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.
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.
Table 23.2 Radiation dose (in mSv) of imaging modalities.
| Type of imaging | mSv |
|---|---|
| Chest X‐ray | 0.02 |
| Abdominal X‐ray | 0.07 |
| IVU | 3 |
| CT KUB (low dose) | <3.5 |
| CT KUB (ultra‐low dose) | <1.9 |
| CT abdomen and pelvis (no contrast) | 10 |
| CT abdomen and pelvis (contrast) | 20 |
| CT Urogram | 15.9 |
Intravenous urogram (IVU) or intravenous pyelogram (IVP) is now becoming somewhat historic, having been supplanted by the superior sensitivity and specificity of CT for the assessment of ureteric colic. The IVU protocol consists of a pre‐contrast control, followed by administration of intravenous (IV) contrast and series of plain KUB X‐rays to assess the uptake of contrast into the kidneys and the excretion. The contrast delineates the shape of the kidney (nephrogram) and can demonstrate hydronephrosis and delayed drainage via a ‘standing column’ of contrast in a poorly draining ureter.
A single shot IVU continues to have a role in the operating theatre for ‘on table’ investigation of renal trauma and suspected collecting system injuries and can occasionally be useful in Shockwave Lithotripsy to help identify the location of a ureteric stone at the distal‐most point of a ‘standing column.’
CT KUB is a non‐contrast, low‐dose CT scan that is used most commonly for the identification of nephrolithiasis. CT KUB offers near 99% sensitivity for urinary tract calculi and allows assessment of concomitant hydronephrosis and hydroureter.
CT KUB allows for reasonable assessment of urinary tract anatomy, and for patients with a contra‐indication to intravenous contrast (e.g., chronic kidney disease) it remains a useful investigation for presentations of other conditions such as haematuria and urinary tract sepsis.
CT urography is employed most commonly for the investigation of visible haematuria and involves three scan phases. A non‐contrast phase (CT KUB), a further scan sequence at 60–90 seconds post‐injection of contrast and a delayed scan sequence at approximately 10–15 minutes. At 60–90 seconds, the uptake of IV within the renal parenchyma produces a ‘nephrographic phase.’ It is in this phase that renal masses may be identified. The delayed sequence allows clinicians to visualise the drainage of the contrast from the kidney to the bladder and can identify filling defects, hydroureter, or delayed drainage.
This scan protocol is used to characterise renal lesions. There is a pre‐contrast phase followed by three further phases: the cortico‐medullary phase, the nephrogenic phase, and excretory phase. The cortico‐medullary phase takes place 25–40 seconds after injection of contrast. The degree of uptake of contrast within a lesion (seen as increased ‘brightness’) is defined as ‘enhancement.’ A change of greater than 20 HU is considered significant. The nephrogenic scan sequence; taken 100 seconds post contrast, allows visualisation of the vascularity of the lesion as well as presence of thrombus within the vein. As with a CT urogram (CTU), the delayed excretory phase allows delineation of the entire urinary tract and is useful in patients where transitional cell carcinoma is suspected within the collecting system.
In patients with significant malignancy, contrast CT scans of the chest, abdomen, and pelvis are performed in order to stage the cancer. Staging is important to determine whether the disease is confined to an organ and thereby establish treatment and prognosis.
Imaging guided percutaneous procedures provide an important tool in diagnostic and interventional urology. Procedures may be ultrasound, fluoroscopy, or CT guided. In most centres the renal procedures are performed by radiologists.
Widely shunned for many years due to concerns about seeding, we are now seeing an increase in renal biopsy. Given the number of renal masses being identified incidentally, especially in younger patients, it offers the benefit of avoiding nephrectomy (partial or radical) in those that are found to benign. This is further discussed in Chapter 20, Renal Cancer.
These are normally ultrasound guided. They are not commonly performed due to the high risk of recurrence as well as the small risk of seeding if the cyst is incorrectly characterised. All cysts must be characterised on CT using the Bosniak classification before aspiration is considered.
Aspiration and sclerosant instillation should be reserved for those who are symptomatic with very large cysts but who are not candidates for surgery either due to patient choice or fitness.
A nephrostomy is a drain placed percutaneously directly into the renal collecting system. It is sited by interventional radiologists under ultrasound and fluoroscopic guidance.
Common indications for nephrostomy insertion include renal obstruction secondary to malignant conditions, ureteric injuries, and impassable structuring of the ureter. Nephrostomy represents a valuable ‘rescue’ option where retrograde stenting has failed.
If a guide‐wire can be advanced into the bladder via a nephrostomy tract, ‘antegrade’ ureteric stent insertion can be attempted.
Nuclear medicine (NM) scans rely on radioactive tracers injected into the body. As the tracer decays, radiation is emitted and can be detected. This allows sensitive measurements of the quantity of tracer within the renal tract, based on the radiation emission and therefore accurate representation of renal uptake and function as well as excretion.
The most commonly used tracer isotope in urology is technetium 99, which decays to emit gamma radiation.
The use of radioactive tracers does expose the patient to a small amount of radiation that does minimally increase their cancer risk. There is a small risk of allergy to the tracer. Nuclear medicine scans are not suitable for those who are pregnant, trying for pregnancy, or breast feeding.
Relying on the tracer 99mTc labelled Mercapto‐Acetyl Triglycine (MAG3) renograms are dynamic scans that allow for the assessment of renal uptake, processing, and excretion.
It is used to diagnose functional renal obstruction, but can also identify ureteric reflux. MAG‐3 provides an estimation of split renal (right vs left) function but this is not as accurate as a dimercaptosuccinic acid (DMSA) (see next section). Perhaps the most common use is for patients with pyelo‐ureteric junction obstruction (PUJO) or for assessment of outcomes in those who have undergone previous pyeloplasty.
Like MAG 3, DMSA is labelled with 99mTc. Unlike MAG3, it is not excreted by the proximal tubules and the image obtained is a static one. By obtaining an image at three to four hours post‐injection, clinicians are able to quantify the number of functioning nephrons in each kidney relative to the other side.
DMSA scans are useful for assessing split function and for monitoring for the presence of scars where nephrons may have been damaged. DMSAs may be used in patients with stag horn calculi or long standing PUJO where benign nephrectomy is being considered, or in those with renal lesions for whom a radical or partial nephrectomy is being pursued.
Another static scan, bone scans are used in urology for assessment of prostatic bony metastases. Patients are injected with technetium labelled methylene diphosphonate (MDP). Methylene diphosphonate is preferentially taken up in areas with increased osteoblastic activity such as metastatic deposits.
A positron emitting tomography (PET) scan or PET/CT scan uses radioactive tracers to identify areas of increased or altered metabolism. It is widely used in the identification and surveillance of malignancy as well as assessing response to treatment. Combining PET scans and CT scans offers both metabolic and anatomical detail. The most widely used radiotracers for PET scans in urology are 18F‐fluorodeoxyglucose (FDG) and 11c‐choline. These radio‐isotopes decay emitting positrons, and as these travel through tissues they slow down. As they slow, they are able to interact with electrons that destroy both of them and produce gamma photons. These gamma photons are detected by a gamma camera. FDG is used in the assessment for metastases in renal and bladder cancer, as well as the staging and spread of testicular cancer. Choline PET can be used for the diagnosis, staging, and surveillance of prostate cancer.