3. Role of Imaging in UTIs
Magnetic resonance imaging
Renal cortical scintigraphy
UTI is mainly diagnosed by a typical history with positive urinalysis result.
Routine imaging is not required for the diagnosis of uncomplicated UTI in adult patients.
The role of imaging is mainly in complicated UTI where it aids in identifying the extent of disease as well as any associated complications.
Ultrasound is usually the first line imaging modality for investigation of complicated, recurrent and atypical UTIs. It is readily available, cheap, easy to perform and does not involve ionising radiation or use of contrast medium.
CT Urography (CTU) has become an important imaging modality for complicated UTIs.
The diagnosis of urinary tract infection (UTI) in adults is primarily based on a typical presentation with urinalysis findings suggestive of UTI. In general, routine radiologic imaging is not required for diagnosis and treatment of uncomplicated cases in adult patients. Appropriate imaging not only defines the extent of disease but also identifies significant complications. This may help in treatment adjustment. It can also guide the interventional radiologist in planning the management of pyonephrosis and abscess .
3.2 Diagnostic Indications
Failure of response to conventional therapy.
Recurrent or unusual presentation.
Diagnostic uncertainty in critically ill patients.
Suspicion of complications.
Possible previously occult structural or functional abnormalities that may require intervention.
Increased susceptibility for more severe life-threatening complications e.g. diabetics, elderly or immunocompromised patients.
Need to characterize the severity of the infection to direct future therapy or intervention.
Need to evaluate the extent of organ damage following an episode of a resolved acute UTI.
3.3 Imaging Modalities
In the past, ultrasound (US) and plain radiographs have been used with intravenous urography (IVU) in the evaluation of patients suspected of having complicated UTI. They allowed detection of calculi, obstruction and incomplete bladder emptying. Their role however was limited in the evaluation of renal inflammation and infection.
The urinary bladder should always be imaged with measurement of post-void residual urinary volume and bladder wall thickness to estimate outflow obstruction.
CTU is performed in different phases of excretion after intravenous contrast medium administration. It not only defines the extent of disease but also identifies significant complications.
The disadvantage of CT is that it exposes the patient to radiation and involves the administration of iodinated contrast medium with its possible side effects .
There are, however, potential pitfalls related to MRI. Gas-forming infections and calculi can cause signal voids which are difficult to interpret. While these are rare in children, they are common in adults and can limit the value of MRI. It is also expensive and requires IV contrast administration. Because of cost restraints, availability limitations and length of scanning; MRI is not routinely used.
It is mainly used as problem solving tool or where other imaging modalities are unsuccessful or unsuitable .
Renal cortical scintigraphy (RCS) using 99mTc-DMSA is limited to use in scar and acute pyelonephritis detection.
Conventional (VCUG) or radionuclide (RNC) voiding cystourethrography can detect vesico-ureteric reflux (VUR) and aids in differentiation of outlet obstruction .
Guidelines have varied over the years due to the changes in accessibility of different modalities and attitudes towards ionizing radiation exposure. Imaging modality choice is dictated by a number of factors e.g. local availability and expertise, renal function, patient’s age and local expertise, and patient factors [1, 3, 4]. Repeated follow up in such cases is usually not required unless there is a new indication.
3.4 Uncomplicated UTI
These patients are usually managed with a course of antibiotics or their symptoms self-resolve and thus imaging for an uncomplicated UTI is unnecessary. Current guidelines state that females should be imaged if they suffer three or more UTIs in a 12 months’ period . Imaging has an important role in diagnosis, especially in atypical presentations.
3.5 Complicated UTI
Imaging is indicated when patients respond poorly to appropriate antibiotic therapy after 3 days. Women aged 60 or above with recurrent or persistent unexplained UTI should be considered for bladder cancer workup.
Prior history of urinary tract surgery or trauma.
Bladder or renal calculi.
Obstructive symptoms such as straining, hesitancy, poor stream.
Urine culture positive for urea splitting organisms with increased predisposition for urinary calculi.
Persistent bacteriuria despite appropriate antibiotic treatment.
History of abdomino-pelvic malignancy with symptoms suggestive of urinary fistula.