Magnetic resonance imaging of soft-tissue tumors: Comparison with computed tomography

Magneric Resonance Imaging. Vol. 3, pp. 345-352. Printed in the USA. All rights reserved.

1985

073a-725X/85 $3.00 + .OO Copyright a 1985 Pergamon Press Ltd.

l Original Contribution Magnetic Resonance Imaging of Soft-Tissue Tumors: Comparison With Computed Tomography* RICHARD

G.

WEEKES, MCLEOD,

M.D.,

M.D.,

THOMAS

H.

BERQUIST,

AND WILLIAM

D.

M.D.,7

ZIMMER,

RICHARD

A.

M.D.

From the Department of Diagnostic Radiology, Mayo Clinic and Mayo Foundation, 200 First Street SW, Rochester, MN 55905. Twenty-seven patients with soft-tissue tumors were examined with a Picker 0.15~tesla resistive magnet and by computed tomography (CT). In all but one patient, MRI was better than or equal to CT in defining the anatomic extent of the tumor. We could determine whether major vascular structures were engulfed by the tumor in 80% of the MRI examinations but only in 62 % of the CT scans. MRI and CT were equally effective in determining the presence or absence of bony invasion. The MRI images of all the tumors showed increased signal intensity relative to normal muscle when spin-echo (SE) pulse sequences with long repeat times were used (SE: echo time [TE], 60 ms; repetition time [TR], 2,000 ms). When Tl weighted pulse sequences were used (SE: TE, 30 ms; TR, 500 ms or inversion recovery: inversion time, 500 ms; TE, 40 ms; TR, 2,000 ms) the malignant tumors showed decreased signal intensity compared to normal muscle. Only lipomas showed high signal intensity on both Tl and T2 weighted pulse sequences. Keywords: MRI; soft-tissue tumors INTRODUCTION

cytomas in 7, liposarcomas in 3, myxoma in 1, lipoma in 1, ganglion cyst in 1, fibrosarcoma in 1, soft-tissue Ewing sarcoma in 1, rhabdomyosarcoma in 1, and neurofibroma in 1. Among patients evaluated for probable tumor recurrence, there were two desmoids, two lipomas, one angiolipoma, one hemangioma, one malignant fibrous histiocytoma, one leiomyosarcoma, one neurofibroma, and one case of postoperative scarring changes. All examinations were performed with a 0.15-tesla resistive magnet (Picker International, Highland Heights, OH). In all cases images were made in the axial plane and in most cases at least one image was in an off-axial plane (sagittal or coronal). A spin-echo (SE) (echo time [TE], 60 ms; repetition time [TR], 2,000 ms) pulse sequence was used in every case (T2 weighted). In most cases images were also made with Tl weighted pulse sequences: either a SE pulse sequence with a short TR (TE, 40 ms; TR, 500 ms) or an inversion-recovery (IR) pulse sequence (inversion time [TI], 500 ms; TE, 40; TR, 2,000 ms). All slices were lo-mm thick and contiguous. The CT examinations were performed on Picker 600 or 1200 or EM1

Magnetic resonance imaging (MRI) is well suited for examining the soft tissues of the extremities, where breathing and peristaltic artifacts are avoided.2,3*8 Computed tomography (CT) is currently the imaging modality of choice for evaluating soft-tissue tumors, but it has limited effectiveness in defining tumor extent in the upper extremity and lower 1eg.‘,417*9Because of its increased contrast capabilities and ability to obtain direct coronal and sagittal images, MRI has the potential for better detection and delineation of soft-tissue tumors. We investigated the utility of MRI in the evaluation of soft-tissue tumors by comparing MRI with CT.

MATERIALS

AND

METHODS

Twenty-seven patients (13 males and 14 females; 1 l-84 yrs of age) with soft-tissue tumors were examined by both MRI and CT. The group included 17 patients in whom no prior operation had been performed and 10 with tumor recurrence. Among the untreated patients there were malignant fibrous histio-

TAuthor to whom reprint requests should be addressed.

RECHVED 8/12/U; ACCEPTED 912185. *Presented in part as an Exhibit at the Radiological Society of North America, Chicago, Illinois, November 17 to 22. 1985. 345

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7070 units. CT slices were IO-mm thick with spacing of 10 mm. Pathologic confirmation of the diagnosis was made in each case. All imaging studies were reviewed retrospectively and independently by three radiologists (R.G.W., T.H.B., R.A.M.), without knowledge of the patient’s diagnosis or history. In addition, the MRI image of each patient was evaluated without reference to the corresponding CT of the same lesion and vice versa. For each MRI and CT scan we evaluated whether the anatomic extent of the tumor was well defined, whether major neural or vascular structures were involved, and whether bone was involved. The MRI signal intensity of each tumor was compared with normal muscle and subcutaneous fat for each pulse sequence. RESULTS

Lesion Detection and Anatomic Extent In every case, both CT and MRI demonstrated an abnormality; however, in all but one, MRI defined the anatomic extent of the lesion better than or equal to CT. MRI was clearly superior to CT in four of the cases. These included two recurrent desmoid tumors of the forearm (Fig. 1) and two cases in which extensive streak artifacts from metallic devices caused significant degradation of the CT image-one was a recurrent malignant fibrous histiocytoma of the thigh and the other was a recurrent neurofibroma of the pelvis (Fig. 2). In most cases, similar anatomic information was available on the MRI scan as was present on the CT scan; however, in seven of these cases the MRI scan was easier to interpret because of the sharp difference in contrast between tumor and normal muscle and the ability to obtain off-axial images (Fig. 3). Four of these tumors involved the calf, one the forearm, one the thigh, and one the back. A rhabdomyosar-

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coma of the proximal forearm was poorly defined by both imaging modalities. A recurrent leiomyosarcoma of the pelvis and buttock was the only tumor for which CT provided better anatomic information than MRI. In two cases when scanning for probable tumor recurrence, a lesion that displayed high signal intensity was detected on the MRI scan (SE: TE, 60 ms; TR, 2,000 ms). The CT scan showed the lesion to be the density of fat, consistent with a lipoma (Fig. 4). A cavernous hemangioma of the chest wall was the only tumor that contained calcification. The small punctate calcifications were identified on the CT scan, but not on the MRI scan; however, MRI showed the extent better. Vascular and Bony Involvement In 80% of the MRI scans it was possible to determine whether or not major vascular structures were involved by the tumor; however, we were able to make this determination in only 62% of the CT scans (Fig. 5). The relationship of major vascular structures to tumor mass is clear on CT only when vascular structures contain abundant calcification or when they are obviously separated from the tumor mass by normal tissue. However, the CT scans were performed without administering a bolus of contrast material intravenously and doing dynamic scanning. Because of small vessel size, both imaging methods consistently failed to define the relationship of major vascular structures to tumor mass in the distal extremities. MRI and CT were equally effective in demonstrating the presence or absence of bony involvement. This was present in only one case, a malignant fibrous histiocytoma of the calf. MRI Characteristics All tumors showed increased signal intensity compared with normal muscle when imaged by a SE pulse sequence with TE, 60 ms and TR, 2,000 ms. When a

Fig. 1, Recurrent desmoid tumor of left forearm. (A) SE (TE, 60 ms; TR, 2,000 ms) axial image clearly shows extent of recurrent desmoid tumor on extensor surface of forearm with extension to interosseous membrane. (B) CT scan demonstrates some soft-tissue prominence, but tumor cannot be distinguished from normal muscle.

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Fig. 2. Recurrent neurofibroma in pelvis. (A) SE (TE, 60 ms; TR, 2,000 ms) shows large recurrent neurofibroma in left side of pelvis extending through sciatic notch. (B) Considerable degradation of CT image is apparent due to streak artifacts from surgical clips placed at time of initial resection.

SE pulse sequence with a much shorter TR (5500 ms) was used (more Tl weighted), the signal intensity of nearly all the tumors decreased significantly. Most showed less intensity than normal muscle and several showed only slightly greater intensity; all were much less intense than subcutaneous fat (Fig. 6). Only the lipomas continued to show high signal intensity when pulse sequences with increased Tl weighting were used (Fig. 7). The images of five tumors were recorded with an IR pulse sequence (Tl weighted): four malignant tumors and one lipoma. All four malignant tumors had decreased signal intensity compared with normal muscle and subcutaneous fat, and the lipoma had increased signal intensity compared with normal muscle and was equal in intensity to that of subcutaneous fat (Fig. 8). Fat maintained a similar high signal intensity with all of the above pulse sequences.

DISCUSSION

The emphasis in the surgical treatment of soft-tissue tumors is limb salvage. In order to accomplish this goal, precise localization of the margins of the tumor, determination of its relationship to neurovascular structures, and indication of the presence or absence of bony involvement are critical. CT is currently the imaging method of choice to evaluate soft-tissue lesions. We recently evaluated a large group of patients with soft-tissue tumors and described CT characteristics of benign and malignant tumors.’ As a result of that study, we found CT did not provide precise anatomic detail of lesions in the upper extremity and lower leg, and we could not differentiate engulfment of neurovascular structures by tumor from neurovascular structures which were simply displaced by the tumor mass. The density of many soft-tissue tumors is not

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Fig. 3. Fibrosarcoma in posterior left thigh. (A) CT slice through lower thigh clearly shows large mass in posterior left thigh. (B-I) Consecutive IO-mm slices centered about knee joint show mass on upper slices laterally, but distal extent is difficult to determine because tumor blends imperceptibly with normal muscle. (J) SE (TE, 60 ms; TR, 2,000 ms) sagittal image slightly lateral to midline clearly defines large mass in posterior thigh and its distal extent just posterior to lateral head of gastrocnemius.

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Fig. 4. Lipoma in posterior left thigh. (A) SE (TE, 60 ms; TR, 2,000 ms) axial image of upper left thigh shows a high-intensity image of lesion lying between adductor and hamstring muscles. (B) Same lesion shown on CT scan is of very low density, pathognomonic of a fatty tumor.

much different from normal muscle on CT. Therefore, identification of the tumor may depend solely on muscle enlargement or the displacement or disruption of normal tissue planes. In the distal extremities the presence of a tumor may be appreciated only when comparison with the normal side is made. Muscle sizes in the upper extremity and lower leg are small and the paucity of fat does not clearly separate different muscle groups. The sharp difference in contrast between tumor and muscle makes MRI a superior imaging modality in both lesion detection and in definition of tumor extent. On T2 weighted images, all tumors we examined showed greatly increased signal intensity compared with normal muscle. As pulse sequences with progressively more Tl weighting were used, the signal intenFig. 5. Malignant fibrous histiocytoma of upper leg. (A) SE (TE, 30 ms; TR, 4,000 ms) axial image of upper legs shows extensive invasion of musculature of upper left leg and engulfment of popliteal artery and vein. (B) The tumor is demonstrated on CT scan of same lesion, but vascular structures cannot be identified.

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Fig. 6. (A) Myoma within adductor magnus of right thigh. Left, SE (TE, 60 ms; TR, 2,000 ms). Note high signal intensity when a pulse sequence with a long relaxation time is used. Right, SE (TE, 40 ms; TR, 500 ms). Sagittal image was obtained by using a much shorter TR; signal intensity has decreased so much that it is now even less intense than normal muscle. (B) Malignant fibrous histiocytoma (MFH) of medial gastrocnemius of left calf. Leff, SE (TE, 60 ms; TR, 2,000 ms). MFH shows high signal intensity when a long TR is used. Right, SE (TE, 40 ms; TR, 500 ms). Tumor is barely discernable on this image produced with a short TR; it is nearly isointense with normal muscle.

sity difference between tumor and normal muscle became progressively less. When a pulse sequence was used which was Tl weighted (IR), the signal intensity decreased so much that the tumor displayed less intensity than normal muscle. Fatty lesions were the exception to this trend. Lipomas and fatty infiltration of muscle showed increased signal intensity on T2 images, and if only a T2 weighted pulse sequence was used, they could not be distinguished from other softtissue tumors. However, with Tl weighted pulse lipomas continued to show high signal sequences, intensity relative to normal muscle. During our early experience with MRI the significance of this was not appreciated. As a result we made several diagnostic errors when the MRI scans were initially interpreted. Specifically, MRI scans were performed in two patients who had prior resections for soft-tissue malignancies and tumor recurrences were suspected. When scanned by MRI, only T2 weighted pulse sequences were used and in each case a small area of increased Fig. 7. Lipomas. (A) CT of lipomas involving left adductor muscle and left groin just lateral to sartorius muscle. (B) SE (TE, 40 ms; TR, 500 ms) axial image; note high signal intensity of lipomas. This high intensity with a relatively short TR was seen only with lipomas.

MRI of soft-tissue tumors l RICHARD G. WEEKFS ETAL.

Fig. 8. Malignant fibrous histiocytoma of semitendinous and semimembranous muscle. posterior thigh. (B) The tumor has high signal intensity when recorded with a SE pulse TR (TE, 60 ms; TR, 2,000 ms). (C) Coronal image just behind femur shows tumor intensity when recorded with a shorter TR (TE, 40 ms; TR, 600 ms). (D) IR (TI, 400 ms; of same lesion shows low signal intensity, even less than muscle.

signal intensity was noted near the site of the prior resection. Therefore, a local recurrence of the malignant soft-tissue tumor was suspected. However, CT scans of the same lesions showed that these “tumor recurrences” were areas of fatty tissue (see Fig. 4). This emphasizes the need for using both T2 and Tl weighted sequences. Liposarcomas, on the other hand, do not image like lipomas but rather like the other soft-tissue malignancies. Only one tumor in this study contained calcification; it was detected on the CT scan but not the MRI scan. This insensitivity of MRI to small amounts of calcium does not affect lesion detection, but in a recent study of soft-tissue tumors we found that the type or amount of calcification present tended to be of some diagnostic importance in predicting tumor type.9 The presence or absence of vascular involvement is shown on MRI without the risks of a bolus of contrast material administered intravenously. Blood vessels may appear black, white, or target-like depending on flow velocity and pulse sequence. MRI also provides direct coronal and sagittal images. This is frequently

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(A) CT of tumor in

sequence with a long has much less signal TR, 2,000 ms) image

useful for providing better definition of the proximal and distal extent of tumors, particularly in the upper extremity and lower leg, where it is often difficult to tell with axial images alone where a tumor begins or ends. MRI also is not hampered by the marked image degradation that metallic prostheses cause on CT and it is the procedure of choice for detecting recurrent tumor in postoperative patients with nonferromagnetic metallic prostheses, fixation devices, and surgical clips. Finally, MRI has the advantage of not using ionizing radiation. In conclusion, we suggest that MRI should be the imaging modality of choice in evaluating tumors of the extremities. An examination should consist of contiguous lo-mm axial cuts and the use of a T2 weighted pulse sequence (e.g., SE: TE, 260 ms; TR, ~2,000 ms). Several coronal or sagittal images should be made by using the same pulse sequence to better define the proximal and distal extent. Finally, at least one Tl weighted pulse sequence (SE: TR, 500 ms or less or IR) should be obtained through the lesion to determine if it is fatty or not.

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Magnetic Resonance Imaging 0 Volume 3, Number 4, 1985 REFERENCES

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