Urinary system imaging
Imaging of the kidneys and urinary tract are done with different techniques depending mainly upon clinical symptoms and signs. Ultrasound is fast, and a very useful screening-tool in experienced hands. It has a high sensitivity to detect focal pathological changes mainly in the kidneys and bladder. Urography is frequently used to detect calculi in the collecting system and may provide some information about functional impairment of the kidney. CT is able to characterise mass lesions in the renal parenchyma and to detect tiny calcified stones not detected otherwise. MRI shows promise in detecting abnormalities in the pelvis, especially in the prostate. NM is used for kidney function studies and to evaluate the significance of urinary outflow tract stenosis.
Stones in renal parenchyma or collecting system
Stones (calculi) contain varying amounts of calcium. The higher the calcium content, the better they can be seen on X-ray. Stones can be detected with plain X-ray, but it is not possible to determine the precise location of the stone with this technique, or if it is obstructing part of the renal pelvis or urinary tract further downstream (ureter).
Urography, which is a X-ray examination with injection of an iv contrast medium, is still the most widely used examination. Urography gives a detailed visualisation of the renal calyces, pelvis, ureter and urinary bladder, because the injected contrast medium is excreted by the kidneys into the collecting system after just a few minutes. A calculi (or any other space-occupying lesion for that matter), will show up as a kind of a filling defect. Urography can rule out obstruction or assess degree of obstruction, which is a very important issue to recognise when treating the patient.
CT without contrast media is increasingly used to detect smaller stones with less calcium not visualised otherwise.
Ultrasound can detect stones above about 3-4 mm in the kidneys or pelvis, but is, for practical purposes, insensitive regarding to stones more downstream. On the other hand, ultrasound can, in most cases, rule out any significant obstruction resulting in distension of the collecting system. Persistent obstruction will in time damage the kidney function on the side in question. This can be monitored by NM split function studies.
Tumours and masses in the renal parenchyma and collecting system
Renal cysts (benign and very common lesions) are readily detected and often also classified by ultrasound. Assuming good visualisation of the kidney, ultrasound is a sensitive tool to detect tumours of renal origin. Normal ultrasound findings in a well visualised kidney rule out presence of tumours. Real life is, however, a little more complex. Because there are some pit-falls in the interpretation (e.g. anatomic variants), additional CT examinations will be performed if there is any doubt.
If a true tumour or suspicious lesion is detected, CT will always be performed, as this modality gives an overview and can characterise tumours regarding their precise location, involvement of capsule or collecting system and invasiveness into neighbouring structures and vessels.
Small tumours in the renal pelvis are, as a rule, well detected by urography. So if presence of tumours is strongly suspected in spite of normal findings with ultrasound and CT, urography will be performed. On the other hand, small tumours in the parenchyma are not detected with urography.
Tumours in urinary bladder
Small tumours and polyps are often detected by ultrasound (only larger ones with urography)
If suspicious symptoms persist and diagnostic imaging is normal, cystoscopy is performed. This is an examination with a fibre-optic device done by a urologist which allows a direct visualisation of the mucous membrane.
Prostate tumours
Ordinary trans-abdominal ultrasound, CT, or urography can not rule out tumours.
In practice there are two imaging options: a special ultrasound procedure with a probe inside rectum, or with MRI. The former is widely used by urologists and some radiologists, and has quite a high sensitivity to detect small lesions. In addition the technique is very suitable for guidance of fine-needle biopsies. MRI is somewhat more experimental.
Infections
Different imaging modalities can rule out predisposing causes for urinary tract infections, but (except in rare conditions) the infection per se can not be visualised. Some predisposing factors are: congenital abnormalities of the collecting system like obstruction and duplications, stone disease with or without obstruction, contemporary tumours.
Especially in recurrent upper urinary tract infections it is important to do diagnostic imaging to rule out occult predisposing disease.
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