Issei Mori

Photon starvation artifacts of X-ray CT: their true cause and a solution

When too few photons reach detector elements, strong streaks appear through paths of high X-ray attenuation and an image becomes completely useless. This photon starvation artifact phenomenon occurs frequently when a pelvis or shoulder is scanned with thin slices. The common understanding regarding photon starvation streaks is that they are a manifestation of irregularities caused by noise in the raw data profile. Therefore, the common countermeasure is local raw-data filtering, which detects and smoothes out the highly noisy part of the raw data. However, the photon starvation artifact can be solved only partly with such a method and a more effective solution is necessary. Here, we examined the mean level shift of raw data attributable to the nonlinear nature of logarithmic conversion, which is the process required for generating raw data from detected X-ray data. We judge that the real culprit of the photon starvation artifact is this mean level shift. When the noise level is very high or the photon level is very low, this mean level shift can become prominent and can become manifest as thick streaks against which the conventional local raw data filtering has no power. To solve this problem, we propose a new scheme of local raw data filtering, which consists of reverting log-converted raw data to a form that is equivalent to pre-log detector data. With this method, not only fine streaks, but also thick streaks are removed effectively. A better image quality with lower X-ray doses is possible with this method.

Evaluating the performance of a MOSFET dosimeter at diagnostic X-ray energies for interventional radiology

For reducing the risk of skin injury during interventional radiology (IR) procedures, it has been suggested that physicians track patients’ exposure doses. The metal-oxide semiconductor field effect transistor (MOSFET) dosimeter is designed to measure patient exposure dose during radiotherapy applications at megavoltage photon energies. Our purpose in this study was to evaluate the feasibility of using a MOSFET dosimeter (OneDose system) to measure patients’ skin dose during exposure to diagnostic X-ray energies used in IR. The response of the OneDose system was almost constant at diagnostic X-ray energies, although the sensitivity was higher than that at megavoltage photon energies. We found that the angular dependence was minimal at diagnostic X-ray energies. The OneDose is almost invisible on X-ray images at diagnostic energies. Furthermore, the OneDose is easy to handle. The OneDose sensor performs well at diagnostic X-ray energies, although real-time measurements are not feasible. Thus, the OneDose system may prove useful in measuring patient exposure dose during IR.

Deriving the modulation transfer function of CT from extremely noisy edge profiles

The point spread function (PSF) method is currently the one predominantly used to determine the modulation transfer function (MTF) of an X-ray CT system. However, the image examined with the PSF method must have a very high contrast-to-noise ratio (CNR); it must also be reconstructed with a fine pixel pitch using a zooming reconstruction. Therefore, the PSF method is often inappropriate for describing the MTF of clinical operating conditions when image linearity is not guaranteed. The edge spread function (ESF) method requires no zooming reconstruction, but its susceptibility to image noise is no better than that of the PSF method. We describe a technique for rendering the ESF method robust to image noise. We smooth out the noisy ESF through multiple stages of filtering. Invariably, the line spread function (LSF) obtained from the smoothed ESF is blurred, and the MTF obtained from the LSF is incorrect. However, because the filtering that has been applied is known, much of the LSF blurring can be corrected. An estimate of the true LSF is obtainable from the blurred LSF, assuming that the true LSF is not very different from either a Gaussian or a composite of multiple Gaussians. For an image reconstructed with a kernel for soft-tissue imaging, the MTF obtained by our method is sufficiently consistent with the theoretical MTF, even when the CNR is as low as 2.

Reduced compression mammography to reduce breast pain

This study evaluated whether reduced compression mammography to relieve breast tenderness is feasible. Women can better tolerate a compression force of approximately 90 N in mammography. The breast thickness increased approximately 3 mm when the compression force was reduced to 90 N, and although the radiation dose increased approximately 20%, the image quality was identical to that with standard compression. Many patients experience breast pain with a compression force of 120 N. Reduced compression force mammography is acceptable in women whose breasts are particularly sensitive.

Development of a micro-imaging probe for functional brain imaging.

Multicellular neuronal activities should be investigated to reveal the dynamics of the neuronal circuit. Optical recording from neuronal populations is suitable for recording multicellular activities. We fabricated the prototype of the micro-imaging probe in combination with a gradient index lens and image fiber. This probe has a smaller diameter than traditional probes. We found an optimal optical configuration for maximizing the efficiency of the imaging probe. Using this optical configuration with the prototype of the imaging probe, the fluorescence images were captured from neurons expressing green fluorescent protein in a cerebellar block preparation, and the calcium-dependent images were sampled in a mouse brain slice preparation. Our optical system would facilitate the in vivo imaging studies with less invasive manners using thinner optic fiber than previously made.

Effect of radiation monitoring method and formula differences on estimated physician dose during percutaneous coronary intervention.

Background: Currently, one or two dosimeters are used to monitor radiation exposure in most cardiac laboratories. In addition, several different formulas are used to convert exposure data into an effective dose (ED). Purpose: To clarify the effect of monitoring methods and formula selection on the estimated ED for physicians during percutaneous coronary interventions (PCIs). Material and Methods: The ED of physicians during cardiac catheterization was determined using an optically stimulated luminescence dosimeter (Luxel badge). Two Luxel badges were worn: one beneath a personal lead apron (0.35-mm lead equivalent) at the chest and one outside of the apron at the neck. Results: The difference in the average ED of seven physicians was approximately fivefold (range 1.13–5.43 mSv/year) using the six different formulas in the clinical evaluation. The estimated physician ED differed markedly according to both the monitoring method and formula selected. Conclusion: ED estimation is dependent on both the monitoring method and the formula used. Therefore, it is important that comparisons among laboratories are based on the same monitoring method and same formula for calculating the ED.

Validation of the use of calibration factors between the iodine concentration and the computed tomography number measured outside the objects for estimation of iodine concentration inside the objects: phantom experiment

PurposeThe aim of this study was to validate the use of a calibration factor measured outside the object for estimating the iodine concentration inside the object to improve the accuracy of the quantitative contrastenhanced computed tomography (CT).Materials and methodsSeveral known concentrations (0, 6, 9, and 12 mg I/ml) of iodine contrast material (CM) samples were placed inside and outside cylindrical acrylic phantoms of two sizes and were imaged under various combinations of the tube voltages and currents (kV/mAs–80/200, 100/200, 120/200, 140/200) to obtain K factors. The K factors were compared between the phantoms and among the tube voltages. Each CM concentration was estimated from the CT number using the K factor measured outside the phantom.ResultsThe K factors varied between the phantoms or among the tube voltages (P < 0.05). Although there were statistically significant variations in K factors among the different regions in a phantom, the mean variation coefficient was 3%–4%. The mean error of the estimated concentration was −5.5%.ConclusionThe CM concentration should be accurately estimated at the region within a patient’s body using the K factor measured at the surface of the body regardless of body size and tube voltage.

Both neurons and astrocytes exhibited tetrodotoxin-resistant metabotropic glutamate receptor-dependent spontaneous slow Ca2+ oscillations in striatum.

The striatum plays an important role in linking cortical activity to basal ganglia outputs. Group I metabotropic glutamate receptors (mGluRs) are densely expressed in the medium spiny projection neurons and may be a therapeutic target for Parkinsons disease. The group I mGluRs are known to modulate the intracellular Ca2+ signaling. To characterize Ca2+ signaling in striatal cells, spontaneous cytoplasmic Ca2+ transients were examined in acute slice preparations from transgenic mice expressing green fluorescent protein (GFP) in the astrocytes. In both the GFP-negative cells (putative-neurons) and astrocytes of the striatum, spontaneous slow and long-lasting intracellular Ca2+ transients (referred to as slow Ca2+ oscillations), which lasted up to approximately 200 s, were found. Neither the inhibition of action potentials nor ionotropic glutamate receptors blocked the slow Ca2+ oscillation. Depletion of the intracellular Ca2+ store and the blockade of inositol 1,4,5-trisphosphate receptors greatly reduced the transient rate of the slow Ca2+ oscillation, and the application of an antagonist against mGluR5 also blocked the slow Ca2+ oscillation in both putative-neurons and astrocytes. Thus, the mGluR5-inositol 1,4,5-trisphosphate signal cascade is the primary contributor to the slow Ca2+ oscillation in both putative-neurons and astrocytes. The slow Ca2+ oscillation features multicellular synchrony, and both putative-neurons and astrocytes participate in the synchronous activity. Therefore, the mGluR5-dependent slow Ca2+ oscillation may involve in the neuron-glia interaction in the striatum.

Noise Power Spectrum in PROPELLER MR Imaging

The noise power spectrum (NPS), an index for noise evaluation, represents the frequency characteristics of image noise. We measured the NPS in PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) magnetic resonance (MR) imaging, a nonuniform data sampling technique, as an initial study for practical MR image evaluation using the NPS. The 2-dimensional (2D) NPS reflected the k-space sampling density and showed agreement with the shape of the k-space trajectory as expected theoretically. Additionally, the 2D NPS allowed visualization of a part of the image reconstruction process, such as filtering and motion correction.

Feasibility of cardiac MR examination during quantitative isometric muscular exercise

Objective — MR examination during quantitative muscular exercise is currently unavailable because the MR instrument has a long gantry and exercise machines are made from ferromagnetic materials such as iron. The purpose of this study was to develop a quantitative, continuously monitored, voluntary isometric exercise device (QIED) for MR examination and to analyse the feasibility of performing MR examination using the new device. Methods and results — The QIED, which consists of 1) a handgrip (rubber cuff), 2) an extension plastic tube, 3) a pressure transducer, and 4) a pressure digital display that includes the power supply. Components 1 and 2 are non-magnetic. Although components 3 and 4 are ferromagnetic, they can be set up outside the MR examination room using component 2 to prevent them from influencing the MR instrument and examination.We did not observe MR image noise or artifacts in the phantom study using the QIED. MR examination and low sensitivity 31P-MRS could be feasible during quantitative isometric exercise using the QIED. Conclusion — Exercise MR examination using the QIED will provide useful information for the detailed evaluation of cardiac patients.