Urinary tract

Published on 12/06/2015 by admin

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Last modified 12/06/2015

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Urinary tract

Methods of imaging the urinary tract

1. Plain film radiography

2. Excretion urography (IVU)

3. Ultrasound (US)

4. Computed tomography (CT):

5. Magnetic resonance imaging (MRI):

6. Micturating cystography and cystourethrography

7. Ascending urethrography

8. Retrograde pyeloureterography

9. Percutaneous renal procedures:

10. Arteriography

11. Venography

12. Conduitogram

13. Radionuclide imaging:

Intravenous excretion urography (IVU)

The technique is less frequently used than in the past and is now largely replaced by CT, MRI or US.

Contraindications

See Chapter 2 – general contraindications to intravenous (i.v.) water-soluble contrast media and ionizing radiation. In patients with contrast medium allergies alternative modalities such as ultrasound or MR can be considered. Patients with impaired renal function, particularly those with diabetes should be prepared with oral or intravenous hydration, or an alternative imaging modality considered. See Chapter 2.

Images

1. Immediate film. AP of the renal areas. This film is exposed 10–14 s after the injection (approximate ‘arm-to-kidney’ time). It aims to show the nephrogram at its most dense, i.e. the renal parenchyma opacified by contrast medium in the renal tubules. Tomography may assist in evaluation of the renal outline or possible masses (or ultrasound if subsequently available).

2. 5-min film. AP of the renal areas. This film gives an initial assessment of pathology – specifically the presence or absence of obstruction before administering compression.

    A compression band is then applied positioned midway between the anterior superior iliac spines, i.e. precisely over the ureters as they cross the pelvic brim. The aim is to produce pelvicalyceal distension. Compression is contraindicated:

3. 10-min film. AP of the renal areas. There is usually adequate distension of the pelvicalyceal systems with opaque urine by this time. Compression is released when satisfactory demonstration of the pelvicalyceal system has been achieved. If the compression film is inadequate the compression should be checked and repositioned if necessary and a further 50 ml of contrast medium administered and the film after 5 minutes.

4. Release film. Supine AP abdomen taken immediately after release of compression. This film is taken to show the ureters. If this film is satisfactory, the patient is asked to empty their bladder.

5. After micturition film. Full-length supine AP abdomen. The aims of this film are to assess bladder emptying, to demonstrate drainage of the upper tracts, to aid the diagnosis of bladder tumours, to confirm ureterovesical junction calculi and, uncommonly, to demonstrate a urethral diverticulum in females.

Ultrasound of the urinary tract

Technique

1. Patient supine, right (RAO) and left anterior oblique (LAO) positions or lateral for kidneys. The kidneys are scanned longitudinally in an oblique coronal plane supplemented by transverse sections perpendicular to the axis. The right kidney may be scanned through the liver and posteriorly in the right loin. The left kidney is harder to visualize anteriorly, but can be visualized from a lateral approach. In difficult cases the patient should lie on their side with a pillow under the left loin to widen the space between the rib cage and pelvis.

2. The length of the kidney measured by US is 1–2 cm smaller than that measured at excretion urography because there is no geometric magnification. With US measurement, care must be taken to ensure that the true longitudinal lengthwise measurement is obtained. The range of lengths of the normal kidneys is 9–12 cm, and the difference between each kidney should be less than 1–2 cm.

3. The bladder is scanned suprapubically in transverse and longitudinal planes. Measurements taken of the three orthogonal diameters before and after micturition enable an approximate volume to be calculated by multiplying the three diameters and applying a conversion factor. (A conversion factor is usually pre-programmed into modern ultrasound machines.)

4. Renal transplants are usually located in the right or left iliac fossa. These usually lie fairly superficially and are easy to evaluate using oblique planes and gentle pressure to displace overlying bowel loops.

5. The native or transplant kidneys can be evaluated for vascular pathology using Doppler techniques.

• Renal artery stenosis is diagnosed by direct Doppler interrogation of the main renal arteries from a transabdominal approach. Elevated peak systolic velocities greater than 200 cm s–1 are suggestive of a >50% stenosis. Alternatively, as the main renal arteries in the native kidneys are often hard to visualize, the intrarenal arteries can be evaluated from a flank approach for downstream changes in waveform – the tardusparvus pattern;1 a slow rise (tardus) to a reduced peak (parvus) producing a prolonged acceleration time (a value >70 ms is indicative of a severe stenosis).

• Renal vein thrombosis is diagnosed by absent colour Doppler venous flow, direct visualization of thrombus within the distended vein, and a raised resistive index with reversal of arterial diastolic flow within the intrarenal arteries.

Computed tomography of the urinary tract

Techniques

CT KUB

Plain CT (commonly referred to as CT KUB – kidneys, ureters, bladder) is useful to assess possible stone disease. It is used in many centres as the primary investigation of renal colic (replacing plain KUB radiograph):

CT urogram (CTU)

This technique uses a combination of unenhanced, nephrographic and delayed scans following i.v. contrast to sequentially allow examination of renal parenchyma and collecting systems.

Suggested protocol:

1. An oral water load of 500–1000 ml 30–60 minutes before injection is recommended to ensure a diuresis and collecting system dilatation. No positive oral contrast

2. Patient supine

3. Initial low-dose unenhanced scans of urinary tract (CT KUB) to determine if renal tract calculus disease is present

4. Low osmolar contrast material (LOCM) 300 mg I ml–1 100 ml is given as bolus intravenously

5. Thin-section (usually 1 mm) scans are obtained from diaphragm to lower poles of kidneys during nephrographic/parenchymal enhancement phase (100 s following start of bolus injection). Alternatively, scan may instead be acquired during the portal venous phase (70 s) but normal corticomedullary differentiation may make small tumours difficult to appreciate

6. Delayed thin-section (1 mm) scans are acquired from upper pole of kidneys to bladder base 20 min after contrast injection, to examine collecting systems and ureters

7. Source images are reviewed along with multiplanar reconstructions. Post-processing with maximum-intensity projections and surface-shaded displays may be helpful, especially for demonstration.

CT angiography

Magnetic resonance imaging of the urinary tract

Magnetic resonance imaging of the prostate

Technique/example protocol

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