In this study, we investigated the relationship between the noise characteristics and the number of projected images in tomosynthesis using a digital phantom.
Methods:The digital phantom consisted of a columnar phantom in the center of the image and a spherical phantom with a diameter of 80 pixels. A virtual scan was performed, and 128 projected images (Tomo_w/o) of the phantoms were obtained. The image noise according to the Poisson distribution was added to the projected images (Tomo_×1). Furthermore, another projected image with additional noise was prepared (Tomo_×1/2). For each dataset, we created datasets with 64 (half) and 32 (quarter) projections by removing the even-numbered images twice from the 128 (fully) projected images. Tomosynthesis images were reconstructed by filtered back projection (FBP). The modulation transfer function (MTF) was estimated using the sphere method, and the noise power spectrum (NPS) was estimated using the two-dimensional Fourier transform method.Results: The MTFs did not change between datasets, and the NPSs improved as the number of projected images increased. The noise characteristics of the Tomo_×1_half images were the same as those of the Tomo_×1/2_full.
Conclusions:To achieve a reduction in the patient dose in tomosynthesis acquisition, we recommend reducing the number of projected images rather than reducing the dose per projection.
We performed a non-electrocardiogram-gated and non-contrast-enhanced magnetic resonance angiography (MRA) of the lower limb arteries using three-dimensional multishot T 1 -weighted fast-field echo-echo planar imaging (3D multishot T 1 -FFE-EPI), and it was optimized the protocol. The image distortion for the change in the EPI factor was calculated using 3.0 T-MRI and MRI phantom. We also calculated the signal-to-noise ratio (SNR) of the femoral artery with a change in the flip angle on images of 8 healthy volunteers. Furthermore, the optimal EPI factor was determined from the SNR of the femoral artery and the contrast ratio between the femoral artery and the adductor magnus. Two radiological technologists performed a retrospective visual assessment of the pelvis, thigh, and leg of 10 patients who underwent lower limb non-contrast-enhanced MRA and contrast-enhanced tomography angiography (CTA). The optimum flip angle and EPI factor were 25° and 3, respectively. In the visual assessment of clinical cases, there was no significant difference between the non-contrast-enhanced MRA and contrast-enhanced CTA in the pelvis and the leg (p=0.52 and p=0.88, respectively). In the thigh, non-contrastenhanced MRA was significantly higher (p=0.02), namely, the ability to visualize the lower limb arteries was not much difference between this method and contrast-enhanced CTA. Our method without electrocardiogram gated and contrast medium is expected for screening tests or detailed examinations.
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