Optimisation of protocol for low dose CT-derived attenuation correction in myocardial perfusion SPECT imaging

In clinical routine, attenuation correction (AC) using X-ray CT is a relatively new method for reducing attenuation artefacts. We evaluated the quality of attenuation maps generated with very low tube current to minimise exposure due to transmission scanning.

SPECT/CT acquisitions were performed with a Millenium VG3 gamma camera with the Hawkeye CT device (GE Medical Systems). In phantom studies, determination of linear absorption coefficients (mu) for air, water and Teflon was carried out. The attenuation maps in both stress and resting studies from 62 patients (21 females and 41 males, age 63.7 +/- 11.0 years, BMI 30.0 +/- 5.7 kg/m(2)) were compared. All patients underwent exercise or pharmacologic stress testing and a resting study for comparison using Tc-99m MIBI or Tc-99m Tetrofosmin. AC in stress studies was performed using 2.5 mA tube current (set as default), whereas 1.0 mA was used in resting studies.

In both phantom and patient studies, differences of linear absorption coefficients were not significant (p > 0.05). Effective dose decreased from 0.90 mSv down to 0.36 mSv, respectively.

Our results indicate that reliable attenuation maps (mu-maps) of the thorax can be obtained even with the use of very low tube current. In our study, radiation exposure in CT-based AC for myocardial perfusion SPECT was substantially lowered (60% reduction). This is of particular importance in high-risk patients who may have to undergo follow-up scans and in research studies on volunteers. The procedure introduced is relatively simple and can be transferred to other SPECT/CT devices, which allow adjustment of tube current.