Lecture #10: GENITOURINARY SYSTEM

RENAL IMAGING

Renal imaging is a reliable method of evaluating the structure, location and function of the kidneys. The kidneys regulate the volume and composition of the body’s extracellular fluid through their excretory function. The kidneys are retroperitoneal organs located between 12^th thoracic and 4^th lumbar vertebrae. The right kidney is positioned slightly lower than the left due to the presence of the liver superiorly. The kidneys receive blood from the right and left renal arteries, which branch directly off the descending aorta.

There are three processes involved in urine formation

1.      Glomerular filtration – involves the filtration of water and solutes out of the glomeruli and into Bowman’s capsule.

2.      Tubular reabsorption – occurs in the walls of renal tubules and collecting systems, where most of the water and other physiologically important substances are reabsorbed into the blood.

3.      Tubular secretion – also occurs in the tubular cells and involves the secretion of certain substances out of the blood and into the filtrate.

Clinical indications of renal imaging

1.      Evaluation of renal perfusion and function

2.      Evaluation of renal trauma

3.      Diagnosis of renovascular hypertension

4.      Detection and evaluation of renal collecting system obstruction

5.      Evaluation of renal transplant

6.      Diagnosis of acute and chronic pyelonephritis

7.      Differentiation of renal masses from normal variants

8.      Quantification of regional renal function

A.    GLOMERULAR FILTRATION RATE (GFR)

GFR – is defined as the volume of plasma filtrate produced in 1 minute by renal glomeruli of both kidneys. The normal value is 125 ml/minute.

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Radiopharmaceutical used:

1.      Tc99m–DTPA (diethylenetriaminepentaacetic acid)

2.      Tc99m – Gluceptate

3.      Dose: 3 – 10 mCi

Energy window             :           20% window centered at 140 keV

Matrix size                      :           128 x 128

Patient preparation:

1.      The patient should hydrate by drinking at least one large glass of water 30 minutes prior to the study. One suggested protocol is 7 ml of water/kg body weight.

2.      The patient should void before beginning the study.

3.      Get the patient’s age, height and weight as it is included in the quantitation

Interventional procedure:

1.      Transplanted kidney

a.      The patient is positioned supine.

b.      Images are acquired in the anterior projection.

c.       The field of view includes the transplanted kidney and bladder (usually imaging the entire pelvis will accomplish this).

2.      Diuretic renography

a.      The purpose of this is to rule out any urinary tract obstruction. Furosemide is administered intravenously to increase urine production. Increased urine flow will promptly washout the residual tracer activity if there is no obstruction.

3.      Angiotensin converting enzyme (ACE) inhibitor renal study – for renal artery stenosis

a.      Enaprilat:             0.04 mg/kg (40 µg/kg) but not exceeding 2.5 mg but not

exceeding 2.5 mg infused over 5 minutes.

Note:                    Enaprilat has a higher incidence of hypotension so an

                              intravenous line with normal saline is suggested

           

b.      Captopril:            25 – 50 mg orally, preferably crushed in 150 – 250 ml water

Note:                    Since food in the gastrointestinal tract delays absorption, the

                              patient should fast for 4 hours prior to the study if captopril

                              will be used.

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c.       Blood pressure should be monitored every 15 minutes for 1 hour

d.     A baseline quantitative renal study should be acquired before captopril study or 2 days after captopril study.

Normal findings

Normal kidneys demonstrate prompt tracer uptake with peak uptake at 3–5 minutes. The kidney activity then gradually decreases as the tracer is excreted. Renal pelvis and bladder activity is usually seen by 3–6 minutes. The computer generated curves represent three phases:

1.      Vascular phase – reflects the arrival of the bolus of activity at the renal area.

2.      Secretory phase – tracer is extracted from the renal tubules and to a smaller extent, filtered by the glomerulus. The peak transit time, usually 3 – 5 minutes following injection, is the time at which the tracer reaches its maximum concentration in the kidneys.

3.      Excretory phase – follows with a fairly rapid drop in the activity curve as the tracer is excreted from the kidney into the bladder.

Abnormal findings

1.      Abnormalities of the renogram curve are usually reflected in the 2^nd and 3^rd phase of the renogram curve. An activity curve that exhibits an adequate upslope but no subsequent fall in activity is usually due to obstruction and indicates that the renal tubules take up the material but cannot excrete the activity.

2.      A curve that displays a below normal level of activity throughout the curve denotes poor renal function. Serial images corresponding to the curve provide additional visual demonstration of the abnormalities.

Renovascular hypertension is estimated to affect 1 – 3% of unselected hypertension population and up to 15 – 30% of patient’s referral to a subspecialty center because of refractory hypertension.

Renal artery stenosis is common in non–hypertensive elderly persons and is an associated but non–etiologic finding in a number of hypertensive patients.

Renovascular hypertension is defined as an elevated blood pressure caused by renal hypoperfusion, usually due to anatomic stenosis of the renal artery and activation of the renin–angiotensin system. The goal of a screening test is to detect those patients who have renal artery stenosis as the cause of hypertension and hopefully to predict curability of hypertension following hypertension.

Clinical features associated with moderate to high risk renovascular hypertension includes:

1.      Abrupt or severe hypertension

2.      Hypertension resistant to medical therapy

3.      Abdominal or flank bruits

4.      Unexplained azotemia

5.      Worsening renal function during therapy with ACE inhibitors

6.      Grade 3 or 4 hypertensive retinopathy

7.      Occlusive disease in other vascular beds

8.      Onset of hypertension under age 30 or over age 55

Image acquisition

1.      When the 2 days protocol is performed, ACEI renography should be done on the first day. If the ACEI renogram is normal, the chance that the patient has renovascular hypertension is low and there is no need to have the patient return on the second day for a baseline study.

2.      When the one day protocol is performed, baseline renography should be performed first with only 1 mCi of Tc99m DTPA or MAG₃. The administered activity for the ACEI renogram should be 5 – 10 mCi in order to overwhelm any counts from the baseline study.

Precautionary measures

1.      Tc99m MAG₃ is preferred over DTPA in patients with elevated creatinine because of its higher extraction

2.      Since the furosemide is a loop diuretic, it can wash the radiopharmaceutical out of the calyces and pelvis and improve evaluation of cortical retention of tubular agents MAG₃ and OIH. It can also cause volume depletion and increase the risk of hypotension.

3.      The renal uptake of MAG₃ and OIH, should be measured in each kidney in the 1 – 2.5 minutes (3 minutes for DTPA) interval. Since after 2.5 minutes, a portion of the tracer have already left renal parenchyma and accumulated in the collecting system thus leading to incorrect estimates. Exclusion of the pelvis and calyces is important if there is retention of activity in these structures.

Interpretation of results

ACEI renography has a sensitivity and specificity of about 90% for diagnosis of renal artery stenosis. Most importantly, ACEI–induced renographic findings of renovascular hypertension indicate a high probability that blood pressure will reduce after intervention.

1.      Low probability

Abnormal baseline findings that improve after ACE inhibition

2.      Intermediate probability

Abnormal baseline findings but the renogram is unchanged after ACE inhibition. This group includes some azotemic patients and hypertensive patients who have a small, poorly functioning kidney.

3.      High probability

a.      For MAG₃ and OIH – a change in 20 min/peak ratio of 0.15 or greater or a significantly prolonged transit time or by a change in the renogram grade. It can also be detected as a delay in the excretion of the tracer into the renal pelvis >2 minutes following ACEI or an increase in the T_max of at least 2 minutes or 40%.

b.      For DTPA – reduction in relative uptake greater than 10% after ACEI or 10% decrease in calculated GFR of the ipsilateral kidney after ACEI or marked unilateral parenchymal retention after ACEI compared to baseline study.

Renographic curve pattern

            0 – Normal

            1 – Minor abnormalities

            2 – Marked delay excretion rate with present washout phase

            3 – Delayed excretion without washout phase

            4 – Renal failure pattern with measurable kidney uptake

            5 – Renal failure pattern without measurable kidney uptake (blood background

                        Type curve)

B.     TUBULAR FUNCTION

Effective renal plasma flow (ERPF) – is a measurement of renal tubular function. The “term” effective is used to describe the measurement of renal plasma flow.

Radiopharmaceutical used:

1.      Tc99m–DMSA (dimercaptosuccinic acid)

2.      Orthoiodohippurate (I¹³¹ OIH)

Energy window:                        20% window centered at 140 keV

Procedure of the test:

1.      An injection of 5 mCi Tc99m–DMSA is instituted.

2.      After 3 hours, patient is positioned supine.

3.      Acquire the following view: anterior, posterior, right anterior oblique, left anterior oblique, right posterior oblique, left posterior oblique at 400k.

4.      Acquire SPECT images with the following parameters:

Degree of rotation                :           360^o

Number of images               :           64

Time per image                    :           20 seconds

Normal findings

Normal static images obtained 1 hour or more after injection demonstrates a smooth renal contour. Tracer distribution within each kidney should be uniform and both kidneys should have equal tracer concentration.

Abnormal findings

Congenital malformations such as fetal lobulations and horshoe kidneys are easily identified on the images as areas of activity outside the normal renal outline. Areas of increased or decreased radioactivity may represent cyst, neoplasms, infarcts or renal trauma.

Evidence of renal scarring

Technical considerations

If the patient is in severe renal failure, images obtained with DMSA may need to be delayed up to 24 hours to improve visualization of the kidneys. Horshoe kidneys and pelvic kidneys are located more anteriorly than the normal renal position. If these anomalies are a consideration, an anterior projection may be helpful.

C.    TUBULAR SECRETION

MAG₃ is handled solely by tubular secretion and has an overall clearance that is less than that of OIH. It has the better physical characteristics of Tc99m. Millicurie amounts of MAG₃ can be administered to patients, thus resulting in higher counting statistics and better image resolution.

The renal tubular secretion images Tc99m–MAG₃ as it passes through the vascular system, renal tubular cells, tubular lumens and collecting systems. This series of images allows the sequential evaluation of renal perfusion, renal clearance by tubular secretion, renal parenchymal transit time and is passage of urine through the renal collecting system. In addition, the study provides high contrast images for evaluation of renal anatomy.

Radiopharmaceutical used:

1.      Tc99m–mercaptoacetyltriglycerine (MAG₃)

Energy window              :           20% centered at 140 keV

Matrix size                      :           128 x 128

Procedure of the test     :           Same as GFR

Quantitative renal studies:

Quantitative techniques provide numerical values that indicate the level of tubular and glomerular function. In addition to the renogram curve and nuclear medicine images such values are useful in monitoring the course of patients with many different types of renal pathology. Quantitative studies are based on either a blood sampling technique to estimate tracer clearance from the blood or a camera method to determine the amount of tracer accumulated in the kidneys. The advantage of radionuclide technique over chemical measurements such as creatinine clearance is that the function of each kidney (differential function) can be determined.

RADIONUCLIDE CYSTOGRAPHY

Radionuclide cystography is performed for the evaluation of vesicourethral reflux, which is common in children. This condition is often responsible for recurrent urinary tract infection, which may damage the kidney.

This study is more sensitive than radiographic methods because of high contrast between instilled radioactivity in the bladder and the lack of radioactivity outside the bladder.

Radiopharmaceutical used

1.      Tc99m–sulfur colloid

2.      Tc99m–sodium pertechnetate

3.      Tc99m–DTPA

Two methods of performing cystography

1.      Indirect method – involves intravenous administration of a renal agent. After the agent is cleared from the kidneys into the bladder, the patient is asked to void while being imaged. Reflux that occurs during voiding can then be observed.

2.      Direct method – most commonly used

a.      Have patient void before the study.

b.      Catheterize the patient.

c.       Connect the catheter to a bottle of normal saline to which 1 mCi pertechnate has been added.

d.     Position the patient supine on the imaging table with the upper portion of the bladder in the lower part of the field of view.

e.      Obtain multiple sequential images as the bladder is being filled with radionuclide saline solution.

f.        Discontinue saline infusion when bladder capacity is reached. Obtain a posterior pre–void image that includes the entire bladder and upper urinary tracts.

g.      Obtain voiding images with the patient in the seated position and the camera against the patient’s back. Remove the catheter and encourage the patient to void into a bed pan or urinal.

Image findings

The normal exam will demonstrate increasing activity in the bladder without reflux into the ureters. The abnormal exam will demonstrate ureteral reflux, especially during micturition. Reflux usually increases as the study progresses, although transient reflux may occur.

Certain quantitative information, such as the reflux bladder volume and volume of reflux into the kidney, can also be calculated.

Technical considerations

1.      Equipment and the surrounding area should be covered with absorbent paper to prevent contamination with radioactive urine.

2.      It is important to establish adequate bladder filling before the voiding portion of the exam. An increase in patient discomfort, leakage of urine around the catheter or cessation of flow from saline infusion bottle may all indicate sufficient bladder filling. Bladder capacities vary according to age and condition of the bladder.

3.      If quantitative results are desired, not that any loss of urine will cause inaccuracies in the calculation.

4.      Children may not cooperate when asked to void, depending on their age.

5.      Catheterization should be performed by someone who is well trained in the technique to ensure correct placement and to avoid physical or psychological trauma to the child.

SCROTAL IMAGING

Scrotal imaging is most often performed to differentiate acute torsion (twisting) of the spermatic cord and epididymitis. Torsion of the spermatic cord is often spontaneous and occurs most commonly in young men, with an acute onset of pain. The distinction is important since torsion requires immediate surgical intervention while epididymitis requires antibiotic therapy.

Tc99m pertechnetate is the usual tracer used. Sequential images of the scrotum are acquired for 5 minutes each for up to 30 minutes. Lead is also put beneath the scrotum to obscure background uptake.