Cardiac imaging
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
This chapter presents a brief overview of cardiac imaging techniques. For a detailed discussion of each of the imaging technoloies, please view the chapter on that imaging technology.
Assessment of LV function
Echocardiography
- Least expensive, most versatile.
- Portable, immediately available
- Preferred initial technique to diagnose heart muscle disease of unknown etiology.
- Regional thickening can be assessed on ECHO and not other techniques and this is a better marker of regional function than is regional wall motion (cant be seen on nuclear or angio studies, MRI can assess though)
- ECHO is not as good at assessing quantitative ejection fraction (SPECT, angio, RVG are better at this).
- More sensitive than EKG in diagnosis of LVH.
- Excellent in estimating LV mass
Radionuclide RVG
- Good for quantitating LV and RV EFs. Excellent for following the wall motion in patients treated with chemotherapeutic agents.
- Good for wall motion.
- In MI does not tell you about infarct expansion, MR, LV thrombus, regional thickening abnormalities.
Gated SPECT Imaging
- Permits evaluation of regional thickening, global LV function and myocardial perfusion.
Cardiac MRI
- Good regional and global cardiac function.
- With contrast agents, good perfusion data
- Superior for congenital, aortic disease, anomalous coronary arteries, and RV dysplasia
- Cost benefit ratios of echo and nuclear make them superior for LV function assessment
- May be best technique for quantitating LV mass
- As a research tool may be useful in the assessment of LV remodeling
Left Ventriculogram
- Gold standard in the assessment of wall motion but not in the assessment of wall thickening.
- LVEF and absolute volumes are highly reproducible
- LVH and LV mass are better quantitated with echo and MRI
- Left to right shunts are most accurately quantitated with cardiac cath over echo and MRI
Techniques to Assess CAD and Prognosis
Exercise EKG
- Strengths
- Low cost
- Short duration
- Functional status evaluation
- High sensitivity in 3 VD or left main disease
- Useful prognostic information
- Limitations
- Suboptimal sensitivity in the detection of single vessel disease (50%), 85% in the presence of three vessel disease
- In all patients, overall sensitivity 68%, specificity 77%
- Beta blocker use is associated with a higher rate of false negatives (fail to achieve rate pressure product)
- Non diagnostic in patients with abnormal baseline EKG (dig, LVH, WPW)
- Poor specificity in certain patient populations: premenopausal women, LVH, dig, IVCD, hypokalemia, hyperventilation, severe hypertension, resting ST abnormalities
- The negative predictive value in women of low to intermediate risk is high, the positive predictive value in men is high
- Need to achieve > 85% of maximum heart rate for maximizing accuracy
- Its main values lies in excluding CAD in patients with a low pre test probability of CAD based on gender and age.
Stress Radionuclide Myocardial Perfusion Imaging
- Strengths
- Simultaneous evaluation of perfusion and function with gated SPECT
- Higher sensitivity and specificity than exercise EKG: For exercise or pharmacologic SPECT imaging with Tl or Tc, in patients with chest pain the sensitivity for the detection of CAD is 85% to 90%. Specificity for excluding CAD is in the 90% range. Good in patients with LVH, dig, IVCD etc. ST depression response higher rate pressure product than does a perfusion abnormality with tracers. Therefore they are more sensitive. Adding stress perfusion imaging to the exercise ECG stress test greatly assists in differentiating true positive from false positive ETT ST segment depression. For single vessel disease, the sensitivity is 25% higher with SPECT imaging compared with exercise testing. The sensitivity for detecting 3VD with exercise SPECT is 95% to 100%.
- High specificity with Tc labeled agents: Half life is shorter than Tl, therefore dose is higher, therefore image is brighter and better. Also allows gated assessment of LV thickening.
- Studies can be performed in almost all patients
- Significant additional prognostic information, can quantitate LV function
- Comparable accuracy with pharmacologic stress testing
- Viability and ischemia when assessed simultaneously
- Quantitative image analysis
- Limitations
- Suboptimal specificity with thallium imaging, with a high false positive rate in many labs, particularly among women and obese patients.
- Long procedure time with Tc agents, higher costs than ETT
- Radiation exposure
- Poor images in obese patients
- Pharmacologic stress testing: sensitivity and specificity are similar for persantine and adenosine. Dobutamine is used in those patients with a history of bronchospasm, or for those patients who have consumed coffee before the procedure. Pharmacologic testing is the preferred method in patients with LBBB.
- Women with chest pain who are referred for exercise or pharmacologic stress testing benefit the most from the enhanced accuracy of Tc imaging. Both Tl and Tc had a sensitivity of about 70%, but the specificity rose to 92% with Tc. Most labs now use Tc because of its improved specificity, the ability to gait the images and assess regional wall thickening. Mild non reversible defects that show preserved systolic thickening usually represent attenuation artifacts, however, if there is abnormal wall thickening, then this is most likely scar.
Exercise / Pharmacologic Stress Echocardiography
- Strengths
- Higher sensitivity and specificity than exercise EKG: Metanalysis showed sensitivity of 84%, specificity 86%. Marked variation across trials though, highly operator dependent. If the max heart rate is < 85% of age predicted, then sensitivity drops to 42%. Sensitivity is 10% lower in women than in men, specificity is the same across genders. In women with single vessel disease the sensitivity was only 40%, if there was 2 or 3 vessel disease, this number increased to 60%.
- Additional prognostic value over exercise EKG
- Dobutamine stress has higher sensitivity than does pharmacologic stress
- Time to complete examination is short
- Identification of co-existent structural cardiac abnormalities (valvular disease)
- Simultaneous evaluation of perfusion with contrast agents
- Relatively lower costs than with other techniques
- No radiation
- Limitations
- Decreased sensitivity for the detection of single vessel disease or mild stenosis with post exercise imaging
- Inability to image the entire ventricle in some patients
- Highly operator dependent in the analysis of images
- No quantitative image analysis
- Poor windows in patients with COPD
- Infarct zone ischemia less well detected
Comparison of exercise SPECT imaging and Exercise Echocardiography
- Both have a higher sensitivity and specificity than regular exercise EKG testing
- Both provide functional information that EKG testing does not
- Both provide information about myocardial viability, which the angiogram does not
Strengths of Stress ECHO over SPECT
- Noninvasive, safe and repeatable, no radiation exposure, quick, little sophisticated equipment and portable, low costs, can identify co-existing valvular heart disease
Limitations of Stress ECHO over SPECT
- Images are difficult to obtain at peak exercise, an ischemic response is required to observe wall motion abnormalities, wall motion can recover quickly in the presence of mild ischemia, detection of residual ischemia is difficult in an akinetic wall zone, the technique is highly operator dependent, good quality images were only acquired in 70% of cases.
Strengths of SPECT over stress ECHO
- Does not require an ischemic response to be abnormal, just requires an abnormality in flow reserve, sensitivity is slightly higher by about 8-10 percentage points (mostly because the ability to detect single vessel disease or mild stenoses of 50-70% is not as good with stress Echo), can see defects in areas that contain scar and viable myocardium, acquisition of images is not operator dependent, in virtually 100% of patients diagnostic images are obtained, with Tc simultaneous assessment of perfusion and function is obtained, resting LV ejection fraction can be obtained, vasodilator SPECT has significantly higher sensitivity than vasodilator stress ECHO, dobutamine ECHO is associated with higher sensitivity and specificity than vasodilator ECHO.
Limitations of SPECT imaging in relation to stress ECHO
- Longer imaging protocols, greater expense of equipment, must inject and store radiopharmaceuticals, inability to visualize the heart in real time, lower spatial resolution than ECHO, higher costs to patients.
In general, the sensitivity is lower for stress ECHO while the specificity is higher.
Prognosis
Exercise Tolerance Testing
- 1 mm or more of horizontal or downsloping ST depression is associated with a poor prognosis
- Failure to achieve 6 METS is associated with an elevated mortality rate over the next 2.5 years.
- Failure of heart rate to rise is associated with higher mortality, even after adjusting for perfusion defects.
- Failure to reach 85% of age adjusted max HR is associated with a RR of 1.85 in mortality.
- Limitation of ETT is the fact that the magnitude of ST depression is not strongly associated with the extent of CAD
- Exercise testing alone has excellent prognostic ability among patients with atypical chest pain or non anginal pain who have a normal EKG at baseline. If these patients have a normal ETT, the prognosis is excellent.
Stress Myocardial Perfusion
- The following are associated with a poor prognosis:
- 20% of the LV is a perfusion defect
- Defects in more than one distribution suggestive of multivessel CAD
- A large number of non reversible defects
- Transient LV cavitary dilation
- Increased lung uptake
- Resting LVEF of < 40%
- Normal thallium: Mortality 1% per year
- Normal Tc: annual mortality 0.6%, 12 fold higher if there is a Tc defect
- The positive predictive value of stress myocardial perfusion imaging and stress ECHO is low: That is the percentage of people who die or sustain an MI is low among patients with abnormal findings. On the other hand the negative predictive value is high and exceeds 95%.
Techniques used to Assess Myocardial Viability
- Tl Imaging
- Rest and delayed redistribution is the most common radionuclide method used to assess viability. Uptake of Tl is related not only to blood flow, but also to membrane integrity. Myocardial stunning or hibernation does not result in a reduction in Tl extraction as long as the sarcolemmal membrane does not sustain irreversible ischemic damage. 60 to 70% of asynergistic segments will show > a 50% improvement after revascularization.
- Tc Imaging
- Same as above, as usual a better signal with Tc, can also assess regional wall thickening. If thickening is present, then viability is likely.
- PET
- Considered by many to be the gold standard. Can be used to assess perfusion and metabolism simultaneously. If there is mismatch in perfusion and metabolism, then the tissue is viable. If there is a match, then there is scar.
Dobutamine: Enhanced systolic contractility with low dose dobutamine is associated with recovery.