Imaging technology (pixel, voxel, resolution) 13/01/2023 Flashcards
What does radiographic density mean?
the degree of blackness on the image
What increases radiographic density?
Greater radiographic density may be produced by increasing
(1) mA the total number of x-rays that reach the film,
(2) s – the length of time the exposure takes place over,
3) kVp the penetrating power of the x-rays so how much energy you are giving the x-ray photons to reach the IR.
It can also be caused by post-processing data manipulation techniques such as windowing.
What is contrast? technology
Radiographic contrast, sometimes referred to as greyscale, is the difference in the displayed brightness between two areas of interest e.g. a lesion and background tissue.
This contrast is caused by the difference in absorption densities between different structures in the body, so in a chest x-ray bone and air for example.
What affects contrast? technology
Some ways to affect image contrast are by;
Exposure factors (kV only) – increase in kVp will decrease the contrast in the image because if you give the photons more energy, less will be absorbed by the tissue.
Windowing / post processing
How does analogue work? technology
In an analogue system, x-ray energy is converted to light, and the light waves are recorded just as they are. As a result we achieved continuous images, seeing various levels of brightness and colour – a complete representation of the information collected.
How does digital work? technology
When we acquire information using DR, the analogue signals are converted into numbers that are recorded and sampled (or counted) multiple times. These multiple numerical values are then divided into smaller elements that can be processed in different ways. A digital signal looks different to an analogue signal as you can see – more blocky in its nature as each numerical value is represented.
What is a matrix?
An arrangement of rows and columns on which pixels are arranged in order to create a displayed image . The pixels of information that we collect are displayed in a matrix.
Larger matrixes have better spatial resolution, but produce bigger quantities of data (and take longer to process).
What is Field of view?
The size of the displayed image
The amount of body part or patient included within this can vary
The larger the FOV, the larger the area that is imaged
What is a pixel?
Pixel
2-Dimensional
The basic component of an image matrix
Part of any digital picture we see (even non-radiographic).
The size of a pixel is determined via: pixel size = FOV/matrix
When keeping the field of view the same, if you had a larger matrix, you would have a greater number of smaller individual pixels.
What is a voxel?
3-Dimensional
Adds depth as a dimension
Used in volumetric scanning (such as in CT and sometimes MRI)
Why is pixel bit depth important?
The greyscales that cause differences in image contrast are determined by the Pixel bit depth. This affects the number of grey tones a pixel can produce.
When a pixel has a higher bit depth, the more grey scale the pixel can produce (i.e. black to white and all the greys in between). This means pixels with higher bit depths increase the number of different shades of grey that can be displayed on an image.
This allows us to differentiate different tissues
What is spatial resolution?
Spatial Resolution
The ability of an imaging modality to differentiate two adjacent structures as being distinct from one another (Radiopaedia, 2020)
How can we improve spatial resolution?
Keeping the same matrix size but reducing FOV results in increased spatial resolution.
More pixels are dedicated to a smaller physical area of your patient so more detail is possible.
This is why good collimation is important for image quality.
This is why “coned views” give better detail.
What is Contrast Resolution?
The ability of any imaging modality to distinguish between differences in image intensity (brightness)
What is temporal resolution?
A modalities ability to see fast moving objects in a rested or still state.
Poor temporal resolution can result in blurring and misregistered data.
Crucial in process where we image moving objects like the heart and blood.
Can be improved by reducing the time it takes to acquire a single slice/set of images.
Types of radiation?
Primary- from the target (useful)
Secondary- created when the primary beam interacts with matter (less penetrating, increases patient dose).
Scatter- multidirectional
Leakage- from tube housing.
