Radiological imaging of the urinary tract

Radiological imaging of the urinary tract

Ultrasound

Non-invasive method of urinary tract imaging. While it provides good images of the kidneys and bladder, anatomical detail of the ureter is poor and the mid-ureter cannot be imaged at all by ultrasound because of overlying bowel gas.

Uses of ultrasound

Renal ultrasound

• Assessment of haematuria
• Determination of nature of renal masses
• can differentiate simple cysts (smooth, well-demarcated wall, reflecting no echoes; benign) from solid masses (almost always malignant; cystic masses with solid components or multiple septae or calcification may be malignant), from those casting an acoustic shadow (stones) 
• Can determine the presence/absence of hydronephrosis (dilatation of the collecting system) in patients with abnormal renal function 
• Allows ultrasound guided nephrostomy insertion in patients with hydronephrosis and renal impairment or with infected, obstructed kidneys

Bladder ultrasound

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  Measurement of post-void residual urine volume
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  Allows ultrasound guided placement of a suprapubic catheter

Prostate: TRUS (transrectal ultrasound)

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  Measurement of prostate size (where gross prostatic enlargement is suspected on the basis of a DRE, and surgery, in the form of open prostatectomy, is contemplated)
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  To assist prostate biopsy (allows biopsy of hypoechoic or hyperechoic lesions)
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  Investigation of azoospermia (can establish the presence of ejaculatory duct obstruction)

Urethra ultrasound

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  Can image the urethra and establish the depth and extent of spongiofibrosis in urethral stricture disease

Testes ultrasound

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  Assessment of the patient complaining of a lump in the testicle (or scrotum) can differentiate benign lesions (hydrocele, epididymal cyst) from malignant testicular tumours (solid, echo poor or with abnormal echo pattern)
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  When combined with power doppler can establish the presence/ absence of testicular blood flow in suspected torsion
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  Assessment of testicular trauma (rupture is indicated by abnormal echo pattern, due to blood within the body of the testis; surrounding haematoma may be seen blood within the scrotal soft tissues that has escaped through a tear in the tunica albuginea and the visceral and parietal layers of the tunica vaginalis; haematocele blood contained by an intact parietal layer of the tunica vaginalis)
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  Investigation of infertility varicoceles and testicular atrophy may be identified

Uses of plain abdominal radiography XRay (the KUB X-ray kidneys, ureters, bladder)

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  For detection of stones and determination of their size and (to an extent) their location within the kidneys, ureters, and bladder 
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  Renal calculi a calcification overlying the kidneys is intrarenal if it maintains its relationship to the kidney on inspiratory and expiratory films (i.e. if it moves with the kidney). If in doubt as to whether an opacity overlying the outline of the kidney is intrarenal or not, get an ultrasound (look for the characteristic acoustic shadow within the kidney), IVU, or CTU.
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  Ureteric calculi sensitivity for detection of renal calculi is in the order of 50 to70% (i.e. the false -ve rate is between 30 to 50%; it misses ureteric stones when these are present in 30 to 50% of cases). CTU or IVU, which relate the position of the opacity to the anatomical location of the ureters, are required to make a definitive diagnosis of a ureteric stone. However, once the presence of a ureteric stone has been confirmed by another imaging study (CTU or IVU), and as long as it is radio-opaque enough and large enough to be seen, plain radiography is a good way of following the patient to establish whether the stone is progressing distally, down the ureter. Not useful for following ureteric stones which are radiolucent (e.g. uric acid), small (generally a stone must be 3 to 4mm to be visible on plain X-ray), or when the stones pass through the ureter as it lies over the sacrum. Ability of KUB X-ray to see stones is also dependent on amount of overlying bowel gas.
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  Plain tomography (a plain X-ray taken of a fixed coronal plane through the kidneys) can be useful, but is rarely done nowadays with the availability of ultrasound and CT.
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  Opacities that may be confused with stones (renal, ureteric) on plain radiography: calcified lymph nodes; pelvic phleboliths (round, lucent centre, usually below the ischial spines).
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  Look for the psoas shadow obscured where there is retroperitoneal fluid (pus or blood) 

Intravenous urography (IVU)

Also known as intravenous pyelography (IVP). A control film is obtained before contrast is given. Intravascular contrast is administered followed by a series of X-rays of the kidneys, ureters, and bladder over the following 30 min or so, to image their anatomy and pathology, and to give some indication of renal function.

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  Radio-opacity of contrast agents depends on the presence of a tri-iodinated benzene ring in the molecule.
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  Ionic monomers (sodium and meglumine salts) ionize, thereby producing high osmolality solutions (e.g. iothalamate Conray), diatrizoate Hypaque, Urografin).
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  Non-ionic monomers low osmolality (e.g. iopamidol Niopam, iohexol Omnipaque).
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  At a concentration of 300 mg of iodine per ml, ionic monomers have an osmolality 5x higher than plasma, compared with non-ionic monomers which have an osmolality 2x that of plasma.
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  Excreted from plasma by glomerular filtration.

Films and phases of the IVU

Plain film

Looking for calcification overlying the region of the kidneys, ureters, and bladder

Nephrogram phase

First phase of IVU; film taken immediately following intravenous administration of contrast (peak nephrogram density). The nephrogram is produced by filtered contrast within the lumen of the proximal convoluted tubule (it is a proximal tubular, rather than distal tubular phenomenon).

Pyelogram phase

As the contrast passes along the renal tubule (into the distal tubule) it is concentrated (as water is absorbed, but the contrast agent is not). As a consequence, the contrast medium is concentrated in the pelvicalyceal system, and thus this pyelogram phase is much denser than the nephrogram phase. The pyelogram phase can be made more dense by dehydrating the patient prior to contrast administration. Pelvic compression can be used to distend the pelvicalyceal system and demonstrate their anatomy more precisely. Compression is released and a film taken (20 to 30 min).

Side-effects of administration of intravenous contrast media

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  Occur in 1% of patients given non-ionic and 5% given ionic contrast media.
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  The most serious reactions represent an anaphylactic reaction hypotension with flushing of the skin (marked peripheral vasodilatation), odema (face, neck, body, and limbs), bronchospasm, urticaria. Rarely, cardiac arrest can occur. The death rate, as a consequence of these reactions, is ~1 in 40,000 to 1 in 70,000 with the ionic media, and ~1 in 200,000 with non-ionic contrast agents.

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  A contrast reaction is more likely to occur in patients with an iodine allergy, previous contrast reaction, asthma, multiple other allergies, and heart disease and is less likely with non-ionic contrast media. Steroid premedication (at least 12h before) can reduce the risk of a contrast reaction.
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  Contrast media are also nephrotoxic. 10% of patients with a raised creatinine will develop an increase in creatinine after an IVU (more likely in diabetics, with dehydration, and with large contrast doses). The increase in creatinine usually resolves spontaneously.

Uses of the IVU

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  Investigation of haematuria detection of renal masses, filling defects within the collecting system of the kidney and within the ureters (stones, TCCs)
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  Localization of calcification overlying the urinary tract (i.e. is it a stone or not?)
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  Investigation of patients with loin pain (e.g. suspected ureteric colic). Increasingly being replaced with CTU, which has superior sensitivity and specificity
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  Very good for identification of congenital urinary tract abnormalities (e.g. ureteric anatomy in duplex systems); malrotation; horseshoe kidneys
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  Used for follow-up post-ureteric surgery to identify strictures.

Other urological contrast studies

Videocystourethrography (VCUG)

To identify presence of vesicoureteric reflux during filling and emptying of the bladder and presence and site of obstruction in the outlet of the bladder and within the urethra, particularly in patients with neuropathic bladder problems (e.g. spinal cord injury).

Cystography

Retrograde filling of the bladder, via a catheter, with contrast. Identifies vesicocolic and vesicovaginal fistulae and bladder rupture (extraperitoneal and intraperitoneal).

Urethrography

Retrograde filling of the urethra with contrast, to identify the site and length of urethral strictures or presence, extent, and site of urethral injury (in pelvic fracture, for example).

Ileal loopogram

Retrograde filling of an ileal conduit with contrast to establish the presence of free reflux into the ureters (a normal finding; absence of free reflux suggests obstruction at the ureteroileal junction due to ischaemic stenosis or recurrent TCC in the ureters at the ureteroileal junction) and the presence of TCCs in the ureters or renal pelvis (an occasional finding in patients who have had a cystectomy for bladder TCC with ileal conduit urinary diversion).

Retrograde ureterography

Retrograde instillation of contrast into the ureters by a ureteric catheter inserted into the ureter via a cystocope (rigid or flexible). Provides excellent definition of the ureter and renal pelvis for detection of ureteric and renal pelvic TCCs or radiolucent stones in patients with persistent haematuria where other tests have shown no abnormality. Also used to diagnose presence and site of ureteric injury (obstruction, ureteric leak) in cases of ureteric injury (e.g. post hysterectomy or caesarean section).

Computed tomography (CT) and magnetic resonance imaging (MRI)

Computed tomography

Widely used for investigation of urological symptoms and disease. It can detect very small differences in X-ray absorption values of tissues, providing a very wide range of densities (and therefore differentiation between tissues) when compared with plain radiography. The computer calculates the absorption value (attenuation) of each pixel, and reconstructs this into an image. The attenuation values are expressed on a scale from -1000 to +1000 Hounsfield units (water = 0, air = 1000, bone = +1000). More recently, advances in computing power have enabled the data to be reformatted so that images can be produced in sagittal and coronal planes as well as in the more familiar horizontal plane.

Plain CT scans (without contrast) can detect calcification and calculi within the urinary tract and administration of intravenous contrast is used to evaluate the nature of solid renal lesions and to determine the nature of soft tissue masses (e.g. to differentiate bowel from lymph nodes in cancer staging CTs). Spiral or helical CT is very rapid scanning while the table on which the patient is lying is moved though the scanner. A large volume of the body can be imaged in a single breath hold, thus eliminating movement artifact particularly useful for identifying suspected ureteric stones in patients with acute loin pain.

Uses of CT

Renal

• Investigation of renal masses characterizes solid from cystic lesions; differentiates benign (e.g. angiomyolipoma) from malignant solid masses (e.g. renal cell carcinoma)
• Staging of renal cancer (establishes local, nodal, and distant spread)
• Assessment of stone size and location (within the collecting system or within the parenchyma of the kidney)
• Detection and localization of site of intrarenal and perirenal collections of pus (pyonephrosis, perinephric abscess)
• Staging (grading) of renal trauma
• Determination of cause of hydronephrosis

Ureters

• Locates and measures size of ureteric stones

Bladder

• Bladder cancer staging (establishes local, nodal, and distant spread)

Uses of MRI

• Staging of pelvic cancer bladder and prostate cancer staging (establishes local, nodal, and distant spread). As with CT, oedema and fibrosis cannot be reliably distinguished from tumour within the bladder wall, leading to overstaging of cancer. Again, as with CT, microscopic disease cannot be identified, leading to understaging of cancer.
• Localization of undescended testes.
• Identification of ureteric stones, where ionizing radiation is best avoided (e.g. pregnant women with loin pain).

Radioisotope imaging

A variety of organic compounds can be labelled with a radioactive isotope which emits gamma rays, allowing the radiation to penetrate through tissues and reach a gamma camera placed adjacent to the patient. The most commonly used radioisotope is technetium ^99mTc (half-life 6h, gamma ray emission energy 0.14MeV). The excretion characteristics of the organic compound to which the ^99mTc is bound determine the clinical use.

MAG3 renogram

^99mTc is bound to mercapto acetyl triglycine. Over 90% of MAG3 becomes bound to plasma proteins following intravenous injection. It is excreted from the kidneys principally by tubular secretion (glomerular filtration is minimal). Following intravenous injection MAG3 is very rapidly excreted (appearing in the kidney within 15s of the injection and starting to appear in the bladder within about 3 min). Approximately two-thirds of the injected dose of MAG3 is taken up by the kidneys with each passage of blood through the kidney. The radioactivity over each kidney thus increases rapidly. The peak of radioactivity represents the point at which delivery of MAG3 to the kidney from the renal artery is equivalent to excretion of MAG3. The radioactivity starts to decline as excretion outstrips supply. Thus, a time activity curve can be recorded for each kidney. This time activity curve is known as a renogram.

Images are collected onto a film at 30-s intervals for the first 3 min and then at 5 min intervals for the remainder of the study (usually a total of 30 min).

A normal renogram has 3 phases

• First phase: a steeply rising curve lasting 20 to 30s.
• Second phase: a more slowly rising curve, rising to a peak. If the curve does not reach a peak the second phase is said to rise continually. A normal second phase ends with a sharp peak.
• Third phase: a curve which descends after the peak. There can be no third phase if there is no peak.

Description of the renogram

No comment is made about the first phase. The second phase is described as being absent, impaired, or normal. The third phase is described as being absent, impaired, or normal.

The time to the peak depends on urine flow and level of hydration and is a crude measure of the time it takes the tracer to travel through the parenchyma of the kidney and through the renal pelvis. The time to the peak of the renogram normally varies between 2 and 4.5 min.

If the renogram continues beyond the time at which the peak should normally occur, then there may be a distal obstruction (e.g. at the PUJ or lower down the ureter). In this situation, an injection of 40mg of frusemide is given (at about 18 min) and if the curves start to fall rapidly, this is taken as proof that there is no obstruction. If it continues to rise, there is obstruction. If it remains flat (neither rising or falling), this is described as an equivocal result.

Parenchymal transit time can also be measured (PTTI parenchymal transit time index). The normal range for PTTI is 40 to 140s, and averages 70s. PTTI is prolonged (to >156s) in obstruction and in renal ischaemia. A normal PTTI excludes obstruction.

Uses

• Split renal function (i.e. the % function contributed by each kidney)
• Determination of presence of renal obstruction based on shape of renogram curve and PTTI

DMSA scanning

Dimercapto succinic acid (DMSA) is labelled with ^99mTc. It is taken up by the proximal tubules and retained there, with very little being excreted in the urine. A static image of the kidneys is thus obtained (at about 3 to 4h post intravenous injection of radioisotope). It demonstrates whether a lesion contains functioning nephrons or not.

Uses

• Split renal function (i.e. the % function contributed by each kidney)
• Detection of scars in the kidney (these appear as defects in the cortical outline, representing areas in which the radioisotope is not taken up)

Radioisotope bone imaging

^99mTc-labelled methylene disphosphonate (MDP) is taken up by areas of bone where there is increased blood supply and increased osteoblastic activity. There are many causes of a focal increase in isotope uptake bone metastases, site of fractures, osteomyelitis, TB, benign bone lesions (e.g. osteoma). Metastases from urological cancers are characterized by their predilection for the spine and the fact that they are multiple (single foci of metastasis are rare). Prostate cancer classically metastasizes in this way.

Uroflowmetry

Measurement of flow rate. Provides a visual image of the strength of a patient's urinary stream. Urine flow rate is measured in ml/s, and is determined using commercially available electronic flowmeters. These flowmeters are able to provide a print-out recording the voided volume, maximum flow rate, and time taken to complete the void, together with a record of the flow pattern. Maximum flow rate, Qmax, is influenced by the volume of urine voided, by the contractility of the patient's bladder, and by the conductivity (resistance) of their urethra.

A number of nomograms are available which relate voided volume to flow rate.

Interpretation and misinterpretation of urine flow rate

The wag artifact is seen as a sudden, rapid increase in flow rate on the uroflow tracing and is due to the urine flow suddenly being directed at the centre of the flowmeter, producing a sudden artifactual surge in flow rate.

In men with prostatic symptoms, for the same voided volume, flow rate varies substantially on a given day (by as much as 5ml/s if 4 flows are done). Most guidelines recommend measuring at least 2 flow rates, and using the highest as representing the patient's best effort.

What does a low flow mean?

Uroflowmetry alone cannot tell you why the flow is abnormal. It cannot distinguish between low flow due to bladder outlet obstruction and that due to a poorly contractile bladder.

The principal use of urine flow rate measurement is in the assessment of elderly men with suspected prostatic obstruction (LUTS/BPH). It is obligatory prior to undertaking surgical treatment. 

Urine flow rate measurement has insufficient diagnostic accuracy for it to be useful in the assessment of female lower urinary tract dysfunction. It has limited value in younger men because in this age group the bladder can compensate for a marked degree of obstruction by contracting more forcefully. Thus, a young man may have a normal flow rate despite have a marked urethral stricture. 

Post-void residual urine volume measurement

Post-void residual urine (PVR) volume is the volume of urine remaining in the bladder at the end of micturition. In normal individuals there should be no urine remaining in the bladder at the end of micturition. A PVR may be caused by detrusor underactivity (due to ageing as the older bladder is less able to sustain a contraction than the younger bladder, or neurological disease affecting bladder innervation), bladder outlet obstruction, or a combination of both. In clinical practice PVR volume is measured by ultrasound after the patient has attempted to empty their bladder. A commonly used formula for calculating bladder volume is:

Clinical usefulness of PVR volume measurement

PVR volume measurement cannot predict symptomatic outcome from TURP. 

Residual urine volume measurement is useful (along with measurement of serum creatinine) as a safety measure. It indicates the likelihood of back pressure on the kidneys and thus it tells the urologist whether it is safe to offer watchful waiting rather than TURP. In men with moderate LUTS it is safe not to operate where the post-void residual volume is less than 350ml, and this probably holds true for those with higher PVR volumes.

Does an elevated residual urine volume predispose to urinary infection?

Though intuition would suggest yes, what evidence there is relating residual volume to urine infection suggests that an elevated residual urine may not, at least in the neurologically normal adult, predispose to urine infection.

Cystometry, pressure flow studies, and videocystometry

• Cystometry: the recording of bladder pressure during bladder filling.
• Pressure-flow studies (PFS): the simultaneous recording of bladder pressure during voiding.
• Videocystometry: fluoroscopy (X-ray screening) combined with PFS during voiding 

These techniques provide the most precise measurements of bladder and urethral sphincter behaviour during bladder filling and during voiding. Cystometry precedes the pressure-flow study. Bladder pressure (Pves, measured by a urethral or suprapubic catheter) and abdominal pressure (Pabd, measured by a pressure line inserted into the rectum) are recorded as the bladder fills (cystometric phase) and empties (voiding phase), and flow rate is simultaneously measured during the voiding phase. The pressure developed by the detrusor (the bladder muscle), Pdet, cannot be directly measured, but it can be derived by subtracting abdominal pressure from the pressure measured within the bladder (the intravesical pressure). This allows the effect of rises in intra-abdominal pressure caused by coughing or straining to be subtracted from the total (intravesical) pressure, so that a pure detrusor pressure is obtained.