MPI via dedicated cardiac camera cuts scan time, dose

SPECT-MPI imaging with a dedicated cardiac camera may reduce the duration of the acquisition and radiation dose to patients, according to a simulation study. Using reframed list-mode raw data, researchers determined that 1 million counts produced images on par with 8 million counts.

The study was published online Jan. 15 in The Journal of Nuclear Medicine.

Ryo Nakazato, MD, of Cedars-Sinai Heart Institute at the Cedars-Sinai Medical Center Los Angeles, and colleagues designed the simulation to test the feasibility of very low-dose MPI by reducing count levels in the myocardium. They hypothesized that today’s high-sensitivity dedicated camera systems could be leveraged to identify the lower limits for dosing that would still achieve accurate images. That information could be used to reduce the effective radiation dose to patients as well as scanning time.

“Several new dedicated camera systems with optimized acquisition geometry and collimator design resulting in increased count sensitivity have been introduced by vendors,” Nakazato et al wrote. “Although it is convenient for patients to have imaging completed in two to four minutes, the efficiency of these systems provides the opportunity to reduce both the duration of the acquisition and the radiation dose to the patient.”

They evaluated 79 consecutive patients who had been referred for a rest-stress MPI. Each patient underwent MPI with a cadmium-zinc-telluride dedicated cardiac camera (D-SPECT, Spectrum Dynamics) with a weight-adjusted 1-d standard dose 99mTc-sestamibi. Patients had a mean body mass index (BMI) of 30. The imaging time totaled 14 minutes with an average 803 MBq of 99mTc injected at stress.

For the lower-count simulation, Nakazato et al used each patient’s list-mode raw data that was reframed to create a truncated view. They then reconstructed six datasets of various counts, ranging from 3.6 million to 0.5 million. For image analysis, they applied software to quantify total perfusion deficit (TPD) and ejection fraction. Thirty-two patients had abnormal stress TPD.

They determined that left ventricular region counts as low as 1 million retained excellent agreement in quantitative perfusion and function parameters seen at high-count images, even in the presence of perfusion defects. In a 14-minute acquisition at a 1 million count, injected radiation could be reduced by a factor of 8, they calculated.

“Because it is desirable to reduce both radiation dose and scanning time, these findings further imply that by increasing the injected dose to 125.8 MBq (3.4 mCi), a submillisievert stress-only MPI study can be achieved with an acquisition time of 10 minutes,” they wrote. “However, a larger, prospective multicenter trial is warranted for further investigation.”

Estimates would vary, based on an individual patient’s BMI, with higher doses in patients with a high BMI and lower in patients with a low BMI, they added.

The authors suggested that similar simulation studies could be designed using other dedicated cardiac systems. They acknowledged some limitations, including the reliance on a single center and a single type of dedicated cardiac system. Their use of a fully automated quantitative analysis of MPI studies eliminated observer variability and bias, though, which they reasoned increased the probability that their results could be repeated elsewhere.