IMAGE NOISE 2: A radiograph is taken of a patient\'s upper leg. The x-ray tube i

Question

IMAGE NOISE 2: A radiograph is taken of a patient's upper leg. The x-ray tube is run at 100 kVp which results in an effective energy of 60 keV. You need to obtain a reliable measurement of the optical density in a region of soft tissue (OD 2.3) beside the bone, and a region that includes bone. For this purpose, you digitize the radiograph, and you can now work with the pixel values of the image. How many pixels do you need at least for your measurement of optical density in each of the two regions if your error is to be no greater than 2%? Assume the following parameters: Thickness of the leg: Thickness of bone: 10 cm 2 cm 3.0 sensitivity 200 Noise of optical density values (std dev. of population): ±0.5 ot ssue cm- 5.360 0.808 0.380 0.274 0.233 0.212 0.189 0.176 sts lerm Energy [keV 10 20 30 40 32.654 4.542 1.573 0.840 0.573 0.450 0.344 0.297 80 100 Density of bone: 1.9 g/cm Density of soft tissue: 1.04 g/cm

Explanation / Answer

A radiograph is taken of a patients upper leg. The x-ray tube is run at 100 kVp which results in an effective energy of 60 keV. You need to obtain a reliable measurement of the optical density in a region of soft tissue (OD 2.3) beside the bone, and a region that includes bone. For this purpose, you digitize the radiograph, and you can now work with the pixel values of the image. How many pixels do you need at least for your measurement of optical density in each of the two regions if your error is to be no greater than 2%

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