1. Myocardial Perfusion Imaging
2008
Zoll Firm Lecture Series

2. Basic principles of myocardial perfusion imaging
Inject radioactive tracers whose distribution in the heart are proportional to the amount of blood flow to the area
Gamma camera detects the photons given off by the radioactive tracers and thus the relative blood flow to the different areas during rest and stress
Detects RELATIVE, not ABSOLUTE blood flow
May miss “balanced ischemia” if blood flow to all areas are reduced in the case of 3 vessel disease.
2008
Zoll Firm Lecture Series

3. Example of a normal scan (uniform uptake during rest and stress)
Short axis
base
apex
Vertical long axis
anterior
base
apex
inferior
Septum
lateral
Horizontal long axis
apex
septal
lateral
base
Posterior
Anterior
2008
Zoll Firm Lecture Series

4. Zoll Firm Lecture Series
Infarct
Decreased uptake at rest
Decreased uptake during stress
Defects are described by
Size (this is large)
Severity (this is severe)
Reversibility (fixed)
Location (inferior and lateral wall)
Can infer coronary distribution (dom LCx or LCx and RCA lesion)
2008
Zoll Firm Lecture Series

5. Example of a reversible defect
Uniform uptake at rest
Inferior defect during stress
Implies ischemia
This defect is
Medium-sized
Moderate in intensity
Reversible
Located in the inferior wall
Probable RCA lesion
2008
Zoll Firm Lecture Series

6. Zoll Firm Lecture Series
Commonly used tracers
Technetium 99m Sestamibi
Technetium 99m is a radioactive compound, sestamibi is a small cation that has been labeled with Technetium 99m. Binds the mitochondrial membrane
½ life of 6 hours
Better imaging characteristics (intrinsically better and also because can inject higher dose since ½ life is shorter)
Does not show “redistribution”
Thallium
Radioactive K analog. Enters/exits cell via Na/K ATPase
½ life of 3 days (more radioactive exposure to patients)
Worse imaging characteristics (intrinsically worse and also because need to limit dose to limit radioactive exposure)
Shows “redistribution”- after injection, the tracer moves to viable parts of the myocardium that initially shows less uptake
2008
Zoll Firm Lecture Series

7. Commonly used protocols
Low dose, high dose Tc99m sestamibi same day protocol:
Inject low dose of the tracer at rest
Acquire images
wait several hours for the tracers to decay (so that the rest pictures wouldn’t interfere with the interpretation of the stress pictures)
stress the patient, iinject a higher dose of the tracer at peak stress
Acquire images again
Compare the pictures
Most commonly used here.
Dual isotope (thallium, Tc99m sestamibi) same day protocol:
Inject thallium 201 at rest
Acquire images
Stress the patient, inject Tc99m sestamibi
Advantage is that thallium and Tc99m gives off slightly different energy of photons so pictures from the rest wouldn’t “contaminate” the picture at stress
Problems includes more radioactive exposure and difficulty in comparing thallium with Tc99m images.
2008
Zoll Firm Lecture Series

8. Zoll Firm Lecture Series
Other protocols
Two days Tc99m sestamibi protocol -same sequence as the 1 day protocol except
Inject high dose of tracer for rest and stress (don’t have to worry about high dose at rest contaminating the stress pictures since there is adequate time for the tracer to decay)- gets better quality rest pictures
Wait 1 day between the stress and rest images
Better for obese patients
Obviously, time delay can be inconvenient.
Thallium-redistribution protocol:
Stress the patient and inject the tracer at peak stress
Image the patient immediately
Wait 3-4 hours and image the patient again (because tracer shows redistribution, it will move to areas that are alive there will be a difference between the stress and rest image if there is ischemia)
Can wait 24 hours to assess viability (areas with little life will light up much later)
2008
Zoll Firm Lecture Series

9. Dypyridamole/Adenosine
Adenosine is a small molecule with the following effects:
Activation of the A2A receptors causes coronary vasodilatation- making this a useful agent for stress testing.
Activation of A1 receptors causes atrioventricular (AV) conduction delay, explaining its use in the management of supraventricular arrhythmia
Activation of A2B, A3, A4 receptors can mediate bronchospasm
Methyxanthines (like caffeine, theophylline, aminophylline) are adenosine antagonists.
Dypyridamole blocks the metabolism and therefore causes the build up of adenosine.
Half life of mintues, reversed with aminophylline.
Cautious use in patients with hepatic dysfunction
2008
Zoll Firm Lecture Series

10. Vasodilator stress test
Causes hyperemia but does not increase workload! Can only be performed with nuclear perfusion imaging
Can be safely performed soon after uncomplicated acute coronary syndrome (safety tested 2 days after ACS) Brown KA et al. Circulation 100:2060, 1999
May decrease the amount of false positive anterior-septal defects in patients with LBBB compared to an exercise stress test.
The absence of EKG changes does not indicate the absence of CAD. However, the presence of EKG changes is a very bad prognostic factor.
2008
Zoll Firm Lecture Series

11. Zoll Firm Lecture Series
Diagnosis
Thallium SPECT- sensitivity of 88% and specificty of 77% Ann Intern Med 1999 May 4;130(9):719-28
EKG stress test- sensitivity 68%, specificity 77% Circulation 1989 Jul;80(1):87-98.
Stress echocardiography- sensitivity of 76% and specificity of 88%
In general, nuclear myocardial stress imaging considered more sensitive and less specific then stress echocardiography.
Referral bias- only the people with positive ETT goes on to have the gold standard test. Hence, true sensitivity is probably lower, true specificity higher.
2008
Zoll Firm Lecture Series

12. Zoll Firm Lecture Series
Prognosis
More important than diagnosis is prognosis!
The more severe, more numerous the defects, the worse the prognosis.
Patients with a normal scan have a very good short to intermediate term prognosis.
Among 5183 patients who underwent myocardial perfusion imaging, the presence and severity of an abnormal scan was predictive of cardiac death or a myocardial infarction during a two year follow-up. Data from Hachamovitch, R, Berman, DS, Shaw, LJ, et al, Circulation 1998; 97:535.
2008
Zoll Firm Lecture Series

13. Doesn’t erase exercise performance, symptoms, EKG changes…etc
Patients with a normal scan have a good prognosis but NOT if they have a high duke treadmill score!
Patients with a mildly abnormal scan does not have a good prognosis if they have an intermediate to high duke treadmill score.
Event rate (death/MI) in 2200 conse-
Cutive patients during 1.5 yrs f/u
Data from Hachamovitch, R, Berman, S, Kiat, H, et al, Circulation 1996; 93:905.
2008
Zoll Firm Lecture Series

14. Zoll Firm Lecture Series
Gated Imaging
Divides the cardiac cycle into phases
Data collected during each phase is pooled to form a single image
Images from each phase are put together to compose a series of images called a cine
Further information can then be obtained from this data by applying computer algorithms
2008
Zoll Firm Lecture Series

15. Zoll Firm Lecture Series
Gated Imaging
3D images allow for accurate quantification of volumes in each phase of the cardiac cycle
Calculated by using computerized edge detection to determine the endocardial border
LVEF = 1-(ESV/EDV)- good correlation with echo and CMR.
To get accurate quantification, the computer must be able to accurately detect the endocardium
Needs regular rhythm
Motion or other artifacts that significantly affect the perfusion images can reduce accuracy
Severe defects (real or attenuation) is a problem
No counts, no border
Small hearts- resolution not high enough to be accurate
2008
Zoll Firm Lecture Series

16. Attenuation artifacts
diagphragmatic attenuation or infarct?
Where there is overlying soft tissue, less photons from the heart penetrate these tissue and get to the camera, creating an apparent defect.
Fixed defects can represent either myocardial infarction or an artifact due to soft tissue attenuation
Inferior defects often due to diagphragm, anterior defect due to breast
Difficult to distinguish between attenuation and infarct
Soft tissue attenuation is very common
Major limitation in the specificity of SPECT imaging for the detection of CAD
Breast attenuation or infarct?
2008
Zoll Firm Lecture Series

17. How to deal with attenuation?
Tend to be of mild intensity, but can be moderate
Usually follow one of these typical patterns
Usually evidence of attenuation on the projection images or the attenuation map
Attenuation correction software available
Works better for inferior attenuation defects
Prone Imaging
Inferior attenuation decreased (gets the diagphragm out of the way)
Look at Gated Images
The wall motion with fix defect should be abnormal if it’s an infarct, normal if it is attenuation
Bottom line, interpret mild inferior defects and mild anterior defects in woman with caution
2008
Zoll Firm Lecture Series

18. Zoll Firm Lecture Series
High risk features
TID in a patient with multiple severe defects
Transient ischemic dilatation
RV uptake post stress
Increased pulmonary tracer uptake
Decrease in post-stress LVEF
Increase RV uptake post-stress
2008
Zoll Firm Lecture Series