In digital imaging the image is divided into a matrix of individual pixels, each pixel has a value which is related to the signal intensity of the corresponding part of the image.
As high values displayed dark and low values displayed light'
FOV
Pixel size =
No. of pixels in the matrix
As in C...
Digital Radiography
Image Structure & Size:
In digital imaging the image is divided into a matrix of individual pixels, each pixel has
a value which is related to the signal intensity of the corresponding part of the image.
As high values displayed dark and low values displayed light'
FOV
Pixel size =
No. of pixels in the matrix
As in CT may be (350\512 = about 0.7 mm)
As the pixel size becomes larger, each given area of the image is made up of fewer
pixels so the more coarse and grainy the image becomes.
The bit depth determines the number of values that can be stored in a pixel, ie a greater
bit-depth results in a greater number of levels of grey.
The value on the pixels is stored in binary format, and the maximum value is related to
the bit depth of the pixel
8-bit pixel has a (28) 256 levels of gray, and 12-bit pixel has a (212) 4096 level of gray.
Computer memory expressed as bytes, with single byte carry up to 8 bits, so for pixels
with more than 8-bits use proportionally increased bytes.
Matrix size more than 512x512 will not improve resolution because of the limitation of
detector size and sapling frequency.
MCQ: Matrix size in large plates used for chest & abdomen 2500 x 3000 pixels / Small plates 2000
x 2500 pixels.
MCQ: Images of chest and abdominal radiographs are typically about 1024 x 768 pixels.
MCQ: Analogue images can be displayed on monitors while digital images must be converted.
Dr. Islam Abo Shady 1
,Compression;
Compression of the images is required to reduce the storage requirements and
transmission time.
Lossless (reversible) compression: compression factor of 2 to 3 with images can be
restored to its origin value.
Lossy (irreversible) compression: compression factor up to 40 with some loss of the
details & images cannot be restored to its origin value.
MCQ: Images with large matrix size can tolerate compression > images with small matrix size
Noise Reduction:
1) Frame averaging:
-Adding the signal from successive frames to give a time averaged image.
-This is equivalent to increasing exposure time in radiography.
-Useful technique provided that there is no movement between frames.
2) Filtration (Low-pass spatial filtration):
Average after adding a proportion of the grayscale values of the neighboring pixels, BUT
blur small details and edges smooth the final image.
NB: High-pass spatial filtration (Edge enhancement): adding a proportion of the
difference between the grayscale values of the neighboring pixels, exaggerate edge contrast
BUT increase noise, and may be create false structures.
Calibration:
The values of the pixels are matched to a display curve that is optimized to the
imaging task, with the optimized patterns is consistently displayed on any display
monitor.
Standard display calibration is DICOM (digital imaging & communications in
medicine).
NB: Fourier analysis:
The image signal is broken into a series of sine waves that vary in terms of
spatial resolution & amplitude.
Dr. Islam Abo Shady 2
, MCQ: Fourier analysis of one term is equivalent to an exact reproduction of even a complicated image (F)
…The more terms (additional sine waves at multiples of the original frequency) the closer the composite
signal is to the original. An infinite number of terms would produce a perfect copy of the original signal.
MCQ: Fourier analysis is used for reconstruction of CR images and tomographic images in, for example,
CT and MRI
Sampling:
A complex analogue video signal is composed of Fourier sine wave components of
many frequencies, then it is sampled before digitization with some information is
inevitably lost.
At too low sampling frequency, small structures with sharp edges information will be
lost.
Nyquist criterion: (signal must be sampled at least twice in every cycle or period.)
Sampling frequency must be at least twice the highest frequency present in the signal,
otherwise high frequencies will be recorded erroneously as low (Aliasing).
Nyquist frequency: the maximum signal frequency that can be accurately sampled, and
equal the half of the sampling frequency.
Aliasing: Under-sampling, leading to high frequencies will be recorded as low frequencies.
MCQ: Aliasing is responsible for streak artefacts in CT and reverse flow artefacts seen in Doppler
Computed Radiography (CR):
Imaging Plates (IP):
Replacing the film cassette.
Consist of photostimulable phosphor (thickness = 500Um) (has wide dynamic range
10000:1), commonly used is barium fluorohalide crystals doped with europium
(BaFX:Eu ) , which is the halide consist of bromide 85 % and iodide 15 %
The phosphors is in powdered form laid on polyester base (with thickness 0.3 mm) +
protective surface coat.
Inserted in a tight light cassette.
Photostimulant phosphor layer become worn or scratched with time and use, leading to
desensitization and degradation of the image plate and cause artefacts.
Dr. Islam Abo Shady 3
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