Naotoshi Fujita

Estimation of ¹²³I-IMP arterial blood activity using ¹²³I-IMP acquisition data from the lungs and brain without any blood sampling: validation of its usefulness for quantification of regional cerebral blood flow.

Objective: The conventional methods for the estimation of regional cerebral blood flow (rCBF) using 123I-labeled N-isopropyl-p-iodoamphetamine (123I IMP) autoradiography (ARG) require continuous or 1-point arterial blood sampling. Patients who need rCBF quantification benefit from the avoidance of arterial puncture. In this study, we attempted to develop a method without any blood sampling to estimate 123I IMP activity in the arterial blood sample at 10 minutes after injection of 123I IMP (Ca10) for the purpose of rCBF quantification. For the evaluation of validity of this method, the mean of rCBFs in various regions of the brain (mean CBF) calculated by 123I IMP ARG method using the estimated Ca10 was compared with that calculated using the Ca10 directly measured with the actual arterial blood sample. Both groups of the mean CBF values were also compared with those measured by O-15 H2O PET ARG method. Methods: I-123 IMP ARG study was applied to 23 patients, and O-15 H2O PET ARG was applied to 20 patients of them. Dynamic images of the lungs, time series of static images of the brain, and brain SPECT images were acquired after injection of 123I IMP. Arterial blood sampling was done 10 minutes after injection of 123I IMP. Multiple regression analysis was used to estimate Ca10 using 5 parameters from the lung washout counts, time series of brain static counts, and brain SPECT average counts as the explanatory variables and the Ca10 directly measured with the actual arterial blood sample as the objective variable, and the regression equation was calculated. Results: The regression equation was calculated by multiple regression analysis as follows: Estimated Ca10 = (2.09 × 10−2 · LW3) − (2.29 × 10−4 · Cb5) − (9.87 × 10−3 · Cbpre-SPECT) + (1.06 · CbSPECTav) + (1.03 × 10−2 · Cbpost-SPECT) + 165 (counts/s/g), where LW3: lung washout count at 3 minutes after injection, Cb5: brain count at 5 minutes, Cb pre-SPECT: brain count before SPECT, Cb SPECT av: average brain count during SPECT, and Cb post-SPECT: brain count after SPECT. The estimated Ca10 values closely correlated with the directly measured Ca10 values (r = 0.907, P < 0.01). The mean CBF values (mL/min/100 g) calculated by 123I IMP ARG method using the estimated Ca10 also closely correlated with those calculated using the directly measured Ca10 (r = 0.818, P < 0.01). The mean CBF values calculated by the 123I IMP ARG method using either the directly measured or the estimated Ca10 significantly correlated (r = 0.698 and 0.590, respectively; P < 0.01) with those measured by O-15 H2O PET ARG method. Conclusions: The 123I IMP arterial blood activity can be estimated reliably without any blood sampling using the 123I IMP acquisition data from the lungs and brain. This method can serve for a convenient and noninvasive rCBF quantification technique instead of the conventional methods requiring arterial blood sampling.

Reduction of patient dose in digital mammography: Simulation of low-dose image using computed radiography system and flat panel detector system

To reduce the patients’ exposure, several low-dose images are necessary to obtain an image that can be used for diagnosis. However, it is clinically undesirable to expose a patient to multiple exposures in order to obtain an optimal image. The purpose of this study was to simulate a low-dose image from the image generated by a routine-dose. Images of acrylic steps were obtained using multiple doses in digital mammography to generate additional noise. The additional noise was calculated as three different noise sources. This study used the digital mammography system with different detectors. It is computed radiography (CR) system and flat panel detector (FPD) system. This noise was added to take into account the resolution of the X-ray detector using the following filters. The filters were designed based on the presampled modulation transfer function (MTF) and digital MTF containing aliasing. The image simulated using the presampled MTF filter was less similar to an actual low-dose image using the CR system. The image simulated using the digital MTF filter was closer to an actual low-dose image compared to the image simulated using the presampled MTF filter using the CR system. The image simulated using the digital MTF filter of the FPD system was similar to an actual low-dose image. By using the proposed method, we were able to obtain a simulated low-dose image from an image generated by a routine-dose.

Evaluation of the quality of image for various breast composition and exposure conditions in digital mammography

Breast density has a close relationship with breast cancer risk. The exposure parameters must be appropriately chosen for each breast. However, the optimal exposure conditions for digital mammography are uncertain in clinical. The exposure parameters in digital mammography must be optimized with maximization of image quality and minimization of radiation dose. We evaluated image quality under different exposure conditions to investigate the most advantageous tube voltage. For different compressed breast phantom thicknesses and compositions, we measured the Wiener spectrum (WS), noise-equivalent number of quanta (NEQ), and detective quantum efficiency (DQE). In this study, the signal-to-noise ratios were derived from a perceived statistical decision theory model with the internal noise of eye-brain system (SNRi), contrived and studied by Loo et al.1 and Ishida et al.2 These were calculated under a fixed average glandular dose. The WS values were obtained with a fixed image contrast. For 4-cm-thick and 50% glandular breast phantoms, the NEQ showed that high voltages gave a superior noise property of images, especially for thick breasts, but the improvement in the NEQ by tube voltage was not so remarkable. On the other hand, the SNRi value with a Mo filter was larger than that with a Rh filter. The SNRi increased when the tube voltage decreased. The result differed from those of WS and NEQ. In this study, the SNRi depended on the contrast of signal. Accuracy should be high with an intense, low-contrast object.

A simple method for evaluating image quality of screen-film system using a high-performance digital camera

Screen-film systems are used in mammography even now. Therefore, it is important to measure their physical properties such as modulation transfer function (MTF) or noise power spectrum (NPS). The MTF and NPS of screen-film systems are mostly measured by using a microdensitometer. However, since microdensitometers are not commonly used in general hospitals, it is difficult to carry out these measurements regularly. In the past, Ichikawa et al. have measured and evaluated the physical properties of medical liquid crystal displays by using a high-performance digital camera. By this method, the physical properties of screen-film systems can be measured easily without using a microdensitometer. Therefore, we have proposed a simple method for measuring the MTF and NPS of screen-film systems by using a high-performance digital camera. The proposed method is based on the edge method (for evaluating MTF) and the one-dimensional fast Fourier transform (FFT) method (for evaluating NPS), respectively. As a result, the MTF and NPS evaluated by using the high-performance digital camera approximately corresponded with those evaluated by using a microdensitometer. It is possible to substitute the calculation of MTF and NPS by using a high-performance digital camera for that by using a microdensitometer. Further, this method also simplifies the evaluation of the physical properties of screen-film systems.

Detection of alpha radionuclides in air from patients during Ra-223 alpha radionuclide therapy

Ra-223 has recently been introduced to alpha radionuclide therapy. According to the decay scheme of Ra-223, an inert gas, Rn-219 is released from patients during alpha radionuclide therapy and its daughter radionuclides may accumulate around the patient. However, the concentration of these radon daughters during alpha radionuclide therapy was not obvious. Here, we first detected the radon daughters of Rn-219 around patients during alpha radionuclide therapy. While the Ra-223-administered patients were in a room for ~1.5 hours, the radon daughter concentration increased to 4 to 5 times higher than without the patients. When the patients were in the room, the energy spectra of the alpha particles in the air showed the peak of the radon daughter of Rn-219, Bi-211 (6.6 MeV), which was different from that without the patients. We conclude that the daughter radionuclides of Rn-219 are accumulated around the patient, and the concentration was higher than that of the natural radon daughters. However, the increase in levels of alpha emitters, while detectable, is lower than the daily variations and thus is likely not a source of concern for radiation exposure.

Investigation of optimal acquisition time of myocardial perfusion scintigraphy using cardiac focusing-collimator

Recently myocardial perfusion SPECT imaging acquired using the cardiac focusing-collimator (CF) has been developed in the field of nuclear cardiology. Previously we have investigated the basic characteristics of CF using physical phantoms. This study was aimed at determining the acquisition time for CF that enables to acquire the SPECT images equivalent to those acquired by the conventional method in 201TlCl myocardial perfusion SPECT. In this study, Siemens Symbia T6 was used by setting the torso phantom equipped with the cardiac, pulmonary, and hepatic components. 201TlCl solution were filled in the left ventricular (LV) myocardium and liver. Each of CF, the low energy high resolution collimator (LEHR), and the low medium energy general purpose collimator (LMEGP) was set on the SPECT equipment. Data acquisitions were made by regarding the center of the phantom as the center of the heart in CF at various acquisition times. Acquired data were reconstructed, and the polar maps were created from the reconstructed images. Coefficient of variation (CV) was calculated as the mean counts determined on the polar maps with their standard deviations. When CF was used, CV was lower at longer acquisition times. CV calculated from the polar maps acquired using CF at 2.83 min of acquisition time was equivalent to CV calculated from those acquired using LEHR in a 180°acquisition range at 20 min of acquisition time.

Reduction of patient dose in digital mammography: simulation of low-dose image from a routine dose

Several low-dose images are necessary to obtain an image that can be used for diagnosis. However, it is clinically undesirable to expose a patient to multiple exposures in order to obtain an optimal image. The purpose of this study was to simulate a low-dose image from the image generated by a routine dose. Images of acrylic steps were obtained using multiple doses in digital mammography with computed radiography to generate additional noise. This noise was added to take into account the resolution of the X-ray detector using the some filters. The image simulated using the filter based on the WS was similar to an actual low-dose image. The image simulated using the presampled MTF filter was less similar to an actual low-dose image. By using the proposed method, we were able to obtain a simulated low-dose image from an image generated by a routine dose.

Evaluation of image quality characteristics of reduction image in high resolution liquid crystal display

With recent developments, digital mammograms can be obtained with a small pixel size, i.e., high resolution; however, the matrix size increases. Therefore, when the image is thinned out, image information is lost when the image is displayed on a liquid crystal display (LCD). To resolve this issue, we have developed a super high resolution liquid crystal display (SHR-LCD) by using a novel resolution enhancement technology for independent subpixel driving (ISD) with three subpixels in each pixel element. However, the lack of image information caused by thinning of the image cannot be ignored because the matrix size of a phase contrast mammogram (PCM) is very large as compared to that of a conventional mammogram. We obtained noise and edge images by using the geometrical layouts of the PCM (7080 x 9480). We measured the Wiener spectrum (WS), modulation transfer function (MTF), and noise-equivalent number of quanta (NEQ) of the images reduced by the nearest-neighbor, bilinear, and bicubic (sharpness and smooth) interpolations. The reduction rate was approximately 0.14. We measured the WS and MTF when the PCM image was displayed on a 5-megapixel (MP) and 15-MP LCD. The bilinear interpolation technique gave the best image quality. The image quality was further improved by using a 15-MP SHR-LCD.

Study of signal-to-noise ratios considered human visual characteristics

The effects of imaging parameters on detectability have not yet been clarified. Therefore, we investigated the usefulness of signal-to-noise ratios (SNRs) considered as human visual characteristics, such as the visual spatial frequency response and the internal noise in the eye-brain system. We examined the amplitude model (SNRa), matched filter model (SNRm), and internal noise model (SNRi) to study the relationship between these SNRs and the visual image quality for signal detection. The test images were simulated by the superimposition of low-contrast signals on a uniform noisy background. The SNRs were obtained for 15 imaging cases with various signal sizes, signal contrasts, exposure levels, and number of acrylic plates used as breast phantoms. The SNRs were calculated by measuring the spatial frequency characteristics of the signal, modulation transfer function (MTF) of the system, display MTF, and overall Wiener spectrum (WS). In the perceptual evaluation, we applied the 16-alternative forced choice (16-AFC) method. The signal detectability was defined as the number of detected signals divided by the total number of signals. We studied the relationship between SNR and signal detectability using Spearmans rank correlation coefficient. The correlation coefficient of SNRi was 0.93, making it the highest among the three SNR types. That of SNRm was 0.91; it correlated at the same level as SNRi although it is not considered human visual characteristics. That of SNRa was 0.45. SNRi, which incorporated the visual characteristics, explained the visual image quality well.

Comparison of the detection rates in reduced image by difference of interpolation method

In the soft copy diagnosis, each pixel of the detector is displayed to the correspondent pixel of liquid crystal display (LCD). But when the image is displayed at the first time, the entire image may be reduced. We examined the influence that the difference of image reduction rate on LCD exerts on detection performance by using observer performance experiment. Moreover, to find the best interpolation method, we investigated the several interpolation methods. We made a simulation image which is similar to Burger phantom. This image consists of 288 signals, each of a different size and contrast. The matrix size is the same as Phase Contrast Mammography (PCM). We gradated the simulation image by using an MTF of a geometric blur, and the image was added to the noise image which is uniformly exposed with PCM. Then the image was reduced by using the nearest-neighbor, the bilinear, and the bicubic methods. The reduction rates were calculated as the ratios of the number of pixels of LCDs to those of PCM. We displayed the reduced images on LCD and examined the detection performance. Results of physical evaluation examined before showed that sharpness and granularity have worsened both in proportion to the reduction rate. The detection performance deteriorated as the reduction rate becomes high. In the comparison of the interpolation methods, the detection performance of the nearestneighbor method was worse than those of other interpolation methods. The bilinear method is the most suitable for the reduction of the image.