Application‐ and patient size‐dependent optimization of x‐ray spectra for CT

Abstract

Although x-ray computed tomography (CT) has been in clinical use for over 3 decades, spectral optimization has not been a topic of great concern; high voltages around 120 kV have been in use since the beginning of CT. It is the purpose of this study to analyze, in a rigorous manner, the energies at which the patient dose necessary to provide a given contrast-to-noise ratio (CNR) for various diagnostic tasks can be minimized. The authors used cylindrical water phantoms and quasianthropomorphic phantoms of the thorax and the abdomen with inserts of 13 mm diameter mimicking soft tissue, bone, and iodine for simulations and measurements. To provide clearly defined contrasts, these inserts were made of solid water with a 1% difference in density (DD) to represent an energy-independent soft-tissue contrast of 10 Hounsfield units (HU), calcium hydroxyapatite (Ca) representing bone, and iodine (I) representing the typical contrast medium. To evaluate CT of the thorax, an adult thorax phantom (300 x 200 mm2) plus extension rings up to a size of 460 x 300 mm2 to mimic different patient cross sections were used. For CT of the abdomen, we used a phantom of 360 x 200 mm2 and an extension ring of 460 x 300 mm2. The CT scanner that the authors used was a SOMATOM Definition (Siemens Healthcare, Forchheim, Germany) at 80, 100, 120, and 140 kV. Further voltage settings of 60, 75, 90, and 105 kV were available in an experimental mode. The authors determined contrast for the density difference, calcium, and iodine, and noise and 3D dose distributions for the available voltages by measurements. Additional voltage values and monoenergetic sources were evaluated by simulations. The dose-weighted contrast-to-noise ratio (CNRD) was used as the parameter for optimization. Simulations and measurements were in good agreement with respect to absolute values and trends regarding the dependence on energy for the parameters investigated. For soft-tissue imaging, the standard settings of 120-140 kV were found as adequate choices with optimal values increasing for larger cross sections, e.g., for large abdomens voltages higher than 140 kV may be indicated. For bone and iodine imaging the optimum values were generally found at significantly lower voltages of typically below 80 kV. This offers a potential for dose reduction of up to 50%, but demands significantly higher power values in most cases. The authors concluded that voltage settings in CT should be varied more often than is common in practice today and should be chosen not only according to patient size but also according to the substance imaged in order to minimize dose while not compromising image quality. A reduction from 120 to 80 kV, for example, would yield a reduction in patient dose by more than half for coronary CT angiography. The use of lower voltages has to be recommended for contrast medium studies in cardiac and pediatric CT.

Optimizing the target detectability of cone beam CT performed in image‐guided radiation therapy for patients of different body sizes

“…The nested disk of the electron density phantom was made from plastic water (1.016 g/cc). The contrast‐to‐noise ratio (CNR) is an important index for the detection and diagnosis of structure and details of interest in CT,20 so CNR was used in this study to quantify the target detectability in both MDCT and CBCT. A circular region‐of‐interest (ROI) of 31 pixels was placed on the target and background regions in 11 slices (the central slice ±5 slices) to calculate the mean and standard deviation of HUs within ROI.…”

Carotid CTA: Radiation Exposure and Image Quality with the Use of Attenuation-Based, Automated Kilovolt Selection

“…14,15 Besides minimizing tube current, the reduction of the tube voltage is a potent option to reduce radiation dose. Such lowkilovolt protocols are used in cerebral perfusion CT in the evaluation of patients with stroke 16 and in pediatric CT. 17 In phantom 18 as well as in clinical studies, it has been demonstrated that this technique can also be used in chest and abdominal imaging in adults. [19][20][21][22][23][24] Lower tube voltage in CTA results in higher noise levels on one hand, but the attenuation of iodine is increased 25,26 on the other hand.…”

CT protocol management: simplifying the process by using a master protocol concept

“…The equal CNR requirement is not made when making a protocol change on a single scanner. To determine how the image contrast changes with beam energy, we have relied on the CT literature 11 , 16 , 17 . For example, if the CNR is desired to be kept constant but the kV is changed from 100 to 80, the contrast would increase by a factor of 1.32 (282 HU at 80 kV divided by 213 HU at 100 kV for iodine in a 25 cm phantom), according to Yu et al (11) For the example change of 100 to 80 kV, the noise needs to increase by a factor of 1.32 to keep the CNR constant (for iodine contrast imaging) and therefore the mA would need to change by ${\left(\frac{1}{1.32}\right)}^2=0.57$.…”