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Technical Innovation |
1 Department of Radiology, Cappagh National Orthopaedic Hospital, Finglas,
Dublin 11, Ireland.
2 Department of Radiology, Mater Misericordiae Hospital, Eccles St., Dublin 7,
Ireland.
Received October 18, 2001;
accepted after revision March 27, 2002.
Address correspondence to M. J. O'Connell.
Introduction
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We describe the use of a moving tabletop to provide convenient reproducible whole-body imaging without the need to reposition the patient during the study. Automated patient repositioning by the moving tabletop and the acquisition of signal using the body coil at each site ensures that acquired slices in each stack are in exactly the same plane. These features, derived from moving-tabletop MR imaging, allow convenience, rapid image acquisition, and image realignment at the operating console or workstation and enhance the quality and standardization of the derived scan.
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To achieve whole-body coverage, operators acquired coronal slabs at five stations with a field of view of 53 cm. Two scan acquisitions were obtained at each station. The number of coronal slices obtained varied at each station according to patient body habitus. At each site, the slice thickness was 8 mm with interslice spacing of 1 mm. In a typical patient, 15-20 slices allowed full coverage of the head and neck from front to back; and 25 slices allowed full coverage of the thorax, upper abdomen, and thighs. Fifteen slices were acquired to image the knees and the proximal metaphysis of the tibia. The patient's arm can be placed above the head, across the chest, or by the side, with the pronate forearms placed under the posterior aspect of the thigh. Table movement after the acquisition of each coronal slab was automated, which ensured that neither excessive overlap nor impaired imaging of a particular region occurred. Table movement was approximately 11 cm/sec. As slice-select gradients were shifted to matching positions during each coronal slab acquisition, images were acquired in matching planes at each consecutive imaging station, which facilitated image realignment to create a whole-body image at the operator console within minutes of the study. Respiratory triggering was used to limit image degradation due to chest movement, with the excitation pulse in the expiratory phase. Patients were instructed to breathe in a regular pattern, when possible, with inspiratory and expiratory phases of 3 sec each. Although variable and dictated by the respiratory rate and the number of coronal slices, this technique allowed reproducible high-quality whole-body imaging in an average total image acquisition time of less than 12 min for five stations. The examination time, including patient positioning on and off the moving tabletop, planning scans, and image reconstruction, was less than 20 min. Derived images were viewed at an Easy Vision version 4.3 workstation (Philips Medical Systems).
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In a 39-year-old man with shoulder pain, whole-body turbo STIR images showed a destructive mass in the left shoulder that was subsequently confirmed by radiographs, scintigraphy, and focussed MR imaging of the shoulder (Fig. 2). Biopsy showed adenocarcinoma of an unknown primary source. Incidental note was made of a benign enchondroma in the left femur, confirmed by an absence of symptoms, radiographic appearances, and a lack of interval change at a 2-year follow up (biopsy was not obtained). Concordance was shown with traditional staging bone scintigraphy and CT of the thorax. Total acquisition time (excluding planning scans) was 10 min 28 sec. The study was completed in less than 20 min, including patient positioning on and off the imaging table and subsequent image alignment.
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In a 16-year-old boy with thigh pain, whole-body turbo STIR images showed a Ewing's tumor of the mid left thigh (Fig. 3) arising in soft tissues adjacent to the mid diaphysis of the left femur, which was proven at biopsy. No marrow or visceral abnormality was identified. Concordance was noted with conventional staging, which included normal bone scintigraphy and CT of the thorax. Image acquisition time was 9 min 42 sec.
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TopIntroductionSubjects and MethodsResultsDiscussionReferences |
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At first, MR imaging was used to visualize stationary organs such as the brain and spine. Subsequent developments led to imaging of organs and structures affected by respiratory and cardiac motion. These developments resulted in scanning using a breath-hold, gating technology, and motion artifact reduction. The development of fast MR imaging with the rapid acquisition relaxation enhancement sequences led to the possibility of rapid whole-body scanning [6].
Using fast gradient-echo and echoplanar imaging, previous authors have described whole-body imaging in 30 [7] and 18 sec [8] in the axial plane. In contrast to the turbo STIR technique, images acquired by echo-planar imaging and fast gradient-echo imaging are degraded by susceptibility artifacts that limit visualization of the skull base, lungs, and interfaces between soft tissues and air. Reflecting such a limitation, neither technique has been validated in a clinical setting by comparison with other imaging tools and a gold standard. The turbo STIR technique has been compared with conventional whole-body scintigraphy to assess skeletal metastatic disease [1], and it has been compared with cross-sectional imaging of regional body parts to evaluate total burden in breast cancer [2].
Recognizing its value, the American College of Radiology has included the technique in a publication on appropriateness criteria [9], in which whole-body turbo STIR imaging is considered to be a valid alternative to scintigraphy for the assessment of skeletal metastatic disease. Despite this endorsement, the practical difficulty of obtaining an image without a moving tabletop has limited the widespread clinical use and implementation of the technique.
We describe an improved method of performing whole-body MR imaging using the turbo STIR sequence, the body coil, and a moving tabletop with an average image acquisition time of less than 12 min. Using a body coil to acquire a signal avoids delays associated with the use and repositioning of surface coils. Although surface coils would improve derived signal and might improve spatial resolution, the diagnosis using turbo STIR is primarily determined on the basis of contrast resolution. In effect, the diagnosis is established on the basis of an ability to identify hyperintense proton-rich structures adjacent to hypointense suppressed fat. It may not be possible in all cases to characterize lesions with the use of a single sequence (STIR), and therefore correlation with additional T1- and T2-weighted sequences is used when required. Correlation with radiographs is recommended when abnormalities are seen.
Despite the use of the body coil, a 256 matrix, and an 8-mm slice thickness, the spatial resolution of the described technique is 2.7 mm per pixel. The use of a body coil is convenient for both patient and operator and facilitates rapid throughput. Automated image acquisition using a programmed moving tabletop prevents excessive overlap or incomplete coverage between body regions or stations scanned. It also improves standardization of the whole-body MR imaging technique, creating a more reproducible test.
The automated moving tabletop ensures that slices in each coronal slab are exactly aligned and in plane because the slice select gradients are returned to exactly the same plane during each coronal slab acquisition. Matching of slices in both the anteroposterior and coronal planes improves the quality of the derived image because data can be immediately reconstructed and aligned to create a whole-body scan at the standard operator console or at the workstation in a format readily appreciated by referring clinicians without the need to manually crop data. In evaluating images, operators have the option to analyze whole-body images or individual coronal slices reviewed sequentially at the workstation. In the latter format, images can be reviewed in the cine mode, which is preferred by some operators.
Limitations of the technique include the requirement for patient cooperation to ensure steady breathing during acquisition of images of the thorax. Erratic breathing—for example, in a sleeping patient—would lead to image motion and impair visualization of lung parenchyma. In the described technique, respiratory triggering was used to minimize breathing artifact at the penalty of slightly prolonged acquisition times. Pulsatility remains a problem in pericardiac soft tissues.
A field of view of 53 cm allows visualization of the upper extremities in most adult patients, although visualization in larger patients may be difficult unless the arms are placed over the upper torso and additional coronal acquisitions are obtained. In addition, the movement of the existing tabletop limits imaging to a range of 120 cm, which allows imaging from the head to the level of the knee in most adults. A prototype tabletop extender, now in production, is likely to overcome this problem and allow the acquisition of images at an additional two stations, permitting imaging to the feet with an imaging range of more than 200 cm.
In summary, our study outlines a reproducible technique that facilitates the acquisition of high-quality fat-suppressed coronal whole-body images in 12 min. Images acquired in this way may be immediately aligned at either the operator console or at an attached workstation to produce dramatic whole-body images. Although whole-body MR imaging has been validated in a limited form by previous authors [1, 3, 9], further studies are required to determine the sensitivity, specificity, predictive value, and accuracy of the modified turbo STIR technique using a moving tabletop. Caution should therefore be used when applying the technique in routine clinical practice.
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TopIntroductionSubjects and MethodsResultsDiscussionReferences |
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