Yasuhiro Noshi

Improvement in image quality of noncontrast head images in multidetector-row CT by volume helical scanning with a three-dimensional denoising filter

PurposeThe aim of this study was to improve the contrast-to-noise ratio on noncontrast head computed tomography (CT) images, which are crucial for assessing patients with acute ischemic stroke. We applied a technique combining volume helical scanning with a three-dimensional (3D) denoising filter.Materials and methodsWe scanned phantoms for low-contrast resolutions and helical/cone-beam artifacts as well as stroke patients using a 16-row multidetector-row CT (MDCT) unit. Volume helical scans with 1-mm collimation and nonhelical scans with 8-mm thickness were performed. From the 1-mm thick volume data, 8-mm thick contiguous images were generated before and after applying a 3D denoising filter.ResultsOn images stacked from volume data, the contrast-to-noise ratio was significantly improved by the 3D denoising filter and was nearly the same as that on nonhelical images. On stacked volume images, artifacts due to the cone beam and the helical scan were increased with larger helical pitches, but bone-related streak artifacts in the posterior fossa and underneath the calvarium were reduced when compared with nonhelical images.ConclusionVolume helical scan with a 3D denoising filter effectively improves image quality in noncontrast head MDCT images.

Development of Variable Pitch Factor Scanning for Multislice Computed Tomography

RATIONALE AND OBJECTIVES The latest multislice computed tomography (MSCT) scanners permit the chest and abdomen to be scanned continuously. However, conventionally, it has been necessary to perform scanning twice using different pitch factors for the cardiac and abdominal regions. We have developed a new scanning technique known as variable pitch factor scanning, in which the table speed is changed during scanning to obtain continuous images from the heart to the abdomen in a single scan, and have evaluated its physical characteristics. MATERIALS AND METHODS A bead phantom, a comb phantom, and a gold wire placed at an angle were scanned using a 64-row MSCT scanner. The variation in the spatial resolution and continuity of images in the body axis direction because of changes in the pitch factor were evaluated. RESULTS Because reconstruction taking the cone angle into consideration was employed, the spatial resolution in the body axis direction was unchanged and the continuity of images in the body axis direction was maintained at a certain level even when the pitch factor was changed. CONCLUSION Variable pitch factor scanning is a useful technique for obtaining continuous images from the heart to the abdomen in a single scan.

Impacts of reduction of CT radiation dose on PET in PET/CT imaging

We studied the impacts of CT dose reduction on PET quantitation with combination of currently available techniques by phantom and human studies on a Toshiba PET/CT prototype scanner with TOF capability. For the phantom studies, an anthropomorphic torso phantom and an IEC phantom were scanned using both diagnostic quality CT protocols and dose-reduced CT protocols. To achieve reduced-dose CT scans, we used tube current reduction, spectral shaping, helical pitch optimization and AIDR 3DTM (Adaptive Iterative Dose Reduction). Then, both the high and low dose acquired CT images were resampled and used to generate attenuation maps for PET. The CT images, attenuation maps and reconstructed PET images were compared between high and low dose CT protocols. ROI-based analysis for each tissue type was also performed to see the impact of the dose reduction on PET quantitative accuracy among different dose protocols. Finally, the selected optimal low dose CT protocol was used for attenuation correction in whole-body PET/CT imaging on twelve patients. The PET data were reconstructed with fully 3D list-mode TOF ordered-subset expectation maximization algorithm with physical corrections of attenuation, scatter, random, detector normalization, decay, duration and dead time. The reconstructed PET images show that in PET/CT imaging, the PET diagnostic quality could be remained unchanged, while CT dose could be dramatically reduced compared to the diagnostic protocol, when CT is not used for diagnostic purpose. There is big room for CT dose reduction as long as the bias and noise in CT could be controlled in an acceptable range without introducing artifacts through CT-based PET attenuation correction. This study demonstrates that CT could be tailored for the intended purpose in PET/CT imaging to dramatically reduce patient radiation dose, while still maintaining high quality PET images.

A physics-based fast approach to scatter correction for large cone angle computed tomographic systems

An efficient scatter correction model is developed for large cone angle CT systems. The scatter intensities are expressed as a 2D convolution of forward functions with two Gaussian kernels. The kernels are derived from the differential cross sections of the Rayleigh (coherent) and Compton (incoherent) scattering. The results of water phantoms show good CT number uniformity and accuracy. The clinical images demonstrate a significant improvement on image quality.