Role of Ultrasound In Renal Transplantation

Dr. Ahmed RefaeyConsultant Radiologist

Prince Sultan Military Medical City

• Ultrasound is often the initial diagnostic modality as it is noninvasive, relatively inexpensive, does not require intravenous contrast, can be obtained at the bedside, and can often rapidly and accurately depict many of the common complications

Normal anatomy

• Knowledge of the exact renal transplant procedure performed is essential for accurate interpretation of both normal and abnormal findings. Particularly important is knowledge of the vascular anatomy, so that all vessels and anastomoses can be evaluated for patency, stenosis or other complications.

* Vascular supply from end-to-sideanastomosis of donor artery and vein to external iliac artery and vein.

• If multiple arteries, usually joined with single anastomosis to EIA or can be anastomosed separately to the external iliac artery

• Ureter anastomosed to superolateral wall of urinary bladder

• The transplanted kidney is usually placed in an extraperitoneal location in the right or left iliac fossa

• The superficial location makes it ideal for US evaluation.

•

• Normal color doppler findings :

- arteries : brisk upstroke, low resistance with normal RI of 0.6 – 0.75

- normal velocity of main renal artery < 200 cm/s

- veins : may be monophasic with continous flow, or demonstrate some pulsatility with cardiac cycle.

Renal artery Doppler flow pattern

• The transplanted kidney is a solitary functioning kidney, so there is usually a physiological hypertrophy, 15% in first 2weeks and may increase by 40% in first 6 months.

US Evaluation of Complications of Renal Transplantation

• Classically, the complications affecting the transplanted kidney can be categorized as:

- anatomic - functional - vascular.

Complications Anatomic

- Perinephric fluid collections

- hydronephrosis - parenchymal

masses.

Functional • Rejection• Drug toxicity• Acute tubular necrosis

- Renal artery thrombosis - Renal vein thrombosis - Renal artery stenosis - Renal vein stenosis - Arteriovenous fistulas - Pseudoaneurysms.

Vascular

Anatomic Complications

- Perinephric fluid collections - hydronephrosis - parenchymal masses.

Perinephric Fluid collections

• Hematoma• Urinoma• Lymphocele• Abscess

• found in ≤50% of renal transplants.• The clinical relevance of a fluid collection depends

on its composition, size, location and whether or not it is exerting mass effect on the transplant kidney, ureter or other adjacent structures

• Mass effect from perinephric fluid can result in: -hydronephrosis -kinking of the vascular pedicle -edema of the leg, abdominal wall, labia or

scrotum.

• Hematoma

- often present in the immediate postoperative period ≤2 weeks after surgery

- usually located either in the subcutaneous tissue , or around the transplant

- the sonographic characteristics vary with age - acute and chronic hematoma : echogenic - intermediate hematoma: fluid filled, internal

septations.

• Urinoma

- often present in the immediate postoperative period ≤2 weeks after surgery

- serious complication, usually caused by a defect in the uretrovesical anastmosis

- appear as well-defined anechoic collections without septations, unless infected or mixed with blood

• Lymphocele

- a more delayed complication, occurring 4 to 8 weeks after surgery

- usually located between the bladder and the kidney

- cystic, but a majority tend to have septations - due to disruption of the adjacent lymphatic

channels

• Abscess

• uncommon, but can occur in the early postoperative period due to pyelonephritis or bacterial seeding of a urinoma, hematoma or lymphocele

• Suspected when the patients presents with fever and increased WBCs

• Sonographically, can vary from an echo-free to complex echopattern.

Anatomic Complications

- Perinephric fluid collections - hydronephrosis - parenchymal masses.

• Either due to extrinsic compression ( perinephric fluid collection )

• or due to renal calculi, clot, anastomotic edema and ureteral stenosis .

• It should be noted that anastomotic edema often results in transient hydronephrosis of the transplanted kidney.

• Also, apparent hydronephrosis may be the result of an increased hydrostatic pressure due to a full bladder; evaluation after voiding can avoid diagnostic error in this setting.

Hydronephrosis

Parenchymal masses• Focal parenchymal lesions in the renal transplant,

whether hypoechoic or hyperechoic, are non-specific findings

• Differential considerations include: - focal pyelonephritis - hematoma - abscess - infarction - renal cell or transitional cell carcinoma - post-transplantation lymphoproliferative disorder (PTLD)

Anatomic Complications

- Perinephric fluid collections - hydronephrosis - parenchymal masses.

Functional Complications

• Rejection• Drug toxicity• Acute tubular necrosis

• Ultrasound plays a more limited role in the evaluation of functional complications.

• very difficult to distinguish from one another by imaging criteria alone.

Rejection

• Hyperacute rejection• Acute rejection• Chronic rejection

Rejection

- Hyperacute rejection : no role since the diagnosis is typically made immediately after transplant while still in the operating room

• Acute rejection

- Acute rejection takes several days to develop and peaks at 1 to 3 weeks after transplant

- findings have been shown to be unreliable in its diagnosis. In cases of severe acute rejection, the transplanted kidney becomes edematous , globular, hypoechoic mass with poor differentiation of the central renal sinus fat with elevation of the resistive index

Acute transplant rejection• Enlarged, globular,

hypoechoic renal transplant with loss of the normal corticomedullary differentiation and ill definition of renal sinus fat due to severe edema

Acute rejection

• Spectral doppler image of a segmental artery reveals a mildly increased resistive index due to parenchymal edema

Chronic rejection

- most common cause of late graft loss - begins 3 months after the transplantation. - US: cortical thinning , mild hydronephrosis ,

prominent sinus fat, dystrophic calcification, decreased color, normal or increased RI.

Acute tubular necrosis

• More common than rejection• Little sonographic change in parenchyma

pattern.• ATN occurs in the immediate post transplant

period as a result of ischemia, thus more commonly seen in cadaveric transplants

Acute tubular necrosis

• In summary, most cases of functional complications have non-specific imaging findings consisting of parenchymal edema and elevated resistive indices and require tissue analysis with renal biopsy for diagnosis.

Complications Anatomic

- Perinephric fluid collections

- hydronephrosis - parenchymal

masses.

Functional • Rejection• Drug toxicity• Acute tubular necrosis

- Renal artery thrombosis - Renal vein thrombosis - Renal artery stenosis - Renal vein stenosis - Arteriovenous fistulas - Pseudoaneurysms.

Vascular

Vascular Complications

- Renal artery thrombosis - Renal vein thrombosis

- Renal artery stenosis - Renal vein stenosis

- Arteriovenous fistulas - Pseudoaneurysms.

Early complications

Late complications

post biopsy complications

• Vascular complications occur in less than 10% of transplant recipients

• Often correctable • Ultrasound plays a pivotal role in identifying

and quantifying vascular complications of renal transplants.

Early complications

• Renal artery thrombosis and renal vein thrombosis are both devastating complications seen in the early post operative period that can rapidly lead to graft loss.

Renal artery thrombosis

• a rare early complication• can be caused by severe rejection, acute

tubular necrosis or faulty surgical technique.• Doppler US shows absent intrarenal arterial

and venous flow

Renal vein thrombosis

• more common than renal artery thrombosis• typically occurs between the third and eighth

days post transplant• Possible etiologies include poor surgical

technique, compression of the renal vein by a fluid collection or hypovolemia

• On US, The kidney may appear enlarged and hypoechoic with lack of Doppler signal in the renal vein.

• The renal artery shows increased resistance, often with a reversed diastolic flow.

Renal artery stenosis

• usually occurs during the first 3 years after surgery and is the most common vascular complication after renal transplantation, occurring in ≤10% of patients

• Approximately half of stenoses occur at the anastomosis

• Patients often present with severe hypertension, audible bruit over the graft and graft dysfunction.

• Doppler US will show a focal area of color aliasing with peak systolic velocities >200 cm/sec .

• A tardus-parvus waveform may be appreciated in the arcuate and interlobar arteries of the renal parenchyma

• Sharp rise systolic flow

• Delayed systolic upstroke

• Rounding of the systolic peak

• Decrease RI

Renal vein stenosis

• is less common and usually results from extrinsic compression by fluid collections or perivascular fibrosis.

• Doppler US shows focal aliasing with a three- to fourfold increase in velocity indicating a significant stenosis

• Normal velocity in the renal vein at the level of hilum

• At the anastmosis, there is focal color aliasing and elevated velocity

Arteriovenous fistulas and pseudoaneurysms

• are possible complications from percutaneous biopsy of the transplant kidney

• The majority of these lesions are small and clinically insignificant. However, large shunts may lead to renal ischemia, and rupture of large arteriovenous fistulas and pseudoaneurysms can cause hematuria or perigraft hemorrhage

AVM - focal area of mixed colors ( aliazing), with high velocity, and increase diastolic flow, due to AV fistula.

Pseudoaneurysm - may appear as simple cyst on gray scale imaging but with typical swirling arterial flow on color doppler

Complications Anatomic

- Perinephric fluid collections

- hydronephrosis - parenchymal

masses.

Functional • Rejection• Drug toxicity• Acute tubular necrosis

- Renal artery thrombosis - Renal vein thrombosis - Renal artery stenosis - Renal vein stenosis - Arteriovenous fistulas - Pseudoaneurysms.

Vascular

Summary of abnormal renal transplant US findings

US findings Differential DiagnosisIncrease size of the graft

- rejection, infection, venous thrombosis

Decrease size of the graft

- Chronic ischemia, chronic rejection

High RI - Severe rejection, ATN, drug toxicity, hydronphrosis, extrensic compreesion

Low RI - Arterial stenosis, AV fistula, advanced aortic or iliac atherosclerosis

Hydronephrosis - Obstruction ( stone, clot), anastmotic stenosis/edema, neurogenic bladder, bladder outlet obstruction.