Automatic exposure control at single- and dual-heartbeat CTCA on a 320-MDCT volume scanner: Effect of heart rate,exposure phase window setting,and reconstruction algorithm

PurposeTo investigate whether electrocardiogram (ECG)-gated single- and dual-heartbeat computed tomography coronary angiography (CTCA) with automatic exposure control (AEC) yields images with uniform image noise at reduced radiation doses.Materials and methodsUsing an anthropomorphic chest CT phantom we performed prospectively ECG-gated single- and dual-heartbeat CTCA on a second-generation 320-multidetector CT volume scanner. The exposure phase window was set at 75%, 70–80%, 40–80%, and 0–100% and the heart rate at 60 or 80 or corr80 bpm; images were reconstructed with filtered back projection (FBP) or iterative reconstruction (IR, adaptive iterative dose reduction 3D). We applied AEC and set the image noise level to 20 or 25 HU. For each technique we determined the image noise and the radiation dose to the phantom center.ResultsWith half-scan reconstruction at 60 bpm, a 70–80% phase window- and a 20-HU standard deviation (SD) setting, the imagenoise level and -variation along the z axis manifested similar curves with FBP and IR. With half-scan reconstruction, the radiation dose to the phantom center with 70–80% phase window was 18.89 and 12.34 mGy for FBP and 4.61 and 3.10 mGy for IR at an SD setting SD of 20 and 25 HU, respectively. At 80 bpm with two-segment reconstruction the dose was approximately twice that of 60 bpm at both SD settings. However, increasing radiation dose at corr80 bpm was suppressed to 1.39 times compared to 60 bpm.ConclusionAEC at ECG-gated single- and dual-heartbeat CTCA controls the image noise at different radiation dose.     

Transluminal attenuation-gradient coronary CT angiography on a 320-MDCT volume scanner: Effect of scan timing,coronary artery stenosis,and cardiac output using a contrast medium flow phantom

PurposeTransluminal-attenuation-gradient (TAG) may reflect patient characteristics and physiological parameters. Furthermore, TAG may be affected by factors such as the CT scanner speed, scanning method, scan timing after contrast-medium (CM) injection, and the injection methods. The purpose of our study was to investigate quantitative TAG at different scan timing points after CM injection for coronary CT angiography.Materials and methodsUsing a CM flow phantom and two types of connecting tube mimicking 0% and 70% coronary artery stenosis, we performed 320-detector volume scanning. The heart rate was set at 60 bpm and cardiac-output (CO) at 2.0 and 4.0 l/min, respectively. The acquisition time repeated at 0.5-s intervals for 40 and 25 s at a CO of 2.0- and 4.0 l/min. We measured the CT number on the same slice level, calculated the time-density-curve (TDC) and the TAG at each time point.ResultsAt COs of 2.0 and 4.0 l/min at 0% stenosis, TAG exhibited smaller variations (−3.02 to +0.55 HU/cm at 2.0 l/min, −2.63 to +0.43 HU/cm at 4.0 l/min) than at 70% stenosis at each time point along the TDC. Compared with a CO at 2.0 l/min with 70% stenosis, the TAG curve for a CO at 4.0 l/min gradually changed with time (−6.64 to +1.18 HU/cm at 2.0 l/min vs. −3.46 to +2.75 HU/cm at 4.0 l/min).ConclusionThe TAG value was affected by scan timing after CM injection and by CO although the size of the connecting tube with and without stenosis was identical.     

Spectral optimization of iodine-enhanced CT: Quantifying the effect of tube voltage on image quality and radiation exposure determined at an anthropomorphic phantom

PurposeTo provide an experimental basis for spectral optimization of iodine-enhanced CT by a quantitative analysis of image quality and radiation dose characteristics consistently measured for a large variety of scan settings at an anthropomorphic phantom.MethodsCT imaging and thermoluminescent dosimetry were performed at an anthropomorphic whole-body phantom with iodine inserts for different tube voltages (U, 70–140 kV) and current-time products (Q, 60–300 mAs). For all U-Q combinations, the iodine contrast (C), the noise level (N) and, from these, the contrast-to-noise ratio (CNR) of reconstructed CT images were determined and parameterized as a function of U, Q or the measured absorbed dose (D). Finally, two characteristic curves were derived that give the relative increase of CNR at constant D and the relative decrease of D at constant CNR when lowering U.ResultsLowering U affects the measured CNR only slightly but markedly reduces D. For example, reducing U from 120 kV to 70 kV increases the CNR at constant D by a factor of nearly 1.8 or, alternatively, reduces D at constant CNR by a factor of nearly 5.ConclusionSpectral optimization by lowering U is an effective approach to attain the necessary CNR for a specific diagnostic task at hand while at the same time reducing radiation exposure as far as practically achievable. The characteristic curves derived in this study from extensive measurements at a reference ‘person’ can support CT users in an easy-to-use manner to select an appropriate voltage for various clinical scenarios.