Testicular torsion, also termed torsion of the spermatic cord, is a relatively common and potentially devastating acute condition due to obstruction of the arterial blood supply to the testis.  Fortunately, this entity is relatively well known, and it usually occurs with enough discomfort to lead to its diagnosis and subsequent testicular salvage. However, atypical presentations of testicular torsion, delayed recognition of the condition, and its confusion with other causes of acute scrotum can potentially delay diagnosis and lead to testicular necrosis necessitating orchiectomy. Diagnostic imaging, particularly Doppler ultrasonography, plays an important role in the assessment of the patient with acute scrotal pain. (See the images below.) 
In general, laboratory tests are not diagnostically useful in distinguishing torsion from other acute scrotal syndromes. Urinalysis results are negative in 98%, and a mild leukocytosis may occur in as many as 30% of patients.
Within the past decade, ultrasonography with color and power Doppler imaging has emerged as the primary imaging modality for the diagnosis of testicular torsion. [3, 4, 5, 6, 7, 8, 9] It not only helps in corroborating the diagnosis by alteration of testicular echotexture but also provides valuable information on vascular perfusion of the testis. In addition, sonographic findings frequently allow other diagnoses to be made in those patients presenting with an acute scrotum who do not have torsion. 
Guidelines on scrotal ultrasound examination have been published by the American College of Radiology, Society of Pediatric Radiology, and Society of Radiologists in Ultrasound for both testicular and extratesticular structures. 
Prior to the development of high resolution, real-time ultrasonography coupled with sensitive color Doppler, nuclear scintigraphy was the mainstay of tests available to evaluate the acute scrotum. Given associated radiation, less widespread availability, limited ancillary information, and the accuracy of color Doppler imaging, scrotal scintigraphy is no longer used as frequently. [12, 13, 14] In cases with a clinically ambiguous picture or with indeterminate sonographic findings, scintigraphy remains a viable imaging alternative.  Testicular scintigraphy is straightforward, although it requires intravenous access. An infiltrated radionuclide bolus prevents an adequate examination. False-negative results are unusual. False-positive results are more frequent because of the changing scintigraphic appearance of infarction over time and potential interpretation errors.
Color Doppler ultrasonography is highly operator dependent. In the diagnosis of testicular torsion, gray-scale findings are combined with dynamic flow information. Inaccurate results may be obtained in the prepubertal patient with small testicular volume or in cases with multiple imaging and Doppler artifacts. Such imaging artifacts may result from inappropriate gain settings and the non-use of slow-flow techniques. 
Information about the role of MRI in the diagnosis of torsion is limited, although MRI is likely to be highly sensitive. [17, 18] However, with its limited availability, particularly at night, and its cost, MRI is unlikely to become a front-line examination for the patient presenting with acute scrotal pain.
Magnetic Resonance Imaging
Limited information is available on the potential role of magnetic resonance imaging (MRI) in the diagnosis of acute testicular torsion. Findings from small studies to date suggest a high degree of accuracy with MRI, particularly when it is performed with contrast enhancement. These finding are corroborated by results of controlled animal models. In addition, phosphorus-31 magnetic resonance spectroscopy can demonstrate rapidly decreasing levels of adenosine triphosphate (ATP) associated with ischemia.
Degree of confidence
To our knowledge, no adequate, controlled clinical trials have been performed to assess the degree of confidence with MRI as a diagnostic tool for testicular torsion. However, if the torsion knot or whirlpool patterns are recognized in conjunction with testicular enlargement and absent vascularity, the diagnosis is virtually certain.
On normal gray-scale and color Doppler images, the testes are homogeneous and symmetrical in echotexture, as shown on straddle views. The testes are relatively symmetrical in size, but the normal range varies widely.
On color or power Doppler ultrasonogram, flow to the testes and epididymis should be symmetrical. However, flow may be difficult to visualize in young patients. In patients with torsion, gray-scale images may show testicular enlargement due to engorgement; uniformly hypoechoic testicle (early); heterogenous, hypoechoic texture, which indicates necrosis and nonviability; echogenic areas inside the infarcted testis, which may represent hemorrhage; twisting of swollen cord, which gives the appearance of a torsion knot (an echogenic or complex extratesticular mass); or in infarcted testis, tunica albuginea and mediastinum, which have increased echogenicity (ie, target sign, which is more common in neonatal torsion). (See the images below.)
Color and/or power Doppler imaging should be performed in all cases. Flow to the affected testicle is absent, although normal or increased flow may be seen with spontaneous detorsion. The symptomatic side should be compared with the asymptomatic side by using the straddle view obtained with optimal technical settings.
Epididymitis is visualized as an enlarged, hyperemic epididymis, usually with a diffusely affected area. [19, 20] Involvement of the testis also produces enlargement and increased vascularity. A scrotal abscess, whether intratesticular or extratesticular, is typically seen as a complex fluid collection, often with a vascular capsule. Torsion of the epididymal appendage is easily recognized as a mass adjacent to the epididymal head without flow; this mass does not affect the testicular vasculature. Finally, an intratesticular hematoma may mimic a necrotic testis, but it typically has normal surrounding blood flow. An extratesticular hematoma appears as a complex, cystic collection clearly separate from but possibly displacing the testis.
Degree of confidence
An absence of flow in a symptomatic, enlarged testicle, with flow demonstrated in the contralateral testicle, is highly specific. Power Doppler and color Doppler imaging should be used together in prepubertal boys, but it demonstrates flow in only 79-90% of normal cases. [21, 22, 23] Color Doppler and power Doppler sonography both demonstrate flow in almost 100% of postpubertal patients.  Color Doppler and power Doppler imaging have similar sensitivities for demonstrating flow in small testes, although the combination of the 2 techniques has a sensitivity that exceeds the sensitivity of each alone. Overall, the specificity is 77-100%, and the sensitivity is 86-100%.
Posttorsion hyperemia may be confused with epididymo-orchitis, producing a false-negative finding. Capsular blood flow must be distinguished from intratesticular arterial flow; these observations may produce false-negative results. Although flow may be visible in one testis and is usually evident in the other, false-positive findings are possible in the young child. Technical factors (eg, erroneous flow settings, motion artifacts on power Doppler images) may produce false-positive or false-negative results.
A scrotal abscess may cause a false-positive diagnosis of torsion because of the depiction of hyperemia surrounding a fluid core. Ultrasonography can be used to distinguish abscess from testicular torsion because of its combination of characteristic imaging and flow dynamics. 
Technetium-99m pertechnetate is the agent of choice, with an adult dose of 10-20 mCi and a pediatric dose of at least 5 mCi. Typically, immediate radionuclide angiograms are obtained, with subsequent static images as well. In the healthy patient, images show symmetric flow to the testes, and delayed images show uniformly symmetric activity.
The appearance of testicular torsion on scintigraphy depends upon the chronicity. In acute torsion (usually < 7 h), blood flow may range from normal to absent on the involved side, and a nubbin sign may be visible. The nubbin sign is a focal medial projection from the iliac artery representing reactive increased flow in the spermatic cord vessels terminating at the site of torsion. (This sign can also be seen in later stages.) Static images demonstrate a photopenic area in the involved testis. In the subacute and late phases of torsion (missed torsion), there is often increased flow to the affected hemiscrotum via the pudendal artery with a photopenic testis and a rim of surrounding increased activity on static images. This has been called a rim, doughnut, or bull's-eye sign.
Acute epididymitis generally appears as an area of focal or diffuse increased activity in the involved hemiscrotum. Testicular appendix torsion has a variable appearance: it may have a normal scan or a focal area of increased or decreased activity. An abscess, tumor, or hematoma may be indistinguishable from a torsed testicle, demonstrating a hyperemic rim surrounding an area of decreased activity.
Degree of confidence
Scintigraphy has a sensitivity of 90% and a specificity of 60% in the diagnosis of testicular torsion. Color Doppler ultrasonography has distinct advantages in diagnosing nonvascular causes of acute scrotum. Scintigraphy may be more sensitive in the neonatal period than at other times because of the difficulty in detecting flow by means of Doppler imaging. Scrotal scintigraphy may be more sensitive than color or power Doppler imaging to the presence or absence of flow in the prepubescent testicle.
Limitations of techniques
An abscess, tumor, or hematoma may produce false-positive findings (rim sign). A hyperemic epididymis may be misinterpreted as a halo, producing false-positive study. Most false-negative studies are due to technical reasons or interpretative errors.