Urography
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Urography is a radiologic technique used for the evaluation of the genitourinary system—specifically, the kidneys, ureters, and bladder. Although originally performed using plain radiographic techniques, advanced imaging modalities have been progressively refined such that computed tomography (CT) and/or magnetic resonance imaging (MRI) have largely replaced excretory urography (EU) as the optimal way to image the genitourinary system. [1, 2, 3, 4, 5, 6]
Despite the advances in radiologic techniques, no criterion standard exists for the noninvasive imaging evaluation of the urinary collecting system, with each modality having its own set of pitfalls that preclude optimal visualization of the entirety of the urinary system. The following article outlines the indications for urography, discusses the advantages and disadvantages of each technique, and explains the key points in performing a urogram using the different modalities.
Excretory urography
The following are accepted indications for EU, as delineated by the American College of Radiology (ACR): [7]
To evaluate the presence or continuing presence of suspected or known ureteral obstruction.
To assess the integrity of the urinary tract status post trauma (including iatrogenic interventions), particularly in situations in which cross-sectional imaging is unavailable or inappropriate.
To assess the urinary tract for suspected congenital anomalies, particularly in situations in which cross-sectional imaging is unavailable or inappropriate.
To assess the urinary tract for lesions that may explain hematuria or infection. In particular, EU may be used to evaluate for an underlying parenchymal mass or may be used to evaluate for a lesion of the urothelial tract in settings in which cross-sectional imaging is unavailable or inappropriate.
Computed tomographic urography
The indications for computed tomographic urography are similar to those for EU. CT urography is most often used for the evaluation of the urinary collecting system in the setting of hematuria. Multidetector CT (MDCT) has been shown to be more sensitive than excretory urography in detection of transitional cell carcinoma of the upper tracts, with a sensitivity of 95.8% vs 75%. [2, 3, 8, 9, 10, 11, 12]
Other clinical situations in which CT urography may be useful include trauma with suspected ureteral injury, as well as recurrent and/or complex urinary infections to exclude an underlying obstructive etiology or formation of an abscess. CT urography is also used to detect renal stones and may be used in the preoperative planning of percutaneous nephrolithotomy (PCNL). [13] CT urography has also been used in the postoperative setting to evaluate the urinary collecting system following cystectomy. [14]
With continued confirmation of the accuracy and advantages of MDCT urography, MDCT has essentially replaced conventional EU. More specifically, the advantages offered by MDCT urography include high spatial and contrast resolution, with the ability to obtain isotropic volume data in as thin as 0.5 mm acquisitions, which may be reformatted in multiple planes or reconstructed in 3D. Evaluation of enhancement characteristics through measurement of attenuation values allows characterization of renal lesions.
Magnetic resonance urography
To date, magnetic resonance (MR) urography has been used in patients with urinary tract dilation or urinary obstruction who either cannot receive iodinated contrast material or in whom imaging using ionizing radiation is undesirable. [15] As such, the patient populations in which this modality has been most widely used include children and pregnant patients. [15] Note that MR urography is relatively insensitive for the detection of renal and ureteral calculi when compared to CT scanning. [15, 16, 3, 5, 17, 18, 6, 19]
Additionally, MR urography may be indicated in the diagnosis and staging of cancers involving the kidney, bladder, and prostate. This is, in part, due to the fact that MR imaging offers superior soft tissue contrast resolution and better detection of contrast enhancement. [20]
In children, MR urography can be used in the preoperative anatomic assessment of vascular anatomy; in the evaluation of duplicated collecting systems, renal dysplasia, ectopic ureter, retrocaval ureter, and primary megaureter; and to distinguish hydronephrosis from cystic renal disease. [15, 4, 21] Furthermore, MR urography can be used to obtain physiologic information about the collecting systems, such as renal transit time, differential renal function, and estimated glomerular filtration rate (GFR), obviating the need for nuclear scintigraphy and consequently avoiding the associated radiation exposure. [22, 23]
In pregnant patients, MR urography is generally used to distinguish physiologic dilatation of the ureters from an underlying obstructive uropathy. [24]
Excretory urography
The technique used in performing a complete EU examination is discussed in detail later; however, the basic premise involves obtaining plain radiographic images of the abdomen at various time points after the administration of intravenous iodinated contrast material. In addition, a compression device is often used for more optimal visualization of the collecting system. Thus, contraindications to EU pertain to the use of iodinated contrast material as well as the use of the compression paddle. A positive pregnancy test is an additional absolute contraindication to this procedure.
Risk factors pertaining to use of iodinated contrast material are as follows [25] :
Contraindications to compression include the following: [30]
Evidence of obstruction on the 5-minute image
Abdominal aortic aneurysm or other abdominal mass
Severe abdominal pain
Recent abdominal surgery
Suspected urinary tract trauma
Presence of a urinary diversion
Presence of a renal transplant
Computed tomographic urography
The safe use of iodinated contrast material and compression devices pertain to both CT urography and EU. Additionally, interestingly, note that some studies have shown that application of abdominal compression did not improve distention or opacification of the ureters compared to a technique of giving the patient a saline bolus. [31, 32] Furthermore, a CT urogram should not be performed in a pregnant patient.
An additional precaution involved in both techniques, but perhaps more pertinent with CT urography, involves the relatively high doses of ionizing radiation involved in imaging the genitourinary tract. The mean effective dose for patients undergoing MDCT urography has been reported at 14.8 mSv ± 3.1, which is about 1.5 times the exposure of EU. [33] Use of dose modulation with iterative reconstruction techniques is expected to bring further reductions in dose. This issue becomes of special importance in both children and pregnant patients, in whom MR urography may be the preferred imaging modality.
Magnetic resonance urography
One of the techniques used for MR urography requires the use of a gadolinium contrast agent. Information on the risks of gadolinium-based contrast agents can be found here.
The anatomy of the bladder (see the image below) forms an extraperitoneal muscular urine reservoir that lies behind the pubic symphysis in the pelvis. A normal bladder functions through a complex coordination of musculoskeletal, neurologic, and psychological functions that allow filling and emptying of the bladder contents. The prime effector of continence is the synergic relaxation of detrusor muscles and contraction of the bladder neck and pelvic floor muscles.
For more information about the relevant anatomy, see Bladder Anatomy. See also Kidney Anatomy, Female Urinary Organ Anatomy, and Male Urinary Organ Anatomy.
Anesthesia is generally not required for adult patients undergoing urographic examination, regardless of the modality. However, children undergoing MR urography often require sedation. Consequently, many institutions coordinate MR urographic examinations with the anesthesia department.
In excretory urography (EU), radiographs are obtained with standard radiographic equipment to obtain multiple kidney, ureter, bladder (KUB) radiographs. The images may be obtained in a variety of projections, with oblique images providing helpful information in evaluating abnormal calcifications. [34]
Multidetector CT scanners have become increasingly commonplace, and CT urography has been well studied using this equipment. As mentioned earlier, the multidetector arrays enable acquisition of isotropic volume data, which is routinely reformatted into axial, coronal, and sagittal image series. In addition, the data are amenable to multiplanar reformatting and 3D manipulation, either within the CT scanner work console, at standalone computer workstations, or within the PACS environment.
MR urography has been well studied on 1.5 T machines. Studies have been published regarding the use of 3.0 T MR systems for MR urography. No clear difference has been shown between 1.5 and 3.0 T examinations, and 3.0 T examinations have been shown to have inherent problems, including prolonged T1 relaxation times and worsening of some artifacts. [35]
Compression, as has been mentioned, is considered by some to be an essential component of an effective urographic examination, [34] whereas others use alternative techniques to image the ureters. [31] Different types of compression devices exist, some of which are placed around the patient and others that are attached to the examination table. In EU, patients may be placed in various positions, and the positioning is only limited by the patient’s comfort level and the range of the radiographic equipment.
In both CT and MR urographic studies, the patient is preferably placed with their arms above their head while in the supine position. In the CT examination, this limits the effect of beam hardening artifact; in the MR examination, this positioning limits wrap-around artifact.
Details pertaining to patient preparation are institution specific. At the authors’ institution, the authors instruct the patients to consume one bottle of magnesium citrate oral solution the evening before the examination. The 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 ascertained by the technologist prior to starting the procedure. The procedure is explained to the patient, and a consent form is signed. The patient is asked to void prior to commencing the study.
After reviewing the above information and determining that the procedure is not contraindicated, a preliminary KUB radiograph is obtained (see the image below). The radiograph is performed on a standard 14×17-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), a peripheral intravenous line is placed, through which the radiologist briskly injects two 50-mL syringes of Omnipaque 300. 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 present, 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 the 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) prior to this portion of the procedure. If no history of an allergic reaction exists, 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 the administration of intravenous Lasix. UPJ pathology is suspected if the contrast fails to clear the collecting system at the 10-minute mark after injecting Lasix.
Patients are encouraged to maintain good hydration prior to the CT examination to reduce the risk of contrast-induced nephropathy. After studying 176 patients in 2013, Weatherspoon et al concluded that oral hydration is more cost effective and produced ureteral distention and opacification similar to IV techniques. [36]
Similar to EU, the 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 the 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)
Series
kvp
mA
Slice thickness
Reconstruction Algorithm
Noise index
Noncontrast
120
DOSE
MODULATION
3.75 mm
Standard
15.7(increased to 25.4 at the iliac crest)
Alternating the mA as the gantry rotates around the patient according to the shape and attenuation characteristics of the patient obtained from the scout radiograph (ie, dose modulation) is used to decrease the radiation dose. In addition, the noise index is increased at the level of the iliac crest in order 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 noncontrast scan is used to look for stones, which are still visible at the higher noise index images.
We subsequently inject 120 mL of intravenous contrast (or 85 mL if there is only one functioning kidney) via a peripheral intravenous 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 the image 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)
Series
kvp
mA
Slice thickness
Reconstruction algorithm
Noise index
IV contrast 100-second
delay (from start of bolus injection)
120
DOSE
MODULATION
3.75 mm
Standard
15.7
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 they are instructed to lie supine on the CT table with their arms over their head. A second digital scout radiograph spanning the diaphragms through 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 opacity the ureters and bladder. Sagittal and coronal reformats are obtained from the 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)
Series
kvp
mA
Slice thickness
Reconstruction algorithm
Noise index
IV contrast
10-minute delay
120
DOSE
MODULATION
0.625mm
Standard
30
Note that the noise index is increased in order 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 patients younger 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 the subsequent CT acquisition timed so that a single contrast-enhanced scan contains both the nephrographic and excretory phases of renal enhancement.
The initial imaging parameters are the same for both techniques: a scout radiograph and a noncontrast CT scan are 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, which is followed by a 150 mL bolus of saline.
Typically, the authors wait 8 minutes after the 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)
Series
kvp
mA
Slice thickness
Reconstruction algorithm
Noise index
IV contrast
Combined nephrographic/excretory
120
DOSE
MODULATION
0.625 mm
Standard
15.7 (increased to 25.4 at the iliac crest)
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.
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. [20]
Early MR urography relied on T2-weighted techniques to take advantage of the high signal intensity of the urine in the collecting systems, ureters, and bladder. An obvious advantage to this technique is that images can be obtained in any plane and the images can be obtained relatively quickly. However, this technique is limited to use in patients with distended urinary collecting systems. Additional interventions may be introduced to optimize the examination, such as intravenous hydration, ureteral compression, and intravenous diuretics.
Excretory MR urography, on the other hand, is similar to CT and conventional excretory urography. A gadolinium-based contrast agent is administered intravenously, and the 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. [37] Again, intravenous diuretics may be administered to optimize the examination. [15] 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. [38]
Ultimately, most modern MR urographic studies combine T1- and T2-weighted sequences in the axial and coronal planes. Importantly, 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 present, intravenous Lasix (furosemide) is administered to optimize excretion. Comprehensive examinations may take 30-60 minutes, with more tailored examinations taking 15-30 minutes. We routinely obtain our postcontrast images at 3 minutes and at 7-10 minutes. Images at 3 minutes are obtained in both the axial and coronal planes, while the 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.
Complications revolve around contrast administration and the use of compression.
Infiltration of the intravenous access site may occur. Small amounts of extravasated iodinated iso-osmolar contrast are unlikely to cause serious complications and may be treated with warm soaks or ice packs, massage, and/or elevation until the fluid is resorbed, which generally occurs over 2-4 hours. During this time, the patient remains under observation in the imaging department.
The patient should be given specific discharge instructions by the radiologist and followed up in the next 24 hours to confirm that there are no further symptoms or signs that might indicate development of blistering, skin sloughing, cellulitis, or thrombophlebitis. Larger volumes of contrast extravasation may be associated with compartment syndrome, and referral to the ED with a plastic surgery consultation for infiltrations greater than 30 mL is advisable.
Iodinated contrast may cause nephrotoxicity in patients with impaired renal function; thus, it is important to obtain laboratory work on patients prior to the administration of intravenous iodinated contrast. Similarly, impaired renal function may result in severe adverse effects if gadolinium-based contrast is administered—namely, nephrogenic systemic fibrosis.
Complications from the use of compression paddles range from patient discomfort to severe, life-threatening emergencies if used in a patient with an abdominal aortic aneurysm. Additionally, patients with a transplanted kidney should be approached carefully, if at all, with compression devices.
Furosemide works to increase excretion of water by interfering with the chloride-binding cotransport system, which in turn inhibits sodium and chloride reabsorption in the ascending limb of the loop of Henle and the distal renal tubule. Bumetanide does not appear to act in the distal renal tubule. For the purpose of EU, our institution administers Lasix as follows: 0.5 mg/kg of Lasix is injected through a peripheral intravenous line up to a maximum dose of 40 mg.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography (MRA) scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. Great caution should therefore be exercised in evaluating patients with renal insufficiency. For more information, see the FDA Public Health Advisory or Medscape.
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Series
kvp
mA
Slice thickness
Reconstruction Algorithm
Noise index
Noncontrast
120
DOSE
MODULATION
3.75 mm
Standard
15.7(increased to 25.4 at the iliac crest)
Series
kvp
mA
Slice thickness
Reconstruction algorithm
Noise index
IV contrast 100-second
delay (from start of bolus injection)
120
DOSE
MODULATION
3.75 mm
Standard
15.7
Series
kvp
mA
Slice thickness
Reconstruction algorithm
Noise index
IV contrast
10-minute delay
120
DOSE
MODULATION
0.625mm
Standard
30
Series
kvp
mA
Slice thickness
Reconstruction algorithm
Noise index
IV contrast
Combined nephrographic/excretory
120
DOSE
MODULATION
0.625 mm
Standard
15.7 (increased to 25.4 at the iliac crest)
Mahan Mathur, MD Assistant Professor of Radiology and Biomedical Imaging, Yale University School of Medicine; Director, Medical Student Education, Associate Director, Diagnostic Radiology Residency Program, Yale-New Haven Hospital
Mahan Mathur, MD is a member of the following medical societies: American College of Radiology, American Roentgen Ray Society, Radiological Society of North America
Disclosure: Nothing to disclose.
David A Lawrence, MD Resident Physician, Department of Diagnostic Radiology, Yale-New Haven Medical Center
David A Lawrence, MD is a member of the following medical societies: American College of Radiology, American Medical Association, Radiological Society of North America
Disclosure: Nothing to disclose.
Caroline R Taylor, MD Associate Professor, Department of Diagnostic Radiology, Yale University School of Medicine; Chief, Diagnostic Imaging Service, Veterans Affairs Connecticut Health Care System
Caroline R Taylor, MD is a member of the following medical societies: Radiological Society of North America
Disclosure: Nothing to disclose.
Justin A Siegal, MD Radiologist, Department of Radiology, Virginia Mason Medical Center
Disclosure: Nothing to disclose.
Gowthaman Gunabushanam, MD, FRCR Assistant Professor, Department of Diagnostic Radiology, Yale University School of Medicine
Gowthaman Gunabushanam, MD, FRCR is a member of the following medical societies: American Roentgen Ray Society, Connecticut State Medical Society
Disclosure: Nothing to disclose.
Urography
Research & References of Urography |A&C Accounting And Tax Services
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