[A Simple Measurement Method for Verifying the Accuracy of the Displayed Dose on an Over-couch-type X-ray Fluoroscopic System].

The purpose of this study was to establish a simple measurement method to verify the accuracy of incident air kerma (Ka, r) and air kerma area product (PKA) displayed on an over-couch-type X-ray fluoroscopy system. A dosimeter was located at the patient entrance reference point, and the irradiation field size was set to 10×10 cm. A lead plate was placed on the couchtop to protect the image receptor, and the duration of fluoroscopy was set to 1 min. The Ka, r was measured with the proposed method and the Japanese Industrial Standards (JIS) method on three X-ray fluoroscopy units of different manufactures. The effect of backscattered X-rays from the lead plate was calculated using Monte Carlo methods. The errors of the displayed Ka, r and PKA to the measured Ka, r and PKA with our proposed method were calculated. There was no significant difference in the measured Ka, r between the proposed method and the JIS method in all units. The effect of backscattered X-ray was ≤0.5%. The errors of displayed Ka, r and PKA to those measured were in the range of 3.4 to 15.7% and -4.1 to 20.3%, respectively, which met the tolerance for accuracy of ±35% in accordance with the JIS method. We found that our proposed method was simple and that the accuracy of measured values was comparable to that of the JIS method.

Metal artifact reduction for improving quantitative SPECT/CT imaging.

OBJECTIVE: This study aimed to evaluate the effect of the metal artifact reduction (MAR) method on quantitative single-photon emission computed tomography (SPECT)/computed tomography (CT) to reveal the usefulness of MAR in patients with metal implants. METHODS: We performed a phantom experiment simulating patients with artificial hip prostheses using SPECT/CT equipped with the iterative MAR (iMAR). The phantom was filled with Tc-99m solution (29.5 kBq/mL). For the CT scan conditions, tube current time products were applied to obtain volume CT dose indexes (CTDIvols) of 1.4, 2.8, and 5.6 mGy. Six types of quantitative SPECT images were reconstructed using data from different doses of CT processed with and without iMAR for CT attenuation correction. Thirty circular regions of interest (ROIs) were placed in each of the dark-band artifact areas, the white-streak artifact areas, and the non-artifact areas. We calculated radioactivity concentrations from quantitative SPECT images with and without iMAR to evaluate the quantitative accuracy. The differences of the effect of iMAR with different CT doses were also evaluated. RESULTS: The results obtained using CT data with a CTDIvol of 2.8 mGy are described below. For quantitative SPECT data without iMAR, we observed the underestimation of radioactivity concentration in the dark-band artifact areas and overestimation in the white-streak artifact areas. We observed quantification errors ranging from - 41.1% to + 20.0% without iMAR, depending on the ROI localization. When iMAR was used, these errors were reduced to a range of - 22.8% to + 14.2%. The mean absolute error from the true value in the artifact regions was also significantly reduced from 4.00 to 1.74 kBq/mL. In the non-artifact areas, the radioactivity concentrations obtained from the quantitative SPECT data with and without iMAR were equivalent to the true value and did not differ significantly between the two conditions. Similar results were observed for procedures with CTDIvols of 1.4 and 5.6 mGy. CONCLUSIONS: This study indicated that iMAR could improve the quantitative accuracy of SPECT/CT independent of the CT dose. iMAR can serve as a practicable technique for quantitative SPECT/CT in patients with metal implants.