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 Physician.  In most instances, one of the scintigraphy technicians is assigned the responsibility of safe transportation of the dose syringe to the stress lab and injecting at peak stress and transporting back the empty syringe for safe disposal. 

Procedure for Rest inj

• NPO for atleast 4 hrs before tracer inj
• Patient should be lying supine in relaxed state.
• Inject the radiotracer intravenously. (patient should not clench the fist!)
• 1 hour delay before scanning is recommended.
• Fatty food during the delay period helps.  

Procedure for NTG Rest inj

NPO for atleast 4 hrs before tracer inj
Patient should be lying supine in relaxed state.
BP should be recorded.
5 mg sorbitrate is given sublingually.
Wait for 3 minutes (the time required for maximal action).
Inject the radiotracer. ( patient should not clench the fist!)
Wait for 2 minutes (the time needed for tracer clearance from circulation).
Check BP in supine and erect positions.
Warn the patient about the possible side effects of postural hypotension and head ache.
Patient should not be allowed to be ambulant if BP is low.
(NTG can be administered as spray, dermal patch, subcutaneous inj or iv infusion also with specific protocol and close monitoring)
1 hour delay before scanning is recommended.
Fatty food during the delay period helps 

Procedure for all stress injections given separately at the end.

SPECT Acquisition parameters:

Step & shoot mode
Position: Supine with the left arm raised above head.
Matrix : 64 X 64 (128X128 matrix may improve slight improvement in contrast)
No. of  projections: 64
Rotation : 180 from RAO to LPO
Orbit: Circular / Non circular
Collimator: LEHR
ECG gating: 8 fr/cycle.(this should not be used if the heart rate is highly irregular) 

Time per projection:   decided based on
                                    whether low dose inj or high dose inj
                                    whether rest inj or stress inj.
whether Non-gated or Gated acquisition
For Tc99m tracers normally expected to be 20 – 30 sec per projection
For Tl-201 upto  40 sec/stop will be required. 

Image Processing

The acquired projections should be seen as a forward backward cine to look for any motion artifact and to assess the left lobe of liver activity.

Tomographic reconstruction of slices should be done defining the axes and limits correctly. The quality of the slices is the most important thing in the entire study.

The right filters should be employed depending on the raw data. Please note that the “same day” processing protocol supplied by most vendors refer to only “Rest-Stress” protocol. It is important to check the filters and change according to the acquisition protocol employed in a given case. Some of the recommended common filters for different cameras is given below: 

Quantitative Programs:

Different soft ware are supplied along with the system to measure the tracer uptake in the myocardial segments and express as a relative percentage of maximal pixel activity.
 

Polar maps are usually generated for this purpose. Defining the base, apex and the direction of the axis are critical for the result. A 20 segment model is commonly used to score the activity.
 

Optionally patient’s polar map can be compared with a reference normal data base map (supplied by the vendor as part of a third party soft ware package developed from some university) and defects can be highlighted.

LV edge detection is performed by different mathematical formulae in different systems for Gated SPECT processing, LV volume calculations and EF result.

Full details on the soft ware program installed in your system (like ECT, QPS, QGS, 4DM-SPECT etc) should be studied from the manuals (nowadays provided on the computer itself). Help of the application specialists from the company should be sought wherever necessary. 

Image Documentation on Hard Copy for Reports:

It is important to display and document the slices in standard format in short axis, HLA and VLA in color prints for future comparison.

Results from quantitative analysis should be printed in the hard copy final document only if they have been reviewed by the nuclear physician and approved to be correct.

Newer systems provide for exporting the screen images to PC compatible JPEG images and AVI cine files which can be recorded on a CD and issued to the patient. 

 Exercise stress

Instructions:

• Patients should be off Beta blockers for atleast 48 hrs
• Off Calcium channel blockers for atleast 24 hrs
• Off  Nitrates for atleast 12 hours

Preparation:

• NPO for atleast 4 hrs before tracer inj
• Male patients to shave chest hair
• Female patients should change into hospital gowns
• Venous catheter (Venflon) to be placed in the dorsum of one hand to push the radio tracer.

Procedure:

Graded exercise test performed according to standard protocol.

Patency of venous catheter in the dorsum of the hand should be checked before and during the exercise.

Tracer should be pushed as the patient is reaching the target heart rate and exercise continued at the same level for minimum another one minute.

Tracer should be administered immediately if the exercise has to be terminated for any clinical reason like chest pain, breathlessness or hypotension irrespective of heart rate achieved.

Recovery phase can be monitored as long as necessary.

Sublingual nitrates can be administered if necessary by 2 minutes after tracer injection. This will not interfere with the scan information.

Delay before scan:

Minimum 30 minutes delay recommended.
Fatty meal or just a glass of milk will help. 

Dobutamine Stress
 

Test is contraindicated in severe hypertension

Instructions:

• Patients should be off Beta blockers for atleast 48 hrs
• Off Calcium channel blockers for atleast 24 hrs
• Off  Nitrates for atleast 12 hours

Preparations:

• NPO for atleast 4 hrs before tracer inj
• Male patients to shave chest hair
• Female patients should change into hospital gowns
• Two veins should be kept open preferably (one in each hand).
• First one Venous catheter (Venflon) is placed on one side to push the radio tracer.
• A butterfly needle can be placed on the other side hand and connected to dobutamine infusion tubing.
• Patient should be warned about the possible side effects of head ache, palpitation and thumping of the heart.

Dobutamine is available as 250 mg in 5 ml solution.
 

This is added to 250 ml of Normal saline and the diluted preparation gives 1mg/ml (1mg in 60 drops). The infusion tubing is placed through an infusion drip set capable of variable rate from 10 to 299 drops per minute.

Dose calculation:

To start with 10 micro gm/kg/min and to increase the dose by 10 units every 3 min  to reach a maximum of 40 microgram/kg/min.

The infusion should be calculated in drops per minute and set accordingly.

For a 70 kg person, this will be as follows: 

If heart rate is not reached, one ml of atropine is pushed iv and repeated after 3 minutes if necessary to induce heart rate response.

Inject radiopharmaceutical and stop the dobutamine infusion. The effect is likely to last for more than 2 minutes before gradual return to normal state. 

Delay before scan:

One hour delay is recommended for good quality images.
Fatty meal will definitely help. 

Adenosine Vasodilator Stress 

Contraindicated in second degree heart block, Sick Sinus Syndrome and bronchial asthmatics.

Instructions:

• To be off Theophylline for 3 days, Aminophylline (2 days), Pentoxifylline (1 day)
• To avoid Caffeine, cigarettes and chocolates for 24 hrs
• Persantin will interfere with the catabolism of adenosine and hence should be withdrawn for 3 days before the test.

Preparations:

Adenosine Dose calculated as 0.14 mg/min/kg body wt for 6 minutes.
Adenosine available in 3mg/ml in 10 ml amp (Adenoject) or 2 ml vials (Adenocor)
Volume of adenosine required  = (0.84  X ----kg)  / 3   =  0.28 X …..(kg body wt)
For 71 Kg person dose required = 20 ml (range 18 – 22 ml in 20 ml syringe)
Two veins should be kept open preferably (one in each hand).
First one Venous catheter (Venflon) is placed on one side to push the radio tracer.
After positioning the patient butterfly needle should be placed on the other side hand and connected to adenosine syringe pump.
 

Aminophylline should be kept loaded freshly in a syringe in case of an emergency.

Patient to be briefed about the possible side effects of flushing, heaviness in the head, epigastric tightness and chest pain and reassured of the transient nature to avoid panic reaction. ECG monitor and recorder to be connected. BP cuff to be placed on the side of venflon.

For Supine Adenosine Stress:

Patient is set up in a bed with the head end elevated.

For Adenosine Stress couple with bicycle exercise stress:

Erect bicycle stress is preferred (to avoid side effects of adenosine).
Steady pedaling without any work load is advised. Should be started at least 30 seconds before starting adenosine infusion and continued for atleast 30 seconds after completion of adenosine infusion. (Forward backward half pedaling is enough for those who cannot perform full cycling). Sudden interruption of pedaling can cause hypotension and should be avoided by proper counseling of the patient.
 

Select the infusion rate in the syringe pump  ( 180 – 220 ml/min)

I.     Baseline BP, ECG tracing.
       Start infusion.

II.    End of 3 min (end of ~10 ml infusion).
       Inject Radiotracer as bolus thro venflon.
       Let adenosine infusion continue without interruption.
       Record BP & ECG and symptoms if any.

III    End of 6 min (end of ~20 ml infusion)
        Record BP & ECG.

IV    Allow 2 min for recovery.
       Record BP & ECG.

Delay before scan:

One hour delay is recommended.
Fatty meal will be definitely helpful.