Imaging the perfusion/function match with Gated SPECT

Background

Gated Single Photon Emission Computed Tomography (G-SPECT) allows the simultaneous assessment of blood flow distribution and wall motion with negligible additional cost (1).
All cameras produced in the last years can be equipped with the gated acquisition and with the main software protocols able to quantify perfusion and function.
Despite this technical progress, the spectrum of information provided by G-SPECT continues to be under-evaluated in cardiology.

Myocardial perfusion

The assessment of myocardial perfusion has always been a part of nuclear medicine.

Following the Xenon age, Thallium 201 and more recently Technetium tracers have provided noninvasive 3D imaging able to differentiate scar form ischemia when correctly used and interpreted.
The need for absolute perfusion measurement of coronary reserve by PET has been limited to research, and SPECT is today the most widespread and reproducible perfusion technique available in clinical cardiology.
Alternative techniques such as contrast echocardiography or magnetic resonance continue their progress to detect blood flow without sufficient evidence to enter everyday patient management.

However, there will be in the future optimal spatial resolution able to discriminate the subendocardium, with significant progress in pathophysiology and decision-making. Unfortunately this progress has not proved to be fast, and experimental studies continue to be more represented than clinical trials.

Wall motion

Echocardiography continues to be the most used technique to assess regional wall motion analysis, and magnetic resonance strengthens its role as a 3D standard for mass, volume, wall motion and valvular function.
The application of contrast ventriculography is decreasing for several reasons such as reduction of contrast dose and exposure, limited 2D information, overestimation of ejection fraction due to geometrical assumptions, and availability of information on wall motion in more or less all patients undergoing angiography.
G-SPECT provides a full spectrum of wall motion analysis : EF is obtained from a 3D count-based volumetric approach, and closely correlates with EF obtained from magnetic resonance imaging. The lower correspondence with EF obtained from 2D techniques is intrinsically explained with the methodological differences between 2 and 3D methods, that results also in intra- and interobserver agreements generally higher for G-SPECT and magnetic resonance than for echocardiography.
Regionally, qualitative and quantitative data obtained from G-SPECT offer the advantage of reference normal values, simultaneous motion and thickening quantitative analysis, and segment-by-segment correlation with perfusion in the same model at the same time.
However, the output of data on regional and global wall motion of G-SPECT has probably not been sufficiently explained to cardiologists, and in many centres only EF is used as volume information from G-SPECT. It is evident that sometimes this EF is quite different from that obtained by echocardiography, and for this reason, is frequently skipped.

How to interpretate a G-SPECT study?
 

A G-SPECT perfusion study should be interpreted following a standard sequence of steps.
 
As discussed above, perfusion is still the main information provided by a G-SPECT, and from this point of view the qualitative approach is unchanged.
Thus, the cardiologist should localise perfusion defects in stress and rest studies, evaluate the severity and extension of defects and check their reversibility.
At the same time the cardiologist should also verify whether in viability assessments, Thallium 201 has been correctly reinjected and Technetium tracers associated to nitrates in rest conditions.

The next step should be that of semiquantitative measurements. All G-SPECT studies can be analysed by quantitative protocols that provide a Summed Stress Score (SSS), a Summed Rest Score (SRS) and a Summed Differential Score (SDS). Despite some limitations for control subjects, that changes from tracer to tracer and from male to female, these “numbers” have shown to have diagnostic and prognostic power independently of any qualitative analysis (2). Furthermore, the use of SDS may be helpful in identifying false positives, especially in the female population in which attenuation artefacts may be more common (3).
Finally, SSS, SRS and SDS must be checked with standard G-SPECT slices in order to confirm whether findings are normal or pathologic.
The echocardiographic experience should be applied to the analysis of regional and global wall motion. Quantitative analysis should be applied first. EF should be compared with the patient’s medical history or with other EFs obtained from different techniques, if available.
Cardiac volumes should be normalised for body surface area or mass to be compared among different patients. Differences in volumes from stress to rest should be also noticed since an EF drop after stress can be used to increase the sensitivity of G-SPECT independently from reversible perfusion defects (4).
Regionally, G-SPECT provides quantitative measurements such as Summed Motion and Thickening scores (SMS, STS), a G-SPECT alternative to echocardiographic wall motion score index. The differences between WMS and WTS may be useful in differentiating anterior infarct from left bundle branch block, or to discriminate true dyssinergies from conduction abnormalities.
Finally, the analysis of cine mode display, if available at the cardiological site, should be analysed in terms of echocardiographic regional wall motion.

The perfusion/function match

It is evident that after perfusion and wall motion analysis a perfusion/function match can be easily obtained by comparing the relative scores or from the direct analysis of perfusion and wall motion bull’s eyes.
The detection of cardiomyopathies will be enhanced with the discrepancy between a more or less normal distribution of blood flow with an evident, diffuse hypo/akinesis.
At the same time, areas of viable myocardium will be detected as those with maintained uptake and evident wall motion abnormality.
Similarly, conduction disturbances will be separated from true dissinergies.
This direct, 3D and segment by segment analysis, together with a semiquantitative approach may represent a cost saving approach to cardiac diseases, given the fact of a state of the art nuclear analysis and of a full cardiological use (5).

The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.