Renal System
Effective renal plasma flow (ERPF) What is the normal?
Measures renal tubular function. Measure of the ability of kidneys to clear paraaminohippurate (PAH). 90% PAH is secreted from the peritubular capillaries to the tubules (some filtered by glomerulus). Normal 500-600 ml/min.
Glomerular Filtration What percent passes through the renal tubules?
Movement of water and small solutes across the filtration membrane as a result of pressure difference to form filtrate. 20 % passes through to renal tubules
What is the normal filtration fraction?
Normal filtration fraction is 20% (GFR/ERPF)
What is the typical dose for DMSA
1-6 mCi
What percentage of cardiac output goes to the kidneys? How many Liters?
20-30% Cardiac output is 5L/min, therefore the total renal blood flow is about 1.2L/min
What percentage of the total blood volume is plasma?
50% of the TOTAL blood volume is plasma, therefore the effective renal plasma flow (ERPF) is about 600ml/min
Morphological agents
Accumulation in the kidney is the result of a complex interaction of Blood flow GFR Tubular secretion Tubular resorption
Areas of increased or decreased activity:
Acute pyelonephritis ◦ Cortical defects from VUR/UTI ◦ Infarctions (trauma) ◦ Neoplasms ◦ Cysts
Adverse effects for Captopril
Adverse effects: ◦ Orthostatic hypotension ◦ Dizziness◦ Tachycardia◦CP ◦ Loss of taste ◦ Rash
Tubular reabsorption
Begins as filtrate travels through the tubule. Most of the water and useful solutes are reabsorbed into peritubular capillaries and reenter the general circulation.
What is the critical organ?
Bladder wall
what does the Nephron consists of?
Consists of ◦ Afferent arteriole that delivers blood to a web of cappilaries in the glomerulus ◦ The blood leaves the glomerulus through the efferent arteriole ◦ Juxtaglomerular apparatus An important hormone producing mass of cells between the afferent and efferent arterioles The hormone (renin) produced has effects on blood pressure.
What does the outer renal cortex contain?
Contains the majority of nephrons, which are the microscopic functional units of the kidney.
Tc-99m - DMSA function
Cortical Binding (40%)
What if the kidneys do not empty?
Diuretic renal imaging◦ Differentiate a dilated renal collecting system from an obstructed renal collecting system If collecting system is dilated after 20min images- Inject furosemide (Lasix) ◦ Dose Adult 20-40 mg Pediatric 0.5-1 mg/kg Image for another 20min Repeat analysis
What is the dose for the radiopharmaceuticals?
Dose 10-20 mCi, min 500 uCi
Dose for Captopril
Dose 25-50 mg orally Radiopharmaceutical 60 minutes post injection
Imaging - Procedure
Flow- 1 minute ◦ 30 frames at 2 sec/frame Dynamic- immediately following flow ◦ 20 minutes, 20 seconds/frame Post void upright static Analysis
Indications of radiopharmaceutical
Function Perfusion Quant ERPF (MAG 3) and Quant GFR (DTPA)
Two categories of Radiopharmaceutical
Functional agents Morphological agents
Renal Stinography Augmented by ACE Inhibitor Procedure
Give 50mg of Captopril 1 hr prior to imaging ◦ Captopril can cause hypotension so the patients blood pressure should be monitored every 15min and should not fall below 30% of initial blood pressure DOSE and IMAGING the same as functional renal imaging
What is the Glomerular filtration rate?
Glomerular filtration rate (GFR) - volume of plasma filtrate produced in 1 minute by the renal glomeruli of both kidneys. Normal 120-125 ml/min.
What happens if venogram looks more normal or abnormal?
If baseline renogram looks more normal or more abnormal there is a presence of renovascular hypertension Normal post captopril renogram = no RAS Abnormal post captopril renogram = RAS If RAS, patient will most likely benefit from revascularization.
Findings - Diuretic Renal Imaging
If collecting system doesn't empty after being stimulated with Lasix, there is an obstruction False positive ◦ Diseased kidney doesn't respond to Lasix ◦ The collecting system is grossly dilated◦ Vesicoureteral reflux or fully extended bladder
Foreshortening artifact
Kidney appears smaller than its actual size because the long axis of the kidney is not parallel to the surface of the crystal of the gamma camera
Kidney Function
Kidney receives 20% of cardiac output with renal plasma flow (RPF) averaging 600ml/min. Kidney clears the plasma & body of waste products Plasma clearance occurs by glomerular filtration (1st step in production of urine) and by tubular secretion. 20% of RPF is filtered by glomerulus Remaining 80% is filtered by tubular cellskid
Why is it a high radiation dose?
Long half life
99mTc-DTPA
More sensitive in detecting early perfusion decrease in a transplanted kidney
Clinical Information
Number, location and size of kidneys Results of other imaging tests Patient lavatory values ◦ Creatinine ◦ Urea ◦Nitrogen Current medications Physiological conditions under which the test is to be performed
Interpretation peak transit time half time excretion
Peak transit time ◦ Normal 3-5 minutes Relative renal uptake ratios at 2-3 minutes ◦ Ideally 50/50 (value < 40 % considered abnormal) Half-time excretion ◦ Normal is 8-12 minutes
Explain the Blood Flow to the kidneys
Plasma and red blood cells pass the glomerulus, approximately 20% of the plasma pass through the glomerulus into the renal tubules, resulting in a glomerular filtration rate (GFR) of 120ml/min Most of the filtrate is reabsorbed Portion of the plasma that reaches the kidney and is filtered by the glomerulus is the Filtration fraction
99mTc-MAG 3
Preferred and used most often Excreted via tubular secretion Has high first pass rate
Findings
Prompt uptake of tracer in the kidneys Relative renal function ◦ Draw regions of interest around the kidneys and background area Use an early image so that uptake is in the parenchyma instead of the collecting system Divide the background-corrected kidney counts by the sum of the background corrected counts
Renal Stinography Augmented by ACE Inhibitor
Renal artery stenosis (RAS) ◦ Significant contributor to renovascular hypertension Pt given ACE inhibitor which blocks the mechanisms that contribute to hypertension (conversion of angiotensin I to angiotensin II) ACE inhibitor causes loss of the efferent glomerular arteriolar vasoconstriction, resulting in decreased filtration pressure and decrease in GFR Ultimately causing renal failure in some patients
Tubular secretion
Substances are actively secreted from the peritubular capillaries to the filtrate to be eliminated in the urine. Includes waste products suchas ammonia, uric acid, metabolic products of medications.
Imaging - patient positioning
Supine ◦ Xiphoid to bladder (hint: top of kidneys are usually at the same height of the anticubital fossa)
Radiopharmaceutical for Morphological renal imaging?
Tc-99m - DMSA Cortical Binding (40%)
TC99M DMSA
Tc-99m succimer (DMSA) 4 hour shelf life after preparation Binds to renal cortex-50 % bound within 2 hours post injection Remains fixed in renal parenchyma
Tc-99m - MAG-3 function
Tubular (100%)
What does the inner renal medulla contain?
contains renal pyramids, which empty into the minor and major calyces of the renal pelvis.
Critical organ
kidney because it remains in the cortex
Anatomy of the kidneys - Size
◦ 11-12 cm long ◦ 5-7.5cm wide ◦ 2-3 cm thick ◦ **** about the size of a fist
3 Processes involved in urine formation
◦ Glomerular filtration ◦ Tubular reabsorption ◦ Tubular secretion
Congenital abnormalities:
◦ Horseshoe kidney◦ Ectopic kidney◦ Absence of a kidney
Instrumentation
◦ Large field of view ◦ Single head (posterior) ◦ Low energy, all purpose collimator Diverging collimator for children
Indication of Functional Renal Imaging
◦ Measurement of relative renal function ◦ Evaluation of renal transplant ◦ Evaluation of acute renal failure
Time at Activity Curve
◦ Peak at 3-5 min ◦ Decrease to less than 50% after 20 minutes
Anatomy of the kidneys - Location
◦ Posterior ◦ 12th vertebra to the 3rd lumbar vertebra ◦ Rt. kidney lower than left because of the liver
Renal Stinography Augmented by ACE Inhibitor Indication
◦ Renovascular hypertension
Three Phases of time activity curve
◦ Vascular ◦ Secretory (concentration) ◦ Excretory
Morphological Renal imaging what is it used to evaluate?
◦ Viability ◦ Infection ◦ Structural abnormalities
Physiology of the kidneys
Major functions ◦ Excretion of waste products ◦ Resorption of important body constituents ◦ Maintenance of acid-base balance ◦ Maintenenance of fluid balance
Renal Transplant
Looking at function/rejection◦ Also vascular problems, leaks & obstruction ◦ Will have a baseline study Protocol same as renal function EXCEPT you image posterior and Anterior ◦ Transplant kidneys are typically placed in the anterior upper groin area
what is Classified by uptake & clearance mechanisms:
Glomerular Filtration ◦ Tubular Secretion ◦ Cortical Binding
Tc-99m - DTPA function
Glomerular Filtration (100%)
Patient Prep
Pt must be hydrated, 2-3 8oz cups of water or IV fluids. ◦ Void immediately before beginning study
Renal Stinography Augmented by ACE Inhibitor Patient Prep
Pt must be off Captopril for 2 weeks and lisinopril for 1 week ◦ Pt should be well hydrated
Functional agents
Rapidly taken up and excreted by the kidneys by a single, simple physiological mechanism ERPF
Indication for Renal Scintinography
Relative renal function ◦ Quantification of renal function/GFR/ERPF Renal transplant evaluation ◦ Rejection or obstruction Acute renal failure Obstructive uropathy Renovascular hypertension/renal artery stenosis Infection and inflammation Vesicoureteral reflux
