Image quality and radiation dose of renal perfusion CT with low-dose contrast agent: a comparison with conventional CT using a 320-row system.

AIM: To compare perfusion computed tomography (CT) with reconstructed image from source data using low-dose contrast agent and conventional 320-row CT for the evaluation of renal tumours. MATERIALS AND METHODS: Twenty-eight patients underwent conventional CT (C-CT) and 26 patients underwent perfusion CT with low-dose (40 ml) contrast agent. Image noise, arterial visualisation, the sharpness of the corticomedullary junction (CMJ), and overall image quality were each assessed using a four-point scale. The tumour detection rate for lesions <4 cm (n=66) was also evaluated. Quantitative image parameters including image noise and the contrast-to-noise ratios (CNRs) of the renal artery and CMJ were measured. The volume CT dose index (CTDI), dose-length product (DLP), and size-specific dose estimate (SSDE) were also recorded. RESULTS: Although the image noise of perfusion CT was higher than that of C-CT and the overall image quality of perfusion CT was lower than that of C-CT, the arterial visualisation score of perfusion CT was significantly higher than that of C-CT. The CMJ sharpness scores of the two techniques were equivalent. Sensitivity and positive predictive values were also equivalent with respect to tumour detection. The CNRs of both the left and right renal arteries were significantly higher on perfusion CT than on C-CT. The CTDI, DLP, and SSDE of perfusion CT were significantly lower than those of C-CT. CONCLUSION: Perfusion CT using low-dose contrast agent preserved arterial visualisation and the tumour detection rate and achieved a low radiation dose despite image quality degradation and image noise.

[Leakage radiations in a medical electron accelerator facility--X-rays and capture gamma-rays].

Absorbed dose rates of photons are measured in the maze of a medical electron accelerator facility. Radiations in the maze include high energy recoiled electrons other than low energy reflected X-rays. An ionization chamber for low energy X-rays is used for measurement, and is put in a build-up cap thick enough to obstruct the recoiled electrons of which contribution to the chamber output comes sometimes equal to that of X-rays. Dose rates of capture gamma-rays in the maze are estimated applying the calculation of neutron dose equivalent rates reported in the preceding paper. It is proved that capture gamma-rays are dominant in the dose rates measured in the maze at 14 MV X-ray generation. The estimated absorbed dose rates of total photons including capture gamma-rays and reflected X-rays agree with the measured ones in the range from +10% to -27%.

[Leakage radiations in a medical electron accelerator facility--calculation of neutron doses in the facility].

Neutron doses often come dominant in mazes of electron accelerator facilities in which X-rays of energies more than 10 MV are produced. A simple analytical method to calculate neutron doses in such a facility is developed. In the calculation procedure, it is assumed that the irradiation room is spherical in shape and the maze is cylindrical. Multiple reflection of neutrons is also considered using the albedo concept in the calculation. The procedure allows to exist a hanging wall over the entrance of the irradiation room and also multiple legs in the maze. All the parameters used in the calculation are given definitely in the procedure, and any experiment is unnecessary to determine value of the parameters. Comparison of the calculated results with experimental ones will be described in the following report.

[Leakage radiations in a medical electron accelerator facility--measurement of neutrons and comparison with calculation].

Neutron dose equivalent rates are measured in the irradiation room and the maze of a medical electron accelerator facility. An m-counter used for measurement is associated with a gate circuit to reject piled up signals caused by pulsed X-rays, so that it is possible to measure the neutrons without obstruction by the piled up signals even in the irradiation room. Counting loss of neutrons is mathematically corrected, and the correction factor comes nearly equal to 1.5 at dose points close to the entrance to the irradiation room when 14 MV X-rays are generated. The measurement in the maze agrees with the calculation reported in the preceding report in a factor less than 1.5 except for dose points close to the entrance to the irradiation room.

[Leakage neutrons and X-rays in a therapeutic microtron facility].

Measurements of dose equivalent rate distribution in a medical microtron facility were done for neutrons and X-rays originated from 14 MV or 8 MV X-ray irradiation by the microtron. Measured data are described with empirical formulas for convenience of quantitative reconstruction of the data. A formula consisting of a simple power function agrees well with measured data except for thermal neutrons, and is understood to describe the dose rate separating into the scattered and unscattered components in the case of the point source. The thermal neutron distribution in the maze is described with another formula of an exponential function. A discrepancy was observed between the X-ray distribution in the maze from 14 MV X-ray irradiation and that from 8 MV irradiation. This is estimated to be caused by a contribution of capture gamma-rays increasingly emitted in the case of 14 MV X-ray irradiation.

[Measurement of neutrons from an electron accelerator by rem-counter].

In the measurement of neutrons from the medical electron accelerator by a rem counter, two problems disturb accurate measurements. One is the pile-up of signals produced by X-rays during each X-ray burst and the other is the increased counting loss caused by bunched nature of yielded neutrons. The time spectrum of neutrons measured by the rem counter 2202 D (manufactured by Studsvik) rises up to a maximum value by about 20 microseconds and then falls down exponentially with a time constant of about 90 microseconds. On the other hand, that of X-rays is roughly rectangular with several microseconds width. A time discriminating system was prepared to be combined with the rem counter, which was triggered by leading edge of electron beam pulses, rejected pile-up signals due to X-ray bursts, and counted pulses of neutrons in a specified time window. The system discriminated the pile-up enough to measure neutrons at a X-ray dose rate of at least 30 mGy/h. Nonparalyzable counting loss correction was practicable upto about 10 mSv/h for the beam pulse rate of 85 Hz, in which the dead time of the rem counter was estimated as 4 microseconds.

Advanced coded-aperture imaging system for nuclear medicine.

An advanced coded imaging system is described, and some results of phantom experiments are presented. The advanced method uses a pair of coherent codes (+ 1 and -1 codes) and has many advantages compared with conventional ones. One of the greatest advantages is that there are no sidelobes in the focal plane and only a few in other planes. Therefore, when an object can be regarded as two-dimensional, it is perfectly reconstructed with high detecting efficiency, and this is successfully simulated by a thyroid phantom with 99mTc. Moreover, this system has an ability to reconstruct tomograms, which is also shown by using ring phantoms piled on one another with some cold spots in their shells. From these experimental results it may be concluded that the new system is useful for practical applications, for example, to nuclear medicine.

Production system for 18F-2-deoxy-2-fluoro-D-glucose--a trial for automatic production.

We have developed a production system for 18F-2-deoxy-2-fluoro-D-glucose (18F-2FDG), which assures reliable production with easy handling and reduces radiation exposures to the operator. Chemical procedures in this system are the same as manual method developed in NIRS. This system has 2 operation modes; one is remote controlled manual operation mode and the other is microcomputer controlled automatic operation mode. In remote controlled mode, we tested this system 5 times and 18F-2FDG synthesized was supplied for clinical use once. The mean radiochemical yield of 18F-2FDG from the target gas recovery with decay time correction was 8%, that is the same as in the manual synthesis. It took about 2 hours from end of bombardment (EOB) to end of synthesis (EOS). Since this time is shorter than in manual synthesis, the available activity at EOS is increased.