# Renal Transplant Evaluation with Nuclear Medicine

## Overview

Nuclear medicine plays a critical role in evaluating renal transplant function, particularly in the early post-operative period. Tc-99m MAG3 dynamic renography is the primary modality for transplant assessment. It evaluates perfusion, tubular function, collecting system drainage, and urine leak in a single study.

## Transplant Anatomy

The transplant kidney is placed in the iliac fossa on the right or left side, anastomosed to the external iliac artery and vein. The ureter is implanted into the bladder via ureteroneocystostomy. The superficial location of the transplant kidney allows excellent imaging with a gamma camera.

## Radiopharmaceutical and Technique

Tc-99m MAG3 is the preferred agent because of its high extraction efficiency and excellent image quality. The dose is 5 to 10 mCi administered intravenously. Dynamic acquisition uses 1 to 3 second frames for the first 60 seconds (flow phase), followed by 15 to 30 second frames for 30 minutes. The camera is positioned over the transplant kidney and bladder. Regions of interest are placed over the transplant kidney, iliac artery (for the input function), and bladder.

<image>Normal Tc-99m MAG3 transplant renogram showing prompt perfusion, rapid cortical uptake peaking at 4 minutes, and brisk excretion into the bladder with a normal renogram curve</image>

## Normal Transplant Renogram

During the flow phase, transplant kidney perfusion should appear within seconds of iliac artery visualization. The functional phase shows rapid cortical uptake with a peak at 3 to 5 minutes. The excretory phase demonstrates prompt drainage into the ureter and bladder. The transplant perfusion index, defined as the ratio of kidney-to-iliac artery peak counts, should be close to 1.0. A normal transplant renogram essentially mirrors the pattern of a normal native kidney.

## Post-Transplant Complications

### Acute Tubular Necrosis (ATN)

ATN is the most common cause of delayed graft function, especially in deceased donor transplants. Scintigraphic findings include preserved or mildly reduced perfusion with markedly impaired cortical uptake and excretion, producing a cortical retention pattern. On delayed images, the kidney appears "hot" because tracer is retained in the parenchyma while minimal or no urine output reaches the bladder. The renogram curve shows a prolonged rising phase with minimal decline. ATN is expected to improve over days to weeks with supportive care, including dialysis as needed. Serial studies demonstrate gradual improvement in function.

### Acute Rejection

The scintigraphic findings of acute rejection overlap significantly with ATN. Both show decreased perfusion and reduced cortical uptake and excretion. However, key differentiating features include more pronounced perfusion impairment in rejection compared to ATN, and a pattern of progressive deterioration on serial studies (unlike ATN, which improves). Clinical context is important: rejection usually occurs after the first week and is accompanied by fever, graft tenderness, and rising creatinine. Biopsy remains the gold standard for distinguishing ATN from rejection.

### Urine Leak

Urine leak is a post-surgical complication resulting from ureteroneocystostomy dehiscence. On scintigraphy, progressive accumulation of tracer is seen outside the collecting system and bladder, appearing as a crescentic or irregular area adjacent to the kidney or along the expected ureteral course. Tracer may be visible in the peritoneal cavity or perinephric space. Delayed images at 30 to 60 minutes best detect this complication. SPECT/CT improves localization of the leak site.

### Obstruction

Ureteral obstruction from stricture, blood clot, or extrinsic compression produces progressive hydronephrosis on the renogram with delayed or absent drainage despite good cortical function. Activity is retained in a dilated collecting system. Diuretic renography can be performed to confirm obstruction using the same principles as for native kidneys.

<image>Comparison of Tc-99m MAG3 transplant renograms in ATN (preserved perfusion, cortical retention, no excretion) versus acute rejection (decreased perfusion and cortical uptake) versus normal function</image>

| Complication | Perfusion | Cortical Uptake | Excretion | Key Feature |
|---|---|---|---|---|
| Normal | Prompt | Rapid (peak 3–5 min) | Brisk to bladder | Perfusion index ~1.0 |
| ATN | Preserved/mildly reduced | Markedly impaired | Absent/minimal | "Hot kidney"; improves on serial studies |
| Acute rejection | Decreased | Reduced | Reduced | Progressive deterioration; biopsy needed |
| Urine leak | Normal | Normal | Extravasation | Activity outside collecting system |
| Obstruction | Normal | Preserved | Delayed/retained | Dilated collecting system |
| Renal vein thrombosis | Initially present then decreasing | Absent | Absent | "Cold kidney"; surgical emergency |

### Renal Artery Stenosis (Transplant)

Transplant renal artery stenosis occurs at the anastomosis site. Scintigraphic findings include decreased or delayed perfusion, a reduced perfusion index, and possibly a cortical retention pattern. Captopril renography can assess the functional significance of the stenosis using the same principles as for native kidney renal artery stenosis. Doppler ultrasound is usually the first-line screening test.

### Renal Vein Thrombosis

Renal vein thrombosis is a surgical emergency occurring early post-transplant. Perfusion may be initially present but progressively decreases. Absent function and excretion produce a photopenic or "cold" kidney on delayed images. Urgent Doppler ultrasound and surgical intervention are required.

### Peritransplant Fluid Collections

Fluid collections including lymphocele, hematoma, urinoma, and abscess appear as photopenic areas adjacent to the transplant. A urinoma fills with tracer over time, confirming a urine leak. Lymphocele and hematoma remain photopenic throughout the study.

## Perfusion Index

### Calculation

The perfusion index is the ratio of transplant kidney peak counts to iliac artery peak counts during the flow phase. Normal values are approximately 1.0 or within a site-specific reference range. An elevated perfusion index above 1.5 to 2.0 suggests impaired perfusion from rejection or vascular compromise. Trending the perfusion index over serial studies is more useful than a single value.

### Hilson Perfusion Index

The Hilson perfusion index divides the area under the aortic/iliac curve by the area under the kidney curve during the first 1 to 2 minutes. Higher values indicate worse perfusion. Some centers use this for quantitative assessment.

<image>Tc-99m MAG3 transplant renogram showing a urine leak: progressive tracer accumulation in the peritransplant space (arrow) separate from the bladder, confirmed on delayed imaging at 60 minutes</image>

## Serial Monitoring

A baseline renogram within 24 to 48 hours post-transplant establishes a reference. Repeat studies at days 3 to 7 assess for improvement (favoring ATN) or deterioration (suggesting rejection). Improving renogram curves on serial studies favor ATN, while worsening curves despite treatment suggest ongoing rejection or other complications. Long-term renography can assess chronic allograft nephropathy.

## Clinical Pearls

The primary role of nuclear medicine in transplant evaluation is distinguishing between causes of delayed graft function, particularly ATN versus rejection.

Serial studies are far more informative than a single scan. Trending the perfusion index and renogram pattern guides clinical management.

A "hot kidney" with retained cortical activity and no bladder activity is the classic ATN pattern and should improve on follow-up scans.

Urine leak is best detected on delayed images. Imaging should always be extended to at least 30 minutes if a leak is suspected.

Biopsy remains the gold standard for diagnosing rejection. Scintigraphy provides functional information that complements but does not replace biopsy.

Peritransplant photopenic collections that fill with tracer over time indicate urinoma. Those that remain photopenic are lymphocele or hematoma.

## References

- Taylor, A. T. "Radionuclides in Nephrourology: Part 1 and Part 2." *Journal of Nuclear Medicine*, 2014.
- SNMMI Procedure Standard for Renal Transplant Scintigraphy.
- Defined Role of Nuclear Medicine in Renal Transplant Evaluation. *Seminars in Nuclear Medicine*, 2014.
- Defined Role of MAG3 Renography in Transplant Dysfunction. *Clinical Nuclear Medicine*, 2016.