23
Plain X‐Ray, Computed Tomography Scanning, and Nuclear Imaging in Urology

Tharani Mahesan

Imaging and radiological investigation are important tools in the urologist’s armamentarium, and access various modalities and sound working theory for their usage is key to running an ambulatory service. Historically X‐rays were the most widely used imaging modality in urology, however in recent decades computed tomography (CT) scanning is often preferred to ‘plain’ X‐ray imaging. An X‐ray is a type of transmission radiology in which an electromagnetic beam is passed through the body. Tissue‐ energy reactions alter the beam as it is transmitted and energy is absorbed by different tissues, to differing degrees. This varied absorption leads to production of an image at a detector or plate, but could be considered as taking a ‘measurement’ of those differing tissues using X‐ray absorption.

Computerized tomography (CT) scanning employs an X‐ray transmission source and detector that rotate about the patient, essentially taking multiple X‐ray ‘measurements’ from multiple angles. This data is then compiled, reconstituted, and reconstructed as cross‐sectional imaging.

Computerized tomography scanning allows for measurement of tissue or structure density and this is measured in Hounsfield units (HU). The higher the HU, the ‘brighter’ a structure appears on CT. This linear scale assigns the tissue a score relative to distilled water at standard pressure and temperature (being 0 HU) and air at standard pressure and temperature (being −1000 HU).

The Hounsfield scale is only applied to the density of tissues on medical CT scans. (See Table 23.1.)

Non‐contrast CT scanning of the kidneys, ureters and bladder (so‐called CT KUB) is now the gold‐standard imaging modality for suspected ureteric colic. For other diagnoses, the additional use of iodinated contrast allows for further enhancement and delineation of the entire urinary tract, which can assist in identifying mass lesions, ‘filling defects’ or causes of ureteric obstruction. The use of intravenous contrast agents can allow some determination of the function of the kidney; however nuclear medicine (NM) imaging is a far superior modality for this purpose.

Table 23.1 Hounsfield values of tissues on CT scan.

Tissue	HU
Fat	−120 to −90
Bone	+1800 to 1900
Kidney	+20 to +45
Blood	+13 to +50
Blood clot	+50 to +75
Urine	−5 to +15

Clinicians need to be mindful that use of X‐ray and CT is not without risk. As radiation passes through the body it is absorbed. The effect of ionising radiation on human tissues is measured in Sieverts, a derived unit that is representative of the stochastic health risk attached to the radiation. Medical scans typically have their radiation effects defined in millisieverts (mSv). It is worth noting that some tissues absorb more radiation than others. This can mean that the effective dose of radiation (whole body radiation absorbed) is higher for certain studies. (See Table 23.2.)

The ALARA (As Low As Reasonably Achievable) principle should be kept in mind when considering the necessity for use of ionising radiation for the purposes of investigation. In younger patients particularly, it should be considered whether ultrasound could reasonably answer the diagnostic question instead of an X‐ray based scan. Furthermore, intravenous administration of iodinated contrast also poses its own risks – largely due to its nephrotoxicity. Patients who take metformin are at risk of developing metabolic acidosis, but this risk is dependent on level of renal function and volume of contrast given. Radiology departments will have protocols for either omitting metformin prior to or after a scan to reduce this risk. In some cases, it may be safe to continue taking metformin. Anaphylactoid reaction to injected contrast media is a rare but serious event. Previous reactions to IV contrast present a contraindication to a further contrast CT scan.

X‐ray

An X‐ray of the KUB can be used to look for the presence of renal or ureteric calculi. Although around 90% of renal stones are radio‐opaque, most studies confirm the sensitivity of plain KUB X‐ray to be around 50% for detecting stones. Due to the speed and simplicity of plain X‐ray, however, this modality is still commonly used for re‐assessment of a known stone burden or to demonstrate the passage of a known ureteric calculus.

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