Renal Transplantation : Imaging

Ultrasound

Ultrasound (US) remains the mainstay for imaging the kidney in both the short- and long-term management of the renal transplant, its advantages being its portability, cost and lack of ionising radiation and nephrotoxicity ( Fig. 30.2 ). The transplant kidney is ideally suited for US imaging; being superficially placed in the iliac fossa makes it possible for high-frequency probes to provide good-quality images. A full examination of the transplant relies upon a combination of greyscale and Doppler US. The graft length should be measured and evaluated for evidence of hydronephrosis, calculi or focal abnormality. The peri-renal tissues should be evaluated for fluid collections and the bladder examined for wall thickening, calculi or tumour. The orientation of the kidney can vary depending on surgical technique and should be established in the axial plane so that true longitudinal images can be obtained. The presence of ureteric stents should be noted and reported. The bladder should be evaluated both before and after micturition, particularly if there is hydronephrosis or urinary tract infection.

Doppler US should be routinely used. Colour flow and spectral Doppler evaluation of the intra-renal blood flow and the extra-renal vessels should be undertaken as a postoperative baseline and if there is a history of hypertension. The spectral pattern of the normal renal transplant artery is similar to that in the native renal artery. Various measurements of the Doppler trace can be made, although there is limited clinical value in performing extensive measurements as they tend not to be specific enough to differentiate between rejection and acute tubular necrosis (ATN). The normal trace is a low-resistance circulation (ski slope pattern) with the diastolic flow measuring approximately a third of the systolic velocity. A reduction in the diastolic velocity is usually a sign of a pathological process. The resistive index (RI) is a useful simple measurement for qualitative analysis, and is useful for communicating with clinicians. RI is measured by

RIs exceeding 0.8 in the kidney transplant allograft have been defined as abnormal; however, multiple researchers have documented RI's lack of specificity considering the complex interaction between renal vascular resistance and compliance. Therefore, clinical assessment and associated sonographic findings remain integral in the assessment of graft dysfunction. The role of RIs may be useful when monitoring the trends rather than in isolation.

In addition to intra-renal measurement of Doppler flow, a systematic assessment of colour flow in all regions of the kidney should be performed to assess differential blood flow. Although evaluation of colour flow is subjective, it is a useful tool and reassuring when normal. This is particularly important when multiple renal arteries are present at the time of transplant.

Recent advances include the application of contrast-enhanced US to renal transplant assessment. The technique can provide information on the microvascular renal blood flow with the timing of inflow of contrast agent being a measurable variable, which may help to separate cases of ATN from acute rejection (the latter having a longer inflow time). Elastography has been widely used in the assessment of liver fibrosis. The application of this technique to renal transplant assessment remains the subject of research, with the focus being on whether measured parenchymal stiffness offers an acceptable alternative to histological measures. This may prove to be an interesting area for development in the future.

Finally, as with all imaging techniques for assessing a renal transplant, it is important to evaluate the native kidneys, especially if the patient is being investigated for recurrent urinary tract infection, as these may be a source of sepsis or stones. Similarly, the investigation of haematuria post-transplant must include an assessment of the native kidneys for hydronephrosis or renal mass.

Computed Tomography

Computed tomography (CT) is useful in the further evaluation of transplant complications where US cannot fully demonstrate an abnormality due to overlying bowel gas or when patients become systemically unwell. CT can be used to confirm urological, nephrogenic and vascular complications that have been detected by US, or where US has not been able to sufficiently exclude such abnormalities ( Fig. 30.3 ). A dedicated renal CT angiographic or urographic phase protocol with multiplanar reconstruction or a biphasic examination may be required to answer these questions. CT can provide excellent quality images to help guide diagnosis of the full range of complications relating to renal transplantation, with the added advantage of being accessible, quick and cost-effective. The disadvantage of CT relates to the risks associated with ionising radiation and nephrotoxic contrast agents, which are relevant in cases involving younger patients, patients with significant transplant renal dysfunction or in patients requiring repeated follow-up imaging.

Magnetic Resonance Imaging

Magnetic resonance (MR) imaging can provide a valuable alternative to CT in cases where repeated radiation exposures or contrast agent administration are undesirable. As access to MR has improved, MR protocols have been developed that include MR angiography, urography and renography, which now provide excellent capability in imaging cases of obstructive, vascular, nephrogenic and extra-renal transplant complications ( Figs 30.4 and 30.5 ). MR angiography is an evolving technique that can be used to evaluate the transplant artery and vein. It is used predominantly when US evaluation is inconclusive. There are two main techniques of MR angiography—unenhanced and contrast-enhanced. Contrast-enhanced MR angiography relies upon the administration of gadolinium agents, which were widely used in the past in patients with renal dysfunction in preference to iodinated contrast agents. Such gadolinium-based agents are now contraindicated in patients with renal failure or severe dysfunction (i.e. with an estimated glomerular filtration rate of less than 30) due to the risk of nephrogenic systemic fibrosis. Contrast-enhanced MR angiography also relies upon a coronal three-dimensional (3D) fast multiplanar gradient-echo breath-hold sequence. This is performed after a timing bolus of contrast medium.

Many techniques are used to perform unenhanced MR angiography such as time-of-flight (TOF), phase-contrast or steady-state free-precession (SSFP) sequences. TOF exploits the inflow effect of blood protons and uses flow-compensated 2D or 3D gradient-echo sequences. Phase contrast is independent of direction but is motion susceptible and time-consuming. Turbulent flow can cause loss of signal and may result in overestimation of narrowing.

Newer techniques that appear to offer significant improvement in image quality have been recently introduced. SSFP with or without arterial spin labelling can provide high-resolution images of blood vessels, including renal arteries, without the use of contrast agents. It relies upon a gradient-echo-based sequence that maintains steady-state longitudinal and transverse magnetisation by applying serial equidistant radiofrequency pulses. SSFP enables a short acquisition time and produces high signal-to-noise images that are independent of flow and direction. The images are susceptible to field heterogeneity.

Compared with CT angiography, MR angiography has a number of disadvantages including an increased time of imaging, a limited range and a range of complex artefacts.

Catheter-Based Angiography and Fluoroscopy

Conventional angiography is usually reserved as a prelude to intervention, or in cases where US, CT and/or MR imaging are unable to exclude or define an abnormality ( Fig. 30.6 ).

Cystography may be required in the evaluation of bladder abnormality immediately after surgery to evaluate bladder injury, or at a later date to investigate reflux following recurrent pain or infection ( Fig. 30.7 ).