In radiation therapy, the dose absorbed by the target tissue needs to be extremely accurate. In order to obtain the target absorbed dose, radiation dose measurements are performed using a phantom instead of the patient's body, because the target absorbed dose cannot be directly measured. Although water is the best human muscle equivalent phantom, it is not useful for this purpose. Therefore, water equivalent solid phantoms are usually used for the measurements. We compared the following water equivalent solid phantoms for water:Tough water phantom, 457 Solid water phantom, RW-3, Mix-DP, polystyrene resin, polyethylene resin, and acrylic resin. The measurements obtained were 1)ionization current in the phantoms as determined by ionization chamber, 2)tissue-maximum ratio, 3)transmission measurements in water with and without the phantoms, 4)Hounsfield units of the phantoms for uniformity of inside phantoms as determined by computed tomography, and 5)accuracy of the phantoms. Results showed the phantoms to be almost equivalent to water, except for the acrylic resin phantom. However, the phantoms had various characteristics that affected accuracy, and the phantoms underwent change with time. Measurement error was caused by the characteristics of the phantoms. Therefore, it is important to measure the calibration coefficient of phantoms for water, regardless of what is stated on paper.
The brightness of an image intensifier (I.I.) decreases with its use over time. The extent of the decrease can be determined by finding a conversion factor (Gx) for evaluating the sensitivity of the I.I. Although JIS requires that Gx be measured with a luminance meter, we have developed a simplified version. In our newly developed system, a photo sensor rater than a luminance meter is used, and, from its output values, relative I.I. brightness is instantly calculated. Our system is composed of a photo sensor, an AD converter, and a card-sized computer. The gathered data are transferred into a lap-top computer through an RS-232C interface, where they can be processed into numerical values or graphically displayed. It was determined that the Gx measured with our system correlated almost perfectly with those measured according to JIS requirements. Therefore, the system is useful as a simple measuring device for Gx.
In order to determine the optimized cut-off frequency of the Butterworth filter for cerebral perfusion single photon emission computed tomography (SPECT) images, we compared optimized cut-off frequencies assessed by both frequency and real space domains. SPECT images of a brain phantom were obtained using a SPECT 2000H-4 (Hitachi, Tokyo). The optimized cut-off frequencies were determined for nuclides of 99m-Tc and 123-I, and for LEGP and LEHR collimators, and compared. The radius direction distribution function Pr(n) (power spectrum method) was used to evaluate the SPECT images for frequency space analysis, and the normalized mean squared error (NMSE) method was used for the assessment of images in real space. The optimal cut-off frequencies using the power spectrum method for 99m-Tc were 0.656 cycle/cm for LEGP and 0.802 cycle/cm for LEHR. However, those for 123-I were 0.583 cycle/cm for LEGP and 0.656 cycle/cm for LEHR. In both analyses, the optimal cut-off frequencies varied depending on the nuclides and collimators. The optimal cut-off frequencies assessed by the NMSE method were similar to those assessed by the power spectrum method. In conclusion, the cut-off frequency of the Butterworth filter for cerebral perfusion SPECT images should be changed according to collimators and nuclides.
In our positron emission tomography (PET) studies, mesurement is carried out during C^<15>O_2, ^<15>O_2 and C^<15>O gas inhalation. The radiation absorbed dose was estimated by the MIRD method from measured cumulative redioactivity in organs and remainder of the body. The radiation absorbed dose in 22 target organs including pharynx, larynx and trachea walls were estimated using the radioactive concentration in 7 source organs (brain, pharynx-larynx, trachea, lung, heart, liver and ramainder of the body). These radioactive concentrations in organs were measured by PET scan in a normal volunteer during continuous C^<15>O_2 and ^<15>O_2 inhalation. The effective dose equivalents for 22 minutes of inhalation were found to be 5.81×10^<-4> mSv/MBq for C^<15>O_2 at 157 MBq/min and 4.64×10^<-4> mSv/MBq for ^<15>O_2 inhaled at 294 MBq/min.
To calculate the contrast-to-noise ratio (CNR) on magnetic resonance images, an equation selected to match each study is commonly used. The CNR values calculated using these equations may have their own characteristics. Therefore, the characteristics of four commonly calculated CNRs were evaluated in comparison with signal detectability. For the calculation of CNR, a phantom with five different solutions of CuSO_4 was imaged using various scan sequences with different TR and NEX. These images, which had different levels of noise and contrast, were measured for averaged signal intensity and standard deviation of noise in the same ROIs (regions of interest). To define signal detectability, Burger's phantom soaked in the CuSO_4 solution was imaged with the same pulse sequences used to evaluate CNR. Burger's phantom images were evaluated by five observers with a 50% confidence level. The characteristics of each CNR valuewere evaluated by correlating them with signal detectability. The results showed that some calculated CNRs indicated the noise element, but contrast element. From the point of view of signal detectability, the equation using the average of local variance and global variance with respect to coarse pixels was superior to others.
To determine the size of the device used in coronary intervention, quantitative measurement with QCA is required. The authors performed this calibration with a triple-axis rotation arm on a vessel phantom using the IVR-shot methos, and compared the results with 11 clinical images in which catheter calibration and IVUS were used. Measurement of the vessel phantom by the IVR-shot method resulted in the least error, followed in order by IVUS and catheter calibration. We measured the proximal and distal regions of stenosis on clinical images and found differences between catheter calibration and the IVR-shot method : 0.5±0.5 mm in the proximal region and 0.3±0.1 mm in the distal region. To examine accuracy, radiograms of a 40 mmφ lead ball were obtained using both catheter calibration and IVR-shot calibration. The IVR-shot method showed significantly greater accuracy, with an error of 1.0±0.5 mm, compared with 3.4±2.1 mm for catheter calibration (P=0.01). In summary, calibration using the triple-axis rotation arm provides greater accuracy of measurement.
A real-time radiation monitor using fiber optic technology was applied to measure entrance skin doses received during interventional radiology (IVR). The measured entrance doses were 2.1+0.8 mGy/frame and 384+42 mGy for photography and 28+11 mGy/min and 504+77 mGy for fluoroscopy in 20 patients who received IVR or DSA examinations. This monitor is designed to easily measure entrance skin dose, and thus it can be used for the optimization of radiation protection during IVR. This device would be useful for physicians performing IVR, to prevent deterministic effects such as severe skin damage.