Gated SPECT (1) is an acquisition technique where the patients ECG is used to guide the acquisition.

The acquisition computer is used to define the number of bins (frames) in which the RR interval are to be divided, usually 8 frames but 16 can be used. The patient ECG is connected to the computer, and the RR intervals over 30-60 sec are collected and usually displayed graphically as a histogram. A 10-20% window is selected around the mean RR interval, meaning that all data from cardiac cycles within this window will be accepted, while data from RR intervals outside the limits of the window will be rejected.

When the acquisition is started, the computer waits for the first R waves to check if the RR interval is within the defined limits. If the RR interval is within the defined limits, the data are accepted, if no the data are rejected. The data from the first 1/8 of the cardiac cycle are stored in bin 1 (frame 1); the data from second 1/8 is stored in bin 2 etc. When the predefined acquisition time / angle is reached, the camera moves 3 degree to the next angle, waits for the R signal, receive the signal and starts the collection as above. This procedure repeated over 180 degree.

In a std SPECT study (above, left), for example a 180 degree-60 steps-30 sec / step setup, one 30 sec projection image is obtained from each step. These 60 projection images (raw data) are processed with a backprojection algorithm giving the usual tomographic data.

In a gated SPECT (above, right), 8 projection images are obtained at each step. Each image represents what the heart looked like in that 1/8 (fraction) of the cardiac cycle. Each image is obtained over about just 4 sec (30 sec / 8), a very short time and therefore very noicy. A complete gated SPECT acquisition thus consists of 8 files each representing a fraction (eight) of the cardiac cycle.Each file is processed with a backprojection algorithm to generate 8 tomographic sets of data. These data sets are then rearranged to a dynamic study.
A gated SPECT study thus requires:

1. Acquisition software,
2. Software for reconstruction,
3.Lots of disk space,
4.A reasonable fast computer.

To obtain a quality gated SPECT study a good ECG signal with well defined positive R wave and regular cardiac rhythm are required. Premature beats with short and/ or long RR intervals, which fall outside the define window, will prolong the acquisition.
Patient in atrial fibrillation are usually impossible to gate.
Patient with pacemakers can usually be gated, but one have to pay special attention to the ECG.
Bundle branch blocks are not a problem in gated SPECT.

Gated SPECT studies can be displayed and analized as dynamic study.
Some information about regional wall motion abnormalities, and volumes can be extracted from such data.
Regional wall thickening assessment has been attempted, with questionable results.

Reliable and extremely useful information can be obtained from Gated SPECT studies by using specific analytic software like the QGS(2) or Emory Cardiac Toolbox (3) packages. Global EF, regional wall motions, and cardiac volumes are supplied by these packages.

Clinically important functional information is thus supplied by gated SPECT technique in addition to the traditional perfusion information -- at very little extra camera and processing time.

RAO projection. The anterior wall, apex, and the inferior wall are visualized. The endocardial edge in end-diastole is marked green. The endocardial edge in end-systole is marked red.
The regional wall motions are thus clearly assessable (commonly used scoring is: 4=normal, 3=mild hypokinesis (HK), 2=moderate HK, 1=severe HK, 0=akinesis (AK),-1=dyskinesis). In this normal subject all segments shows normal wall motion (WM).

ECHO projection (4-chamber view): Septum, apex, and the lateral wall are visualized. All segments shows normal WM as indicated by the motion of the endocardial edge betwee end-systole and and-diastole.

The total wall motion score based on these two projections in this case is 28+28=56.

RAO projection: Apical and apical anterior moderate HK.

ECHO projection: Septum shows marked HK.

NB The perfusion in septum was normal. Such septal hypokinesis is almost always present in patient who have had open-chest surgery. The etiology is not absolutely clear; it might have to do with changed pericardial support for the right ventricule.

The total wall motion score based on these two projections is in this case: 23+17=40.