X-Rays: Image Processing & Reconstruction Flashcards
What kind of blur arises from back projection reconstruction?
1/r
How is the 1/r blur from back projection removed?
Filter the images first with a convolution kernel to counteract the 1/r blur, then back-project.
How does iterative reconstruction from an image?
Initial guess of image made (e.g. Filtered back projection reconstruction)
Comparison of projection from guess and actual projection made.
Guess corrected to better match projections.
Process repeated until “perfect” match.
Describe digital breast tomosynthesis.
X-Ray tube moves in an arc about a pivot point.
As the breast and detector don’t move, anything at the pivot point will be clear, and everything else will be blurred out.
By acquiring many low dose projects computer reconstruction of the data into various slices at different depth can be obtained.
What problems arise from viewing DR images on a computer screen?
Image must look the same on different display devices.
Not all input levels are perceivable on all displays.
Human visual system has a non-linear response (it is more difficult to see contrast in dark parts of the image.
How do high-end displays fix the problems of viewing DR images on a computer screen?
Calibrate using “standard curve”.
High stable luminance.
High resolution (Exceeding that of the image).
Large “bit-depth” allows up to 16,000 grey levels.
How is the raw imaged created and stored so as to increase the diagnostic information contained within?
Raw image is flat.
Detector has a wide dynamic range.
Whole range of exposure is captured.
Contrast for the clinically relevant part of the image can be adjusted using a look-up table.
Explain how Digital Subtraction Angiography works.
The mask image (without contrast) is taken - only normal anatomy is shown.
Contrast agent is introduced to the patient and a series of live images are taken when the vessels have filled with the contrast medium - showing the vessels superimposed on the normal anatomy.
The mask image is then subtracted pixel by pixel from the live image leaving only the vessels visible.
How can misregistration occur in DSA?
Patient movement between frames, can be corrected by moving the mask (pixel shifting).
How must the two images be subtracted from each other in DSA?
Pixel by pixel.
Taking natural log first.
How does Dual Energy X-Ray Absobimetry (DEXA) calculate the density of patients bones?
Patient scanned with x-rays of two energies.
Tissue density is subtracted out using a tissue baseline image.
The bone density can then be calculated based on the absorption of the two energies of x-rays by the bone.
