A quantitative analysis of signal to noise ratio in comparable regions inside and outside the ROI for the skull phantom study (refer to to see the two rectangular regions) is presented in for different temporal weights outside the ROI. The signal to noise ratio is measured after the images are brightness corrected and spatially different temporal filter is applied with different weights inside and outside the ROI.
Signal to Noise Analysis in similar regions inside and outside the ROI with different temporal weights applied to the region outside the ROI
The kerma area product (KAP) can be used as a measure to gauge the integral dose [4
] assuming the patient intercepts the entire beam. The ratio of the KAP with (K) and without the material filter (K0
) is given below
is the air kerma calculated in the attenuated region of the filter with area (A
) and Knf
is the air kerma calculated in the non-attenuated region of the filter with area a
is the kerma calculated without any material attenuator and A
is the total field of view.
The exposure values were measured using an ion chamber (PTW, Freiburg, Germany) and the kerma calculated in the region of the ROI and in the region under the ROI filter, with no additional attenuating material present in the beam. With the measured values, the ratio calculated is
Physically, the attenuator is a stack of Kodak Lanex Regular gadolinium screen material with a hole in the middle. The purpose of the attenuator is to reduce the number of x-ray quanta and thus the dose to the patient in the region outside the ROI. But the patient acts as additional attenuation which further reduces the number of x-ray quanta reaching the detector outside the ROI. If the attenuation is too high, there is a possibility that the information in the pixels falling under the attenuator might be further compromised due to the instrumentation noise of the detector.
and show the ROI fluoroscopy images obtained with a stack of 4 filter layers with an attenuation factor of 6. and show the ROI fluoroscopy images obtained with a stack of 8 filter layers with an attenuation factor of about 14. The brightness equalized image shows that the region outside the detector is extremely noisy; the exposure reaching the detector was measured to be around 0.33μR/frame, which is well below the instrumentation noise level of the FPD detector estimated to be about 2–3μr/frame [11
]. With a high weight temporal filter applied outside the ROI the noise is reduced so that major structures in the patient should still be visualized and able to be monitored during an interventional procedure. Nevertheless, because the lag is increased, the moving guidewire tip might not be visible in all part of the image outside the ROI.
An image with brightness equalized in both regions and a temporal filter weight of 1.0 applied in the periphery. ROI attenuator stack - 4 layers, attenuation factor – 6 x
An image with brightness equalized in both regions and a temporal filter weightof 0.4 in the periphery and 0.8 in the ROI. ROI attenuator stack - 4 layers, attenuation factor – 6x
An image with brightness equalized in both regions, and a temporal filter weight of 1.0 applied in the periphery. ROI attenuator stack - 8 layers, attenuation factor – 14 x
An image with brightness equalized in both regions, and temporal filter weight of 5 in the periphery and 1.25 in the ROI. ROI attenuator stack -8 layers, attenuation factor – 14 x
Thus the amount of dose reduction possible may be limited by the tradeoff between the effect of noise and image lag on image quality.