Kidney imaging techniques

1. List the most commonly used imaging modalities for the kidneys.

• Radiography (plain film, excretory urography [EU], retrograde pyelography, cystography)

• Ultrasonography (US)

• Computed tomography (CT) scan

• Magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA)

• Radionuclide imaging

• Kidney angiography

2. Describe the information that can be provided about the urinary tract on the plain abdominal radiograph.

The plain abdominal radiograph, also called kidneys, ureters, bladder, can show the following:

• Calcifications: kidney calculus, calcified neoplasm, sloughed papilla, medullary or cortical nephrocalcinosis, ureteric or bladder calculus/tumor

• Air: air within or adjacent to the kidneys from severe infection

• Soft tissue changes: obliteration of the psoas or kidney outline may indicate inflammation or tumor

• Bone: changes of renal osteodystrophy and either lytic or blastic metastasis

3. What is the current role of excretory urography (EU)?

• EU is also known as an intravenous pyelogram

• The EU used to be the initial modality for upper tract imaging in patients with hematuria, flank pain, and other urologic disease

• Now replaced by ultrasound, CT urography, and MR urography in most medical centers

• Less sensitive than US, CT, or MRI for detecting kidney masses

• EU does not allow reliable differentiation of solid masses from cysts

4. What is retrograde pyelography?

The injection of contrast material directly into the distal ureter or the ureteral orifice of the bladder for visualization of the collecting system and ureter, without relying on the ability of the kidneys to excrete contrast media. The primary use of retrograde pyelography is to evaluate suspected ureteral obstruction or ureteral urothelial cancer in a patient whose ability to excrete contrast material is significantly impaired.

This is an adjunctive technique when conventional imaging studies fail to adequately demonstrate the suspected pathology. Retrograde pyelography does not evaluate the kidney parenchyma and requires cystoscopy to place the catheters. This procedure is usually performed by a urologist.

An alternative to the retrograde pyelography is antegrade pyelography, usually performed by an interventional radiologist. A percutaneous needle is placed into the kidney collecting system, and contrast is injected.

5. What are the components of a CT urogram?

Comprehensive upper tract imaging includes the following:

• Unenhanced axial CT of the kidneys—detection of calcification and baseline density measurement to determine enhancement of masses

• Enhanced CT of the abdomen and pelvis with corticomedullary phase (early enhancement of the cortical tissue) and nephrographic phase (delayed imaging to view opacification of the medullary pyramids before calyceal excretion) for detection of enhancement of kidney masses

• Excretory phase imaging of the abdomen and pelvis obtained with delayed imaging once contrast is in the collecting system essential for assessing subtle urothelial abnormalities including urothelial tumors; papillary necrosis; calyceal deformity; ureteral stricture; and inflammatory changes of the kidney collecting systems, ureters, and bladder

• CT images may be reviewed as two-dimensional and three-dimensional reformatted images

6. What is the diagnostic utility of ultrasound?

• Estimating kidney size

• Assessing the echogenicity of the kidney (increased echogenicity may indicate chronic kidney disease but is nonspecific)

• Preferred screening modality for suspected obstruction because it is very sensitive to dilatation of the collecting system, such as from obstruction in kidney failure, pelvic neoplasm, in kidney transplant, and in acute urinary tract infection with pyonephrosis

• Complete ureteral obstruction can be excluded by documenting the presence of a ureteral jet (color flow seen on Doppler ultrasound as urine passes into the bladder from the ureteral orifice)

• Can detect kidney calculi as echogenic foci with shadowing

• Can differentiate solid from cystic mass

• Diagnosing adult polycystic kidney disease and screening involved families

• Guiding interventional procedures such as kidney biopsy and cyst aspiration

• Detecting perinephric fluid collections

• Evaluating kidney transplant allograft

7. What is the diagnostic utility of ultrasound in the evaluation of a kidney transplant?

• Evaluating parenchymal echogenicity and masses

• Detecting perinephric fluid collections (seroma, hematoma, urinoma, lymphocele)

• Looking for hydronephrosis

• Diagnosing ureteral obstruction/stenosis

• Using Doppler to look for vascular compromise/complications

• Used to guide for biopsies, aspiration, and drainage

• Using resistive index to look for rejection.

8. List the strengths of US in the evaluation of kidney disease.

• Sensitive for detection of perirenal fluid collections, pelvicalyceal dilatation, and cysts

• Differentiates cortex and medulla

• Differentiates cystic and solid masses

• Shows the kidney contour and perinephric space

• Demonstrates kidney blood flow by Doppler technique

• Provides good kidney imaging irrespective of kidney function; may be used in patients with elevated serum creatinine

• Can evaluate resistive indices to monitor chronic kidney disease and kidney transplants

• Can be used portably at the bedside in the intensive care unit

• Safe: no ionizing radiation or nephrotoxic contrast medium

• Low cost

9. List the weaknesses of US in the evaluation of kidney disease.

• Does not show fine pelvicalyceal detail

• Does not show the entire normal ureter, although it may occasionally see proximal or distal ureters

• Sometimes there is a limited acoustic window for seeing kidneys, especially on the left

• Does not show the entire retroperitoneum

• Can miss small kidney calculi and most ureteral calculi

• Gives no functional information

• Operator dependent

10. When is CT scan superior to ultrasound for evaluation of kidney disease?

• For evaluation of an indeterminate mass on US or a solid mass when neoplasm is suspected

• CT can define the extent of a neoplasm, evaluate for lymph node involvement, give a more comprehensive view of the perirenal, pararenal spaces, and Gerota fascia, evaluate vasculature (renal vein/inferior vena cava involvement), and stage neoplasms.

• CT is the imaging method of choice in the evaluation of suspected kidney trauma.

• CT provides information on the retroperitoneum and adrenal glands.

• Complications of obstruction, such as infection, calyceal rupture, and kidney cortical atrophy, are readily seen.

11. What is the bosniak classification?

In 1986 Bosniak proposed a classification to characterize cystic kidney masses detected by CT scan as “nonsurgical” (i.e., benign) or “surgical” (i.e., requiring surgery). In his original classification, there were four categories:

• Category I: simple benign cysts (fluid-filled, no perceptible wall)

• Category II: benign cystic lesions that are minimally complicated (mural calcifications, few thin septations)

  • Category IIF—added in 1993 (F = follow-up): more numerous thin septations, slight cyst wall thickening, totally intrarenal, nonenhancing, high-density lesions (i.e., hyperdense cysts)

• Category III: more complicated cystic lesions (calcifications, thickened or numerous septations, enhancement of the septations, mural nodules, thickened, irregular wall)

• Category IV: lesions that are clearly malignant cystic carcinomas (mural nodules with vascularization, enhancement of solid components)

Categories I and II are considered nonsurgical, whereas categories III and IV are surgical. The risk that a category III or IV lesion is malignant is approximately 50% (range, 25% to 100%). The risk of malignancy in a Bosniak IIF cystic lesion is approximately 5%.