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Today’s scintigraphy technician is expected to produce good quality myocardial slice images obtained from the radiotracer distributed in the myocardium.
Radiopharmaceutical
Tc99m Sestamibi and Tc99m Tetrofosmin are used most often and Thallium Tl 201 is rarely used.
Imaging of Myocardial Perfusion under different conditions
Scan reflects the status of myocardial perfusion at the time of injection of the tracer. Tracer inj may be performed under one of the following conditions:
1. Peak exercise stress usually on Treadmill (optionally ergometer bicycle can also be used). This is physiological with heart rate & blood pressure responses and accumulation of adenosine in the body which results in coronary vasodilatation.
2. Dobutamine stress. This is closer to exercise stress with both chronotropic and inotropic responses and secondary response of coronary vasodilatation.
3. Adenosine stress. This is pure vasodilator stress without significant heart rate or BP response.
4. Rest injection. This may be needed to confirm the hypoperfusion defects that may be seen in the stress scan.
5. NTG Rest Injection. Nitroglycerine administration at the time of tracer inj augments the uptake in myocardial segments subtended by severely stenotic vessels. This will help to demonstrate the viable myocardium better.
Low Dose and High Dose Imaging
Both stress inj scan and rest inj scan may be performed on the same day or on different days depending on the physician’s preference.
Imaging can be performed with a dose ranging from 7 mCi to 25 mCi for average Indian adults. (For heavy persons the dose may go upto 12 mCi to 36 mCi).
When two studies have to be performed on the same day, low dose is used first in the morning followed by a second high dose (ie three times the first dose) after an interval of minimum 4 hours. When the studies are performed apart on two days, there is no technical compulsion for such a high dose.
ECG Gated imaging is preferably done with higher dose than with lower dose.
Describing the procedure as Low Dose & High Dose Acquisition is more important for the technician for selecting the acquisition and processing parameters in addition to the label for the study as Rest or Stress.
Patient Scheduling
Nowadays myocardial perfusion scan is performed on any day of the week.
Labs with low volume work load will prefer the same day protocol whereas labs with high turn over of cardiac patients can afford to adopt two day protocol also.
Rest inj scans (without or with Sub-lingual NTG) can be scheduled without screening by the Nuclear Physician whereas all other requisitions which include some form of stress testing should be studied by the Nuclear Physician before scheduling.
IMPORTANT:
Stress procedures should be performed only where there is facility for cardiac monitoring and resuscitation. Normally Stress Lab, ICCU & Emergency Room are preferred but can also be performed in the Gamma Camera room or NM Dept with necessary support.
All cardiac stress testing part of the procedure should be performed only by physicians trained for the purpose. In some labs the Nuclear Physician might perform the same and in some other centers another physician might be available exclusively for this. Informed consent should be obtained before any stress test.
The radio-tracer injection may be performed by the Stress Lab Staff (Physician or Technical assistant) if they have been taught about safe handling of radioactivity and authorized by the in-charge Nuclear
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