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Pictorial Essay |
1 Department of Radiology, Wake Forest Medical Center, Winston-Salem, NC
27157.
2 Present address: Department of Radiology, Mater Misericordiae & Cappagh
National Orthopedic Hospitals, Cappagh, Finglas, Dublin, Ireland.
Received February 8, 2002; accepted after revision June 21, 2002.
Address correspondence to S. Eustace.
Basic Technique
Without a Moving Tabletop
Total body coverage is yielded by four contiguous coronal acquisitions,
each performed using turbo STIR tissue excitation and the following
parameters: TR range/TE effective, 2000-4000/40; inversion time at 1.5 T, 160
msec; echo-train length, 6; and field of view, 45 cm. The TR that was selected
depended on the amount of coverage required. These parameters allow the
acquisition of 24 slices that allow coverage from anterior to posterior with
contiguous 8-mm-thick slices in most adults in 4-min increments for each
coronal station. Using respiratory triggering during the acquisition of images
of the thorax and abdomen increases the acquisition time at these sites.
Coronal scans of the head, neck, and thorax are acquired with the patient in the head-first position, whereas coronal scans of the abdomen, pelvis, and lower extremities are acquired with the patient in the feet-first position. The need to reposition the patient from the head-first position to the feet-first position during whole-body scanning contributes significantly to the total time required to scan the patient and affects both patient and technologist acceptance of the technique. In addition, scans are acquired at four separate stations and require manual realignment, cropping, and pasting to create the visually appealing whole-body scan.
With a Moving Tabletop and Tabletop Extender
The development of the moving tabletop allows sequential movement of the
patient through the bore of the magnet during imaging. This feature overcomes
the requirement to reposition the patient during scanning. The patient enters
the bore of the magnet head-first and is imaged at seven separate contiguous
stations by integrated table movement; the moving tabletop allows imaging of
the head and neck, the thorax, the abdomen, the pelvis, and then the
extremities to be performed. Because each acquisition uses the body coil, the
slice selection gradients match exactly at each station, thus facilitating
immediate image realignment after acquisition to create the whole-body
scan.
When a tabletop extender (prototype; Philips Medical Systems, Best, The Netherlands) is used, the field of view is extended to 200 cm. This larger field of view enables most adult patients to be scanned from head to toe.
Images are acquired by a receive—transmit body coil with a horizontal field of view of 53 cm and a vertical field of view of 26 cm with a 50% rectangular field of view. When a 256 x 256 matrix is used, the derived image has a resolution of 2-2.7 mm per pixel.
With a standard moving tabletop, the maximal field of view as a result of the longitudinal table movement is 120 cm. If a tabletop extender is used, the field of view is extended to 200 cm, making it possible to scan most adult patients from head to toe.
Tissue excitation with turbo STIR MR imaging uses a TR/TE of 3200/60 (total TR = 20,108 msec), an echo-train length of 128, an inversion time of 165 msec, 1 excitation, and half-Fourier mapping of k-space; the total scanning time is 4 min 20 sec. An average of 30 total-body coronal images of each patient are acquired from anterior to posterior with a slice thickness of 8 mm.
The time required for processing after the examination is also significantly reduced using this technique because image realignment (numerically guided) occurs immediately after acquisition at the console, thus creating a true whole-body scan within minutes of imaging (Fig. 1). Currently, image realignment is facilitated by the movement of the tabletop because the images at each of the seven stations are acquired in exactly matching slices; therefore, one can predict numerically which images at each station should align to create the whole-body image. A software function facilitating this technique is in production.
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Current Applications
Evaluation of Skeletal Metastatic Disease
Scintigraphy has long been the standard for the detection of bony
metastases because scintigraphy is more sensitive than radiography or CT.
Technetium 99m—labeled diphosphonate analogues localize to
osteoblast-produced calcium hydroxyapatite and are indirect markers of
disease. In the absence of a reactive osteoblastic response, lesions may be
scintigraphically occult or present as focal cold spots; these areas are often
difficult to recognize.
Although blastic metastases are readily identifiable on scintigraphy, MR imaging offers the ability to discriminate between benign reactive sclerosis and true metastases, thus enabling improved determination of the total tumor burden. Several groups of researchers compared the capabilities of scintigraphy and MR imaging to detect metastatic bony disease [1, 2]. Findings from both of these studies support the use of regional MR imaging for the detection of skeletal metastases. More recent studies have compared whole-body MR imaging with scintigraphy; again, results of these studies indicated that MR imaging is at least as effective as scintigraphy.
Whole-body MR imaging tends to yield better visualization of the pelvis because the radiotracer in the bladder in scintigraphy often obscures osseous structures. In addition, MR imaging is superior to scintigraphy in depicting lesions that do not induce an osteoblastic response. Small lesions can be readily identified using the STIR sequence because of the high contrast resolution from tumor-induced bone marrow edema (Fig. 2A,2B).
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Assessment of Total Tumor Burden in Breast Carcinoma and Other
Malignancies
Developments in chemotherapeutic protocols and stem cell transplantation,
combined with aggressive surgical resection of solitary metastases, have
heightened the need for a convenient and accurate method by which to stage
breast cancer. When examined for metastatic disease, patients routinely
undergo chest radiography, bone scanning, and sonography or CT of the abdomen
to assess the total tumor burden and to plan therapy. In addition, depending
on the patient's symptoms, CT or MR imaging of the brain may be performed.
These techniques are fraught with false-positive and false-negative results,
which can lead to additional unnecessary investigations, including biopsy, and
can cause undue stress on and anxiety in patients.
The development of turbo STIR whole-body MR imaging provides a safe and potentially effective screening tool by which to search for metastatic disease, obviating multiple examinations [3]. Regional MR imaging has been shown to be more sensitive than CT and other imaging modalities in the detection of liver, bone, and central nervous system lesions. Whether turbo STIR whole-body MR imaging can reliably reveal small lesions in the lungs (Fig. 3A,3B,3C), liver, or brain needs to be assessed by additional controlled prospective trials (Fig. 4).
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Detection of an Occult Primary Tumor
The prevalence of unknown primary tumors in patients with metastatic
disease is estimated to be between 3% and 15%
[4]. A primary tumor is
unlikely to be found despite extensive investigation that may include CT,
regional MR imaging, endoscopy, and serologic tests. Even autopsy fails to
yield a primary tumor in at least 16% of the cadavers
[4]. Despite the limited
experience with whole-body turbo STIR MR imaging, this technique may represent
a reasonable alternative to the currently available techniques when examining
patients for unknown primary tumors (Fig.
5). Whole-body turbo STIR MR imaging may facilitate primary tumor
detection and potentially decrease costs incurred by conventional strategies
[5].
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Detection of Multiple Myeloma
Skeletal survey.—Multiple myeloma is the most common primary
bone neoplasm and is characterized by marrow infiltration with neoplastic
cells. Conventional staging of multiple myeloma uses serologic markers in
conjunction with radiography
[6]. Radiographs require the
loss of up to 50% of bone mineral density before lesions can be detected, and
this limitation contributes to the difficulty in diagnosis and treatment of
early multiple myeloma.
Turbo STIR MR imaging.—MR imaging allows direct visualization of multiple myeloma; therefore, MR imaging provides improved sensitivity and enables earlier detection of disease than does skeletal survey. However, standard MR imaging protocols for bone marrow often exclude the sternum, skull, and ribs, sites where myelomatous deposits are frequently found because these areas contain a substantial amount of red marrow. Complete evaluation with whole-body MR imaging may be a convenient and accurate method by which to examine a patient (Fig. 6). In this setting, the sensitivity of whole-body MR imaging may be improved by incorporating a whole-body non—fat-suppressed T1-weighted sequence, in addition to the standard STIR sequence, into the examination.
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Visualization of Polymyositis
Polymyositis is an inflammatory myopathy of striated muscle; when
accompanied by a characteristic skin rash, this entity is called
dermatomyositis and has an increased chance of being associated with
carcinoma. The causes of these disorders are unknown, although viral and
immunologic factors have been proposed. Diagnosis can be difficult, and
because treatment involves high-dose steroids or immunosuppressants, the
treatment cannot be undertaken lightly. Early diagnosis is important because
the mortality rate is high in patients with either disorder—particularly
in children—even among those who undergo treatment, which heightens the
need for early diagnosis. Whole-body turbo STIR MR imaging allows rapid
visualization of edema within involved muscle groups; moreover, in patients
with asymmetric disease, whole-body turbo STIR MR imaging allows
identification of the appropriate involved muscle groups for diagnostic biopsy
[7]
(Fig. 7).
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Staging Malignancy in Pregnant Patients
Neoplastic disorders are uncommon in pregnant women, but occasionally a
pregnant woman presents with a rapidly progressive tumor that necessitates
staging. Both CT and scintigraphy are relatively contraindicated, especially
for women who are in the first trimester. In this setting, the use of a
nonionizing modality, such as whole-body MR imaging, may represent a useful
alternative to conventional staging techniques.
An Adjunct for or Alternative to Autopsy
The dramatic decrease in the number of conventional autopsies performed has
been attributed to an increase in inoculation risks incurred by the spread of
HIV and hepatitis infections and to changes in societal acceptance of the
procedure. In a limited study, Patriquin et al.
[8] outlined potential benefits
of performing postmortem whole-body MR imaging, including facilitating
percutaneously guided biopsies of specific abnormal tissues, rather than
performing formal dissection (Fig.
8A,8B).
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Evolving Applications
At the time of this report, some researchers suggest that whole-body MR imaging may have a role in the assessment of child abuse. In this setting, a single nonionizing tool may be used to assess for brain, visceral, and skeletal contusions. In infants, a single image obtained using a large field of view will allow coverage of the entire body, thus dramatically reducing the overall scanning time and minimizing the effect of motion on image quality.
The development of monoclonal antibodies and tagged contrast agents and the ability to fuse images derived from differing imaging modalities—including positron emission tomography, which has been termed fusion imaging—provide new opportunities for research and development [9]. Although positron emission tomography may prove to be more sensitive for lesion detection than whole-body MR imaging, this issue can be addressed only by additional experience and formal comparisons of both modalities.
Conclusion
The evolution of MR technology has provided a reasonable and practical means of scanning patients with suspected skeletal metastases, assessing total tumor burden, and detecting an occult primary tumor in patients with skeletal metastases. Additional applications include staging multiple myeloma, assessing inflammatory myopathies, and staging tumors in pregnant patients and use as an an adjunct for or alternative to autopsy.
References
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