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Editorial |
1 From the Department of Radiology, National Institutes of Health, Bldg 10, Rm 1C660, 10 Center Dr, MSC 1182, Bethesda, MD 20892-1182. Received June 20, 2000; accepted June 26. Address correspondence to the author (e-mail: pchoyke@nih.gov).
Index terms: Editorials • Epididymitis, 847.214 • Magnetic resonance (MR), contrast enhancement, 847.121411, 847.121415, 847.121416 • Magnetic resonance (MR), rapid imaging, 847.121416 • Magnetic resonance (MR), three-dimensional, 847.121411, 847.121415, 847.121416 • Scrotum, diseases, 847.1437, 847.214, 847.311, 969.756 • Scrotum, MR, 847.121411, 847.121415, 847.121416 • Testis, abnormalities, 847.1437, 847.1477, 847.219, 847.311, 847.91 • Testis, neoplasms, 847.30 • Testis, torsion, 847.1437 • Testis, undescended, 847.1477 • Varicocele, 969.756
Scrotal imaging has been one of the undeniable success stories of modern radiology. Acute scrotal disease presents a notoriously difficult physical examination; pain and swelling are nonspecific and limit the ability to examine the patient (1). Today, color Doppler ultrasonography (US) is used routinely to distinguish testicular from nontesticular disease and vascular, infectious, and neoplastic causes of acute scrotal disease. With its worldwide acceptance and vast clinical use, scrotal US is clearly the first choice for diagnostic imaging. Why, then, even consider scrotal magnetic resonance (MR) imaging?
For all its virtues, scrotal US has some notable flaws. In cryptorchidism, US often is unable to depict high undescended inguinal or intraabdominal testes because these small, ectopic testes tend to blend in with background tissue or are obscured by bowel gas. MR imaging is useful in locating ectopic testes, thus aiding in their removal as a preventive measure against the increased risk of malignancy (2). An additional use for scrotal MR imaging is in patients in whom the sonogram is inconclusive for the presence of a tumor. Dilated rete testes, for instance, can appear masslike at US but are clearly cystic at T2-weighted MR imaging and do not enhance after administration of contrast medium. Serra et al (3) have shown that MR imaging following equivocal scrotal US can be diagnostic and cost-effective. However, only 1.4% of the sonograms in the study by Serra et al (3) required the addition of MR imaging.
Another limitation of US is in the diagnosis of testicular torsion, especially in prepubertal patients. Despite excellent results in most adolescents and adults, color Doppler US can be difficult in prepubertal male patients, with reported sensitivities of 84%–91% (2–4). Reasons for failure include diminished flow in both testes, the preservation of flow in partial torsions, or hyperemia associated with spontaneous detorsion (2,4). Because the decision to perform surgery is time critical and often is performed in the off-hours when second opinions are hard to obtain, an alternative to color Doppler US would be welcome. Thus, the findings by Watanabe and colleagues (5) on dynamic contrast-enhanced scrotal MR imaging in this issue of Radiology should be of great interest to radiologists.
In 42 patients with a variety of scrotal diseases, Watanabe and colleagues performed dynamic contrast medium–enhanced scrotal MR imaging. They examined the degree of enhancement (peak height) and the rate of enhancement (mean slope) for 8 minutes after intravenously administering a bolus of contrast medium. They found that when compared with the normal side, torsion, infarction, and hemorrhagic necrosis showed significant reductions in both parameters, whereas tumors and orchitis showed significant increases in both parameters. These findings point to a potentially practical tool that relies on functional rather than anatomic assessments for improving diagnostic accuracy.
The techniques used by Watanabe and colleagues (5) can be implemented with virtually any MR unit. The test is straightforward and time efficient. The analysis of the enhancement profiles is simple. The image quality is excellent with the use of surface coils. In many respects, this dynamic enhanced MR imaging method recalls earlier radionuclide studies in which serial dynamic projection images were obtained after administration of a bolus of technetium-99m pertechnetate (1). Scrotal MR imaging, however, has substantially better spatial resolution than do radionuclide studies. This MR technique should be useful in distinguishing tumors from nonmalignant lesions and in distinguishing torsion and trauma from other acute causes of pain. Dynamic enhanced scrotal MR imaging combines the advantages of structural imaging of the testes with functional time–signal intensity profiles, but can it supplant color Doppler US?
Enthusiasm for this new technique must be tempered by the realities of clinical practice. Dynamic MR imaging is more expensive than color Doppler US and less likely to have support staff during the off-hours. Its use in prepubertal male patients, a group for whom dynamic MR imaging might be most appropriate, is limited by the need for sedation in young patients (2). However, more rapid and less expensive MR techniques combined with longer MR staffing and open MR systems, all observable trends, could change the equation in favor of MR imaging.
Are there other uses for dynamic contrast-enhanced scrotal MR imaging? Another problem for US remains the relationship between varicoceles and infertility (6). Many believe that varicoceles alter perfusion, which inhibits spermatogenesis. However, many fertile men have asymptomatic varicoceles. Hence, distinguishing the causal from the incidental varicocele is a clinical problem that might be solved by means of dynamic enhanced MR imaging. The slow, steady increase in enhancement of the normal testes demonstrated by Watanabe and colleagues (5) reflects an intact "blood-testis" barrier, which, like the blood-brain barrier, slows the rate at which the testicular parenchyma enhances. Subtle disruptions of the blood-testis barrier could be associated with other causes of primary infertility and could be detected by means of dynamic enhanced MR imaging. The role of dynamic contrast-enhanced MR imaging in this setting is an exciting area for further research.
MR imaging technology is not standing still. Faster and stronger gradients translate into faster and higher resolution images. It might be possible to improve the temporal resolution described in the article by Watanabe and colleagues (currently one three-dimensional imaging set every minute) by at least an order of magnitude. However, it is unclear whether this would improve diagnostic accuracy. Moreover, improved image analysis programs should make it possible to view the testes as three-dimensional images with pixels color-encoded according to peak height or slope to illustrate the specific foci of perfusion deficits. Macromolecular intravenous contrast media may also provide superior diagnostic performance without the need for dynamic images, since they better reflect tissue vascularity than do the current extravascular media.
However, US technology is also evolving. Forthcoming US contrast media and methods to improve their detection are anticipated to yield results comparable to those described by Watanabe and colleagues (5). It is clear that what is learned from one modality feeds and nourishes other modalities. The modality that is actually selected by the diagnostician on the front line of medical care ultimately will depend on its availability, cost, accuracy, and the current standards of care. US will continue to be the method of choice for a while.
ACKNOWLEDGMENTS
The author is indebted to Andrew J. Dwyer, MD, for his critical comments and Barbara McCoy for manuscript preparation.
FOOTNOTES
See also the article by Watanabe et al (pp 219–227 ) in this issue.
REFERENCES
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  V. S. Dogra, R. H. Gottlieb, M. Oka, and D. J. Rubens Sonography of the Scrotum Radiology, April 1, 2003; 227(1): 18 - 36. [Abstract] [Full Text] [PDF]
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