N, Number of subjects; Stone +, Number of subjects with ureteral stone

N, Number of subjects; Stone +, Number of subjects with ureteral stone ence of a 'comet tail' sign strongly suggests that the calcification is a phlebolith rather than a stone. In practice, the presence of two or more secondary signs of obstruction, even without clear visualization of a calcification within the ureter, indicates obstruction. If there is no history of recent stone passage, a cistoscopy and contrast-enhanced study of the upper tracts may be needed to exclude neoplasm.

Intravenous urography is an alternative technique for the detection of urinary tract obstruction [10]. It is safe, relatively inexpensive, and allows evaluation of the entire urinary tract with some functional information. Abnormalities may be subtle during the earliest phases of obstruction. Dilatation of the urinary tract may be minimal or even absent. Delayed opacification of the collecting system, asymmetric persistent nephrograms, and columnization of the ureteral contrast material down to the level of obstruction, indicate ongoing obstruction. Delayed films may be necessary to delineate the level of obstruction using IVU. The accuracy of IVU in diagnosing ureteral obstruction is unknown, but small studies suggest that it is significantly lower than the accuracy derived by noncontrast helical CT studies. Extraurinary causes of acute abdominal pain are not usually detectable with intravenous urography.

US, usually combined with a plain film, is an alternative method for evaluating the obstructed or dilated urinary tract. Although US allows for excellent evaluation of the renal parenchyma and the collecting system to the ureteropelvic junction, it is limited in the evaluation of the ureter and of soft-tissue lesions within the collecting system. The use of renal US in the evaluation of suspected acute ureteral obstruction is limited because dilatation often does not develop for hours, or even days. In these cases, US findings are normal in up to 50% of patients. The use of US Doppler-derived resistive indices may be helpful in detecting acute obstruction before dilatation develops. The usefulness of this finding is controversial [11-14]. Identification of jets at the ureterovesical junctions indicates that obstruction is incomplete and may be used to guide therapy [15].

In diuresis renography, radionuclides are injected to evaluate the urinary tract for obstruction. Considerably less anatomic detail is available with this test than with other radiographic examinations, so it is less useful in the acute setting than for follow-up or evaluation of chronic urinary tract obstruction. Diuresis renography does have the advantage of yielding objective data regarding the significance of hydronephrosis and also allows for evaluation of the function of each kidney. Administration of a diuretic, usually furosemide, augments the standard renogram and is useful in evaluating dilated urinary systems.

Magnetic resonance urography (MRU), using rapid scanning techniques such as HASTE or single-shot fast spin echo sequences, is beginning to be used for evaluation of the urinary tract [16, 17]. The kidneys and dilated ureters are very bright on T2-weighted images and their stable position allows for clear imaging of the level of ob struction. Unfortunately, stones appear as signal voids and can be difficult to identify and measure.

Most stones are radiodense, meaning that confirmation of stone location during conservative therapy is best performed using plain films [18]. US is useful in identifying persistent hydronephrosis or cortical atrophy.

In dealing with the pregnant patient with flank pain, fetal age and estimated radiation dose is of paramount importance. Right hydronephrosis is commonly encountered as the enlarging uterus turns slightly to the right, compressing the ureter. When an obstructing stone is suspected in either the right or left system, ultrasound should first be performed. Some urologists will place a stent based on clinical findings and severe hydronephro-sis. If more imaging information is needed, a limited IVU using a plain scout film followed by a 10 minute post infusion delayed film yields the least radiation in the first trimester patient. After 20-24 weeks, IVU becomes difficult to interpret because of the enlarging uterus, and CT should be considered [19]. The expected fetal dose is approximately 16 mSv, well below that expected to cause anomalies.

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