JNM: Dedicated cardiac SPECT comes of age
One of the innovations of the newer dedicated cardiac cameras involves improved software to filter out noise, according to Ernest V. Garcia, MD, and colleagues from the department of radiology at Emory. Traditionally, there is a loss of resolution inherent in parallel-hole collimators. Newer computing software "allows for a mathematic correction of this resolution degradation known as resolution recovery."
Noise is therefore suppressed "because additional counts are now correctly accounted for instead of being treated as noise."
Consequently, newer SPECT cameras can deliver higher resolution images in a shorter amount of time compared with conventional backprojected images reconstructed from studies acquired for longer times.
Most manufacturers of SPECT cameras have implemented some version of resolution recovery or noise reduction algorithms including Astonish (Philips Healthcare), Flash3D (Siemens Healthcare), Evolution (GE Healthcare), nSPEED (Digirad) and wide-beam reconstruction by a third-party vendor (UltraSPECT).
Researchers noted that these algorithms have been clinically validated, compared with conventional SPECT reconstruction, with findings that included:
- Images may be acquired in half the time without compromising qualitative or quantitative diagnostic performance, with or without attenuation correction;
- Images may even be acquired in a quarter of the time if the reconstruction is optimized for the reduced count density;
- Scanning provides left ventricular volumes that correlate but are significantly smaller;
- Scanning provides left ventricular ejection fractions that correlate but are lower than conventional reconstruction because of a reduction in end-diastolic volume or, for some implementations, an increase in end systolic volume; and
- Scanning provides similar diagnostic quality whether imaging time is reduced by half or a half-dose is injected.
New ultrafast camera designs
In the new camera designs, all available detectors are constrained to imaging just the cardiac field of view. "These new designs vary in the number and type of scanning or stationary detectors and in whether NaI, CsI, or cadmium-zinc-telluride (CZT) solid-state detectors are used."
However, they all have the potential for a "five- to 10-fold increase in count sensitivity at no loss of, or even a gain in, resolution resulting in the potential for acquiring a stress myocardial perfusion scan in two minutes or less if the patient is injected with a standard dose."
The first SPECT system to offer a totally different design was D-SPECT (Spectrum Dynamics), Garcia and colleagues noted.
On this scanner, the patient is imaged sitting in a reclining position, "similar to a dentist's chair, with the patient's left arm placed on top of the detector housing."
One of the first systems developed to take advantage of solid-state electronics and use more than two detectors simultaneously imaging the heart is the Cardius 3 XPO (Digirad). Three detectors are fixed at set angles, while patients are rotated through an arc while sitting on a chair with their arms resting above the detectors.
Researchers noted that "dedicated cardiac SPECT cameras are undergoing a profound change in design for the first time in 50 years."
The innovations have resulted in shorter study times or reduced radiation doses to patients. "Shorter study times promote easier scheduling, higher patient satisfaction and, importantly, less patient motion during acquisition, which translates to higher-quality images," they said.
"The fast acquisition also makes the hybrid SPECT/CT systems more practical since it allows the CT scanner to be used for a longer part of the day," they concluded.