5/5), 0% (0/2), and 71.4% (5/7). The sensitivity, specificity, and accuracy of observer 2 reading fast STIR images were 80.0% (4/5), 0.0% (0/2), A Fig year-old woman with malignant peripheral nerve sheath tumor of right buttock. A, Coronal fast STIR MR image (TR/TE, 4,000/120; inversion time, 150 milliseconds; echo-train length, 10) shows well-defined mass of heterogeneously high signal intensity. Markedly hyperintense areas correspond to cystic necrosis (short arrow). Tumor invasion of surrounding area of high signal intensity (long arrow) was pathologically proven. B, Coronal T1- MR image (400/10) shows strongly heterogeneous enhancement. Multiple unenhanced compartments correspond to cystic necrosis (arrows). A Fig year-old man with schwannoma of left thigh. A, Coronal fast STIR MR image (TR/TE, 4,000/120; inversion time, 150 milliseconds; echo-train length, 10) shows well-defined mass of marked heterogeneously high signal intensity with intermediate hyperintense area. Marked hyperintense area (long arrow) corresponds to Antoni type B pattern. Partial intermediate hyperintense area (short arrow) corresponds to Antoni type A pattern. B, Coronal T1- MR image (400/10) shows weakly heterogeneous enhancement with ill-defined margin. Arrow indicates strongly enhanced area corresponding to Antoni type A pattern. and 57.1% (4/7) and reading T1-weighted fatsuppressed contrast-enhanced images were 100% (5/5), 0% (0/2), and 71.4% (5/7). The weighted kappa values for the ratings of the subjective image contrast in evaluation of soft-tissue tumors in muscle were (95% CI: ) for fast STIR B B 1610 AJR:193, December 2009

5 MRI of Soft-Tissue Tumors A B Fig year-old woman with solitary fibrous tumor of left inguinal region. A, Coronal fast STIR MR image (TR/TE, 4,000/120; inversion time, 150 milliseconds; echo-train length, 10) shows heterogeneous mass of low signal intensity with ill-defined margin (arrow). B, Coronal T1- MR image (400/10) shows strongly heterogeneous enhancement with well-defined margin (arrow). images and (95% CI: ) for T1- images, in bone marrow were (95% CI: ) for fast STIR images and (95% CI: ) for T1-weighted fatsuppressed contrast-enhanced images, and in fat were (95% CI: ) for fast STIR images and (95% CI: ) for T1- images. These findings suggested good interobserver agreement on the ratings of subjective image contrast. The weighted kappa values for differentiation of benign and malignant tumors were (95% CI: ) for the fast STIR images and (95% CI: ) for the T1- images. These findings suggested excellent interobserver agreement for the differentiation between benign and malignant tumors on both types of images. The weighted kappa value for differentiation of benign and TABLE 3: Observer Ratings of STIR and T1-Weighted Fat-Suppressed Contrast-Enhanced Images of Soft-Tissue Tumors Soft-Tissue Tumor Muscle Fast STIR Observer 1 Observer 2 T1-Weighted Fat-Suppressed Contrast-Enhanced 95% CI p Fast STIR malignant tumors by observer 1 was (95% CI: ) and for that by observer 2 was (95% CI: ). These findings suggested excellent intraobserver agreement for both observers for differentiation of benign and malignant tumors. The mean A z for observer 1 was ± (95% CI: ) for the fast STIR images and ± (95% CI: ) for the T1-weighted fat-suppressed contrast-enhanced images (Fig. 4A). The A z values for observer 2 were ± (95% CI: ) for the fast STIR images and ± (95% CI: ) for the T1-weighted fat-suppressed contrast-enhanced images (Fig. 4B). Neither observer had significant differences between the A z values for fast STIR and those for T1- images (observer 1, p = 0.453; observer 2, p = 0.239). Discussion In the STIR technique, an initial 180 inverting radiofrequency pulse is followed by a standard spin-echo sequence. The time allowed to elapse between the inversion pulse and the 90 pulse is chosen to approximate the null point of fat, resulting in suppression of the signal intensity of fat. Most often, the inversion time is determined with field-strength-dependent reference values. A further important point is that high-fieldstrength systems are required for use of the T1-Weighted Fat-Suppressed Contrast-Enhanced 95% CI p Benign tumors 3.26 ± ± to 1.14 < ± ± to 0.95 < 0.05 Malignant tumors 3.52 ± ± to 0.74 < ± ± to 0.77 < 0.05 All tumors 3.39 ± ± to 0.84 < ± ± to 074 < 0.01 Bone marrow Fat Benign tumors 3.32 ± ± to 0.97 < ± ± to Malignant tumors 3.52 ± ± to ± ± to All tumors 3.42 ± ± to 0.68 < ± ± to Benign tumors 3.36 ± ± to 0.90 < ± ± to Malignant tumors 3.45 ± ± to ± ± to All tumors 3.36 ± ± to 0.54 < ± ± to Note Values are mean ± SD. AJR:193, December

6 Tokuda et al. TABLE 4: Differentiation of Benign From Malignant Tumors on Fast STIR and T1-Weighted Fat-Suppressed Contrast-Enhanced MR Images Fast STIR Sequence Observer 1 Observer 2 Mean Sensitivity 93.9 (31/33) 81.8 (27/33) 87.9 Specificity 73.5 (25/34) 52.9 (18/34) 63.2 Accuracy 80.6 (56/67) 67.2 (45/67) 73.9 T1- Sensitivity 97.0 (32/33) 84.8 (28/33) 90.9 Specificity 58.8 (20/34) 47.1 (16/34) 53.0 Accuracy 77.6 (52/67) 65.7 (44/67) 71.7 Note Data are percentages. Numbers in parentheses are raw data. True-Positive Fraction STIR T1WI-FSCE False-Positive Fraction 1.0 A True-Positive Fraction STIR T1WI-FSCE False-Positive Fraction Fig. 4 Receiver operating characteristic (ROC) curves of observer confidence in differentiating benign from malignant tumors on STIR and T1- (T1WI-FSCE) images. A, Graph shows ROC curve for observer 1. B, Graph shows ROC curve for observer frequency-selective fat-suppressed sequence, whereas the fast STIR sequence can be performed on low- or high-field-strength MRI units [21]. Most soft-tissue tumors are visualized as areas of markedly increased signal intensity on both fast STIR and T1-weighted fat-suppressed contrast-enhanced images. Consequently, the most important point of comparison between the fast STIR and T1-weighted fat-suppressed contrast-enhanced sequences for evaluation of soft-tissue tumors is the uniformity of fat suppression. In this study, the mean CNRs of malignant and all tumors in comparison with normal muscle, bone marrow, and fat were significantly higher on fast STIR than on T1-weighted fat-suppressed contrast-enhanced images. There were no significant differences, however, between the mean SNRs of benign, malignant, and all tumors on fast STIR images and those on T1- images. These findings may have occurred because the fast STIR sequence was superior to the T1- sequence in terms of uniformity of fat suppression. Nakatsu et al. [16] reported that the fast STIR sequence afforded fat suppression in the cervical and thoracic regions that was closer to homogeneous than that obtained with the fat-suppressed (chemsat) fast spinecho sequence. In the cervical and thoracic regions, magnetic field inhomogeneity due to the susceptibility effects of complex anatomic relations and air-containing structures often disturbs uniform fat suppression. In addition, magnetic field uniformity can be a problem in MRI of extremities because of the relatively off-center location of the imaging object and the proximity of air tissue boundaries. The sensitivity, specificity, and accuracy for differentiation of benign from malignant bone tumors have been calculated for fast STIR and T1-weighted fat-suppressed contrast-enhanced images [17]. In that study, no significant differences between fast STIR B and T1- images were observed for either observer in regard to sensitivity, specificity, and accuracy for differentiation of benign from malignant bone tumors. In our study, the sensitivity, specificity, and accuracy for differentiation of benign from malignant softtissue tumors on fast STIR and T1-weighted fat-suppressed contrast-enhanced images were comparable for the two observers. In addition, the two observers had no significant differences in A z values for fast STIR and T1- images. These findings suggest that the ability to differentiate benign from malignant soft-tissue tumors on fast STIR images may be almost the same as the ability to differentiate them on T1-weighted fat-suppressed contrast-enhanced images but that for depiction of soft-tissue tumors, as opposed to differentiating benign from malignant, fast STIR imaging is superior. In this study, both observers rated the mean subjective image contrast of soft-tissue tumors in comparison with muscle on fast STIR images significantly higher than on T1- images. Observer 2 rated the mean subjective image contrast of soft-tissue tumors in comparison with bone marrow on fast STIR images comparable with that on T1-weighted fat-suppressed contrast-enhanced images. Furthermore, interobserver and intraobserver agreement in ratings of subjective image contrast was good, and this comparable agreement also suggested that the fast STIR sequence was equivalent to the T1-weighted fat-suppressed contrast-enhanced sequence for evaluation of soft-tissue tumors. The STIR sequence has several disadvantages. First, the SNR tends to be lower with the STIR than with a spin-echo sequence [14]. In our study, however, there were no significant differences between the mean SNR of benign, malignant, and all tumors in the normal muscle, bone marrow, and fat on the fast STIR images and that on T1-weighted fat-suppressed contrast-enhanced images. Second, the fast STIR sequence includes relatively long imaging times for acquisition of a limited number of imaging slices, resulting in vulnerability to motion artifacts and poor SNR [1]. This problem is less severe in imaging of soft-tissue tumors of the extremities than imaging of tumors of the chest and abdomen because the extremities are less affected by motion-induced noise. Motion artifact induced long acquisition times 1612 AJR:193, December 2009

7 MRI of Soft-Tissue Tumors are important problems in the evaluation of soft-tissue tumors of the chest and abdomen. Finally, nonlipid tissue can be suppressed if it has a short T1 similar to that of fat because fat suppression on fast STIR images is based strictly on relaxation values [1]. Although it has several disadvantages, the fast STIR sequence clearly is useful for evaluation of soft-tissue tumors. A further important point is that high-field-strength systems are needed for the frequency-selective fatsuppressed sequence, whereas the fast STIR sequence can be performed on low- or highfield-strength units [21]. According to May et al. [22], the routine use of gadolinium in the initial MRI evaluation of a possible primary musculoskeletal neoplasm is not justified. In several cases, however, contrast enhancement is necessary for evaluation of soft-tissue tumors. First, it is difficult to evaluate cystic and myxoid soft-tissue masses without contrast enhancement. The differential diagnosis of a cystic mass includes ganglion, infection (abscess or pyomyositis), myxoma, and myxoid sarcoma. Ganglion and abscess have regular rim enhancement, but myxoid soft-tissue sarcoma has thick nodular or irregular peripheral enhancement with or without central nodular enhancement. Therefore, the contrast enhancement pattern is important in the evaluation of cystic masses. In the analysis of seven cystic masses in our study, the sensitivity and accuracy of both observers reading fast STIR images were lower than in reading of T1- images. Another reason that contrast enhancement may be helpful in the evaluation of a soft-tissue mass is to help the radiologist plan the biopsy route, especially in evaluation of a cystic or necrotic mass. The area of enhancement representing more viable tumor tissue typically has higher diagnostic yield than the unenhanced necrotic areas of the mass [23]. At some institutions, it is possible for the radiologist to evaluate unenhanced images of soft-tissue tumors and decide whether contrast-enhanced sequences should be performed. Some authors advocate use of dynamic contrast enhancement in the initial evaluation and follow-up of soft-tissue tumors, and some advocate use of MRI spectroscopy in the evaluation of these tumors [24 26]. For practical reasons at busy hospitals and imaging centers, it is not possible to routinely check images before IV contrast administration. Therefore, at those institutions, both STIR and T1-weighted fat-suppressed contrast-enhanced sequences should be included in standard protocols for evaluation of soft-tissue tumors. This study had limitations. First, a consequence of the retrospective design was variability in the MRI parameters. Second, one musculoskeletal radiologist placed a region of interest within the soft-tissue tumors, the nearest normal soft tissue (muscle, bone marrow, and fat), and air near the lesion. To avoid sampling error, all measurements were performed twice, and the mean values were adopted. Third, the patients were not consecutively registered, and the sample size was modest. All of the patients with soft-tissue tumors who underwent MRI fell into the following three groups: both fast STIR and the T1- images available; both T2-weighted fat-suppressed and T1-weighted fat-suppressed contrast-enhanced images available; and only fast STIR images available. Although more than 80% of the patients with soft-tissue tumors who underwent MRI had both fast STIR and T1-weighted fat-suppressed contrast-enhanced images, selection bias is another consideration. This study showed that fast STIR and T1- sequences had comparable results for differentiation of benign from malignant tumors. 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