MYOCARDIAL
PERFUSION SCINTIGRAPHY
1. Detection
of Coronary Artery Disease (CAD)
2. Documentation
of myocardial perfusion abnormalities before and after interventional therapy
3. Detection
of hibernating myocardium
4. Detection
of myocardial perfusion abnormalities secondary to causes other than CAD.
1. The
patient should be fasting for at least 6 hours.
2. Certain
drugs should be held for some time as it may affect exercise response
Drugs Discontinue
prior to test
Nitroglycerine
1 hour
Long–acting
nitrates 1 day
Tranquilizer /
Sedative 1 day
Anti–arrhythmic
agents 2 days
Beta–blockers 2 days
Diuretics 4
days
Anti–hypertensives
4 – 7 days
Digitalis 1 –
2 weeks
Myocardial Perfusion Scintigraphy is a
comparison of a heart in a resting to that of a heart in a stressed state
pictured in different tomographic slices through the aid of
radiopharmaceutical. The stress state is being simulated through Treadmill
Exercise Test and in patients with limited exercise capacity, through Pharmacologic
Stress Inducing Agents. The resting state may be
accomplished before or after stress testing procedure depending on department’s
protocol.
1.
Stress Testing
Procedure
a.
Treadmill Exercise
Test
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| Electrode positions in Treadmill Exercise |
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| ECG patterns of different elctrodes (aVR, aVL and AVF is not included for stress testing) |
The purpose of
this test is to evaluate the potential CAD patient in a controlled setting by
recreating symptoms the patient experiences on exertion. In treadmill testing,
a certain level of exercise must be reached for adequate evaluation of the
patient. This level is usually 85% of the patient’s maximum heart rate.
Example:
Patient is 40 years old.
220 – 40 = 180
bpm x 0.85 = 153 bpm
The 153 bpm
can be seen at the monitor of the Treadmill Machine. It is at this point where
the radiopharmaceutical will be injected.
Types of
Exercise Protocol
(1)
Bruce Protocol
In this
protocol, speed and elevation of the treadmill increases every 3 minutes. These
stages, as they are called represent an in increase in the patient’s workload,
thereby increasing the patient’s heart rate.
(2)
Modified Bruce
Protocol
This is a
variation of the Bruce Protocol in which the elevation increases slightly every
3 minutes, but the speed remains at a slow pace. This reserved for patients
with very limited exercise capacity (obese or senile or female patients).
b.
Pharmacologic
Stress Inducing Agent
Several
patients may not be able to perform the conventional treadmill exercise because
of physical limitations (such as those with amputated legs) or poor motivation.
This patient can be “stressed” pharmacologically.
Types of
Pharmacologic Stress Inducing Agents
(1) Persantin
(Dipyridamole)
Mode of action:
Persantin
increases the level of exogenous adenosine in the patient’s blood, which acts
as powerful vasodilator. This vasodilation creates a “myocardial steal”
phenomenon that diverts blood away from the myocardium that is served by
coronary arteries that may have significant stenosis
Dosage:
0.56 mg/kg
body weight diluted to 50 cc saline
Antidote: Aminophylline
Contraindication:
Patients with
history of bronchospasm, pulmonary disease, prior intubation, systemic
hypotension (BP systolic <90 mmHg), severe mitral valve disease
(2) Adenosine
Mode of action:
It activates
the A2 receptors on the cell membrane of smooth muscle.
Dosage:
200 µg/kg body
weight
Antidote: Aminophylline
Contraindication:
Patient with
advanced (2nd and 3rd degree) atrioventricular block or
sick sinus syndrome
(3) Dobutamine
Mode of action:
It acts as
positive inotropic agent (a substance that affects the force of myocardial
contraction) that stimulates the myocardium’s beta–1 receptors. This
stimulation increases the force and frequency of the contraction.
Dosage:
Initial
infusion starts with 5 µg/kg/min for 3 minutes.
Stepped
increases follow at 5 minutes intervals to infusion rates of 10, 20, 30 &
40 µg/kg/min.
Antidote: Propanolol
Contraindication:
Unstable
angina, obstructive or hypertrophic myopathy
c.
Combined
modified Bruce and Persantin Stress Testing
This is a
combination of a low–level exercise and dipyridamole infusion which shows
decrease side effects, increase target–to–background ratios and enhanced ECG
effects of dipyridamole.
Procedure of
the test
(1) Dipyridamole
is prepared in its usual concentration
(2) Dipyridamole
administration begins when the patient begins walking and continues for the
full 4 minutes.
(3) The
patient continues walking during the second stage.
(4) The
radiopharmaceutical is injected at the end of the second stage.
(5) The
patient continues walking for 1 minute into the third stage at which time
exercise is terminated
(6) Aminophylline
may be administered after dipyridamole infusion without affecting subsequent
images.
2.
Types of
radiopharmaceutical used
a.
Thallium–201
or Thallous Chloride
Thallium is an
analog of the element, potassium, which is used by the myocardium during
contractions. Thallium does not collect in the myocardium permanently but is
constantly pumped in and out of the myocardial cells.
For coronary
blockages, because of the restrictions of blood flow, Thallium cannot enter the
myocardium to the same extent as when blood flow is unrestricted. This
condition is intensified during exercise, when the myocardium’s demand for
oxygen is greater.
Since the
amount of Thallium uptake is based on the amount of blood flow to that area of
the myocardium, ischemic regions will concentrate less Thallium and appear to
as areas of decreased radioactivity on the images.
Click
here for poisonous form of Thallium
Dosage: 2 – 4 mCi
Advantages:
(1) Short
waiting period for the patient prior to imaging
(2) Long
half life of Thallium (72 hours) permits longer delayed imaging
Disadvantages:
(1) High
radiation exposure for the patient
(2) Attenuation
defects found in Thallium makes it less reliable for obese patients and woman
with large breast.
(3) 24
hours delayed imaging is sometimes inconvenient for the patient.
b.
sestaMIBI or
Cardiolite
sestaMIBI,
where MIBI stands for “methoxy–isobutyl–isonitrile” and “sesta” stands for six
number of MIBI compound attached to
Technetium. When diluted with technetium, Tc99m–sestaMIBI provides a very
accurate diagnosis of a potential CAD patient. Unlike Thallium, use of
sestaMIBI requires protocol depending on the one utilized by the department.
Types of
protocol
(1)
Two–day
Stress–Rest Protocol – patient is stressed as in any other stress
testing protocol. If a defect is seen on the initial stress image or if ECG
clinical changes occur during the stress procedure, rest study will be advised
the following day.
Dosage: 20 to
30 mCi (rest and stress)
(2)
One–day
Stress–Rest Protocol – stress procedure is done using 10 mCi
sestaMIBI, imaging follows after 45–90 minutes. Reinjection of 25–30 mCi
sestamIBI is done 3–4 hours later and imaging proceeds after 60– 90 minutes.
(3)
One–day Rest–Stress
Protocol
– this the same as one day Stress–Rest Protocol except that the sequence is
reversed.
Advantages of
sestaMIBI
(1) The
availability of Technetium makes it possible to be done on a random basis.
(2) Less
radiation for the patient.
(3) Suitable
for obese patients and women with large breast.
Disadvantage
of sestaMIBI
(1) Long
waiting period for the patient prior to imaging. Much of the activity not
collected in the myocardium will be collected in the hepatobiliary system. A
fatty meal will resolve this problem thus imaging proceeds after 45 – 90
minutes
(2) The
short half–life of Technetium demands for further reinjection of the isotope.
c.
Dual isotope
procedure
This is a
combination of Thallium and sestaMIBI and has shown a high sensitivity for the
detection of coronary stenosis with much shorter time in the department for the
patient as compared to that for single tracer studies using Thallium or
sestaMIBI.
Procedure of
the test
(1) The
patient at rest is injected with 3 mCi Thallium.
(2) SPECT
imaging follows after 10–30 minutes.
(3) After
rest imaging, the patient is prepared for stress testing.
(4) The
patient undergoes stress testing and 20–30 mCi sestaMIBI are administered at
the peak of exercise.
(5) Imaging
occur 60–90 minutes post–injection.
d. Other
radiopharmaceuticals used:
(1) Tc99m
Tetrofosmin
(2) Tc99m
Teboroxime
(3) Rubidium–82
(50 mCi) – for PET scanners
INFARCT–AVID
IMAGING
1. Myocardial
infarction begins with the denial of blood to the myocardium by a blocked
coronary artery or by blunt chest trauma that injures part of the heart.
2. As
myocardial cells begin to die, tissue necrosis begins. During the cell
necrosis, the cellular mitochondria take in phosphates while the cell releases
large amounts of macromolecules in the form of proteins.
3. The
action of mitochondria and the high binding affinity of polyphosphates to
proteins made Tc99m–pyrophosphate one of the first clinically useful myocardial
imaging agents.
4. In
most cases, this type of imaging is performed on patients for whom other data
such as the ECG or cardiac enzymes are inconclusive.
Procedure of the test
1. Administer
15–25 mCi Tc99m pyrophosphate intravenously
2. As
with any bone agent, encourage the patient to drink fluids and to void
frequently
3. Image
2–3 hours following tracer administration. Obtain 1 million counts
4. Increased
tracer uptake in the region of the myocardium is abnormal. When interpreting
the images, the physician typically compares the activity in the cardiac areas
to bone activity to grade the severity of the infarct.
GATED
EQUILIBRIUM RADIONUCLIDE VENTRICULOGRAPHY
Synonyms of RVG:
1. Gated
cardiac–blood pool imaging
2. Multigated
Acquisition (MUGA)
3. Gated
equilibrium Radionuclide Angiography
Parameters obtained from RVG
includes:
1. Global
ventricular systolic function
2. Regional
wall motion
3. Ventricular
volume (qualitative and quantitative)
4. Responses
of above parameters to exercise or intervention
5. Systolic
and diastolic indices
6. Stroke
volume ratios
Benefits obtained from RVG:
1. In
determining long–term prognosis
2. Assessment
of short–term risk (pre–operative evaluation)
3. Monitoring
the response to surgery or other therapeutic intervention
Clinical indication of RVG:
1. Known
or suspected Coronary Artery Disease (CAD)
a. CAD
without myocardial infarction
b. Remote
myocardial infarction
c. Acute
myocardial infarction
2. Known
or suspected Congestive Heart Failure
a. To
distinguish ischemic from non–ischemic causes
b. To
distinguish systolic from diastolic causes
3. Evaluation
of cardiac function in patients undergoing chemotherapy
4. Assessment
of ventricular function in patients with valvular stenosis and/or insufficiency
Acquisition parameters
1. List
mode acquisition is useful for making a composite cardiac cycle from a
heterogenous population of beats.
2. 16
frames per R–R interval are required for accurate assessment of ventricular
wall motion and assessment of ejection fraction.
3. 32–64
frame per R–R interval is necessary for detailed measurement of diastolic
filling parameters
4. Typical
acquisition are for a total of 3–7 million
Sources of error:
1. Heparin
to chronic renal will decrease labelling efficiency and reduce the target–to–background
ratios.
2. Non–separation
of left ventricle from other cardiac structures will cause inaccuracy in
ejection fraction
3. QRS
complex should the triggering signal
4. Inadequate
counts acquisition
5. Heart
rate variability
6. Inclusion
of non–ventricular activity or exclusion of ventricular activity from
ventricular ROI may cause underestimation or overestimation of ejection
fraction
7. Inclusion
of structures such as the spleen or descending aorta in background may alter
left ventricular ejection fraction.
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