Urography: Practice Essentials, Preparation, Technique (2024)

Excretory urography

Details pertaining to patient preparation are institution specific. At the authors' institution, patients are instructed to consume 1 bottle of magnesium citrate oral solution the evening before the examination. Magnesium citrate acts as a laxative, ensuring that subtle renal, ureteral, or bladder calcifications are not masked by abundant amounts of stool. In addition, the patient is instructed to not consume liquids or solids on the morning of the examination.

Recent laboratory values are screened prior to the procedure to ensure normal renal function, and a negative pregnancy test (as applicable) is obtained. A detailed list of medications and allergies is prepared by the technologist before the procedure is begun. The procedure is explained to the patient, and a consent form is signed. The patient is asked to void prior to the start of the study.

After the above information is reviewed and after it is determinedthat the procedure is not contraindicated, a preliminary KUB radiograph is obtained (see the image below). This radiograph is obtainedon a standard 14x17-inch cassette centered at the iliac crest and taken in full inspiration. For larger patients, the radiograph is centered at the umbilicus. The preliminary radiograph is examined by the radiologist to ensure that the field of view is appropriate (the radiograph should encompass the suprarenal region to a level below the pubic symphysis). Additionally, the radiologist should note the presence of any calcifications. Additional oblique radiographs may be required to localize and delineate suspected calcifications seen on the KUB radiograph.

If satisfied with the preliminary radiograph(s), the radiologist places a peripheral intravenous line through which two 50-mL syringes of Omnipaque 300 are briskly injected. After injection of contrast, a cone-down radiograph focused on the kidneys is obtained during full expiration at the 1-minute mark (see the first image below). A subsequent full KUB of the abdomen is obtained at 3 minutes. At this juncture, if abdominal compression is not contraindicated, the patient is placed prone onto a compression paddle, with the top of the paddle situated just above the superior aspect of the iliac crests.

Abdominal compression allows better visualization of the renal collecting system, especially in situations in which low osmolar contrast is used. Once the compression device is placed, an additional anteroposterior cone-down radiograph of the kidneys and bilateral oblique radiographs are obtained (see the first 3 images below). The radiologist is shown the images, and if optimal opacification of the collecting systems is evident, the compression paddle is released, with a subsequent KUB obtained by the technologist (see the fourth image below). An additional cone-down post-void radiograph of the bladder may be requested in frontal and oblique projections (see final 3 images below).

A modification of the technique may be used in patients with suspected ureteropelvic junction (UPJ) obstruction. The patient is specifically asked if he/she has an allergy to Lasix (furosemide) beforethis portion of the procedure is begun. If no history of an allergic reaction is reported, 15-20 minutes after the initial contrast injection, 0.5 mg/kg of Lasix (up to 40 mg) is injected through the peripheral intravenous line. Subsequent KUB radiographs are obtained at 5, 10, and 15 minutes after administration of intravenous Lasix. UPJ pathology is suspected if the contrast fails to clear the collecting system at the 10-minute mark after Lasix is injected.

Computed tomographic urography

Patients are encouraged to maintain good hydration prior to the CT examination to reduce the risk of contrast-induced nephropathy. After studying 176 patients, Weatherspoon et al concluded that oral hydration is more cost-effective and produced ureteral distention and opacification similar to IV techniques. ^[45]

As with EU, patients answer a questionnaire prior to the examination, highlighting their medications and history of allergic reactions. All metal is removed from the area of interest (to reduce beam hardening artifact from metallic objects). Peripheral intravenous access is ensured prior to commencement of the examination. The patient is placed supine on the table with arms raised over the head.

A digital scout radiograph is obtained to ensure coverage from the diaphragm to the iliac crests. A non-contrast CT scan is obtained (see image below), scanning from the top of the kidneys through to the pubic symphysis using the following parameters:

Table 1. Noncontrast CT Parameters (Open Table in a new window)

The mA is alternated as the gantry rotates around the patient according to shape and attenuation characteristics of the patient obtained from the scout radiograph (ie, dose modulation) to decrease the radiation dose. In addition, the noise index is increased at the level of the iliac crest to minimize the radiation dose to the gonads. The trade-off, inevitably, is poorer-quality images; however, the authors feel this is a reasonable compromise because the non-contrast scan is used to look for stones, which are still visible on higher noise index images.

The authors subsequently inject 120 mL of intravenous contrast (or 85 mL if there is only one functioning kidney) via a peripheral IV line at a rate of 2-3 mL/sec and image through only the kidneys after a 100-second delay (from the start of the bolus injection) to obtain a nephrographic phase of renal enhancement (see theimage below). No oral contrast is administered. The parameters used are shown below.

Table 2. IV Contrast With 100-Second Delay CT Parameters (Open Table in a new window)

After this acquisition, a bolus of 200 mL of saline is administered. The patient is then asked to sit up for approximately 8 minutes (counting from initial bolus injection of contrast), after which the patientis instructed to lie supine on the CT table with arms over thehead. A second digital scout radiograph spanning the diaphragmthrough to the pelvis is now obtained. The patient is then scanned from above the kidneys through the pubic symphysis to obtain a 10-minute delayed excretory image to opacify ureters and bladder. Sagittal and coronal reformats are obtained from axial imaging data, and 3D volumetric images are generated by the radiologist at a separate, dedicated 3D workstation (see the images below). The parameters used are shown below.

Table 3. IV Contrast With 10-Minute Delay CT Parameters (Open Table in a new window)

Note that the noise index is increased to compensate from the increased dose brought on by using the thinner slices needed to accurately evaluate the collecting systems for subtle filling defects.

An alternative technique that is typically used in patientsyounger than 40 years is referred to as the "split dose" technique. The premise of this technique is to administer a divided dose of iodinated contrast, with subsequent CT acquisition timed so that a single contrast-enhanced scan contains both nephrographic and excretory phases of renal enhancement.

Initial imaging parameters are the same for the 2 techniques: A scout radiograph and a non-contrast CT scanare obtained (using dose modulation and increasing the noise index at the iliac crests to decrease the dose to the gonads). Subsequently, 75 mL of intravenous noniodonated contrast is injected via a peripheral line at 2-3 mL/sec; this is followed by a 150-mL bolus of saline.

Typically, the authors wait 8 minutes after injection of contrast and then administer an additional 75 mL of noniodonated contrast at 2-3 mL/sec, followed by a 50-mL bolus of saline. No oral contrast is administered. After a 100-second delay, a CT scan is obtained from the top of the kidneys through to the pubic symphysis using the parameters shown below.

Table 4. IV Contrast Combined Nephrographic/Excretory CT Parameters (Open Table in a new window)

Volume data, consisting of 0.625 mm slices, as well as reformatted 3.75 mm slices, are provided by the technologist to the radiology console for interpretation. 3D volumetric images are subsequently generated by the radiologist at a separate, dedicated 3D workstation.

In a study by Morrison et al, single-bolus CT urography was found to result in significantly fewer repeat excretory phases (28.6% vs 46.3%) and faster scan time (678s vs 978s), with only slightly higher radiation dose, in patients 50 years and older. ^[46]

Magnetic resonance urography

Magnetic resonance (MR) urography has evolved into a comprehensive evaluation of the urinary tract that combines anatomic imaging with functional evaluation in a single test without ionizing radiation. Quantitative functional MR imaging is based on dynamic contrast-enhanced MR acquisitions that provide progressive, visible enhancement of the renal parenchyma and urinary tract. Functional evaluation with MRI has continued to improve as a result of both significant technical advances allowing for faster image acquisition and new tracer kinetic models of renal function. ^[30]

The challenge of MR urography is to obtain diagnostic quality images of the kidneys, ureters, and bladder within a reasonable time frame, while taking into account the effects of respiratory motion, ureteral peristalsis, and flowing urine. ^[25]

Magnetic resonance urography requires patient preparation in the form of pre-examination IV hydration, placement of a urinary catheter, and administration of diuretics at the time of the exam. The imaging protocol is based on T2-weighted images for anatomic assessment and dynamic post-contrast images for functional evaluation. These images are then used to generate quantitative and graphic results, including contrast transit and excretion time, and to calculate differential renal function. ^[47]

Early MR urography relied on T2-weighted techniques to take advantage of the high signal intensity of urine in the collecting systems, ureters, and bladder. Obvious advantages of this technique are that images can be obtained in any plane and images can be obtained relatively quickly. However, this technique is limited to use in patients with distended urinary collecting systems. Additional interventions such as intravenous hydration, ureteral compression, and intravenous diuretics may be introduced to optimize the examination.

Excretory MR urography, on the other hand, is similar to CT urography and conventional EU. A gadolinium-based contrast agent is administered intravenously, and collecting systems are then imaged during the excretory phase. Generally, to avoid T2 effects of concentrated contrast in the urine, low-dose gadolinium-based contrast is administered. ^[48] Again, IV diuretics may be administered to optimize the examination. ^[20] The primary imaging sequence is a 3D gradient-echo, generally with fat suppression. Motion suppression is best achieved with breath-hold acquistions, as opposed to respiratory triggering. ^[49]

Ultimately, most modern MR urographic studies combine T1- and T2-weighted sequences in axial and coronal planes. Contrast administration should not occur until after the T2-weighted sequences are obtained,because the gadolinium-based contrast agent causes decreased signal on T2-weighted sequences. At our institution, the initial set of T2-weighted images are reviewed by the radiologist to evaluate for an underlying obstruction. If no obstruction is noted, intravenous Lasix (furosemide) is administered to optimize excretion. Comprehensive examinations may take 30-60 minutes, with more tailored examinations taking 15-30 minutes. The authorsroutinely obtain our post-contrast images at 3 minutes and at 7-10 minutes. Images at 3 minutes are obtained in both axial and coronal planes; remaining contrast-enhanced images are obtained in the coronal plane (see the images below). A radiologist is present to monitor the case for quality assurance.

Pediatric MR urography can be performed at 1.5 or 3 Tesla (T) in children. The 3 T magnets can provide superior spatial resolution, which can be particularly helpful in younger children and provide improved visualization of small urinary tract structures. The 1.5 T magnets, however, generally allow for more hom*ogeneous fat saturation and are less susceptible to artifacts, such as dielectric effect, T2* effects of excreted gadolinium, and any artifacts from surgical material. Imaging is performed with multi-element phased-array surface coils.Generally, children who are younger than 10 years or those with developmental delay will require the use of anesthesia or sedation to prevent motion artifacts. ^[50]