Renal transplantation has been one of the triumphs of modern surgery and medicine with recipients anticipating and 90-95% graft survival rate. The procedure has almost become a victim of its own success such that demand outstrips supply in almost all parts of the world. One result is that surgeons are prepared to attempt to graft kidneys that would have been rejected as too challenging a decade or so ago, particularly those with multiple arteries. In addition, the development of new immunosuppressives, namely the fungal agents of the calcineurin inhibiting family (e.g. ciclosporin and tacrolimus) that reduce T-cell activation, have improved graft survival by almost eliminating acute rejection. Perhaps ironically, these developments have made the task of imagers more demanding, partly because of an increased workload, but mainly because of a demand for early and reliable diagnosis of vascular problems. In addition, in the important subgroup of diabetic end-stage renal failure patients, combined renal and pancreatic transplants are often performed. In these patients, ultrasound assessment of the pancreatic transplant is also required.
Since the transplant is usually quite superficial, high-resolution transducers can be used and this means that the anatomy is well displayed. It also means that pathological changes are readily demonstrable and Doppler information on flow is easily obtained. Anatomical abnormalities such as changes in size, perinephric collections, dilatation of the collecting system and the layout of the vasculature are routinely detectable. However, a disappointment is the difficulty of interpreting spectral Doppler traces, which have not lived up to their initial promise.
Ultrasound is very good at measuring renal size, detecting dilatation of the collecting system, following perinephric collections, assessing arterial and venous occlusions and in detecting new pathology of the transplant kidney, all clinically useful attributes that underlie its choice as the first, and often the only, imaging test required. It is also the best means of guiding transplant biopsies and insertion of nephrostomy catheters.
Ultrasound can detect arterio-venous fistulas and the end-stage features of the failed transplant, usually because of chronic rejection. However, since these findings do not affect clinical management in most cases, they are clinically irrelevant, though often interesting findings.
A major weakness of ultrasound in renal transplantation is its inability to help differentiate between the causes of failure in the early post-operative phases. Biopsy is still required to distinguish acute rejection from acute tubular necrosis and serum levels of immunosuppressants are required to diagnose over-dosage with immunosupressants.
Two recent technical developments promise to improve ultrasound's effectiveness in renal transplant studies, as well as elsewhere. Microbubble contrast agents are now well established for the detection and characterisation of focal liver lesions in many parts of the world and the technology, both concerning the microbubbles and the scanning methods, has reached a high level of refinement. There has been little work reported in renal transplants but in the native kidneys, contrast is helpful in characterising complex cysts because it allows the microcirculation to be depicted - true cysts are completely devoid of signals whereas cystic tumours have some signals. The same seems likely to be true in transplants. However, contrast has been disappointing in the differentiation of solid lesions because of overlap in the appearances between, for example, angiomyolipomas and carcinomas. It has proved useful in renal trauma where the perfusion defects are sharply etched and in transplants, personal experience indicates a supporting role to Doppler in defining ischaemia caused by branch or accessory artery occlusion.
Microbubbles allow functional assessments by analysis of the time-intensity curves when a bolus injection is given or when microbubbles are destroyed in a scanning plane by turning up the MI for a few seconds and then observing the refill of the slice. The filing phase has an exponential shape whose slope relates to blood inflow rate while the maximum value achieved relates to the fractional vascular volume. These are important haemodynamic features and their product is related to true tissue perfusion. Though not well studied yet, there is hope that this rich information might further improve the usefulness of ultrasound in renal transplants.
The proximity of the renal artery and vein allows a spectral Doppler gate to straddle both, so that the time taken for an injected contrast bolus to cross the renal circulation can be measured. This is the true transit time which can be expected to change in diseases affecting the kidney's blood vessels as occurs in many diffuse diseases. Again, this has not been tested in practice, and so remains an intriguing possibility.
Elastography is another important development in ultrasound. Here a mechanical force is applied to the tissue (the stress) and the tissue's response, i.e. how much it distorts (the strain) is measured, usually by a speckle-tracking algorithm working in two dimensions. The stress can simply be applied by moving the transducer, which could work well in a transplant kidney, or the acoustic radiation force from a conventional transducer can be used to create a shear wave that propagates laterally from the pushing beam. In this way, the stress force is generated within the tissue. Essentially, elastography depicts or measures tissue stiffness, which is increased in many pathologies, both focal and diffuse. It has been applied to the breast and prostate, with good results. It will be of great interest to see it applied to transplant kidneys.