Principles of Imaging Flashcards
What are the 3 elements required in imaging and their relative positioning
- X-ray source
- Object (person)
- Detector
RED
Relative Electron Density.
Describes differing electron density’s of tissues which describes their ability to absorb energy from x-ray photons. This electron denote is relative as it is a comparison with water.
Hounsfield Scale
A scale to quantify RED. measured in HU (Hounsfield units). In shorthand, air = 0, water = 1 and bone = 1.3-1.7.
What is the benefit of CT
Computerized Tomography.
Collects a much more extensive set of data than radiographs. Taken in slices for a better representation of a 3D image. radiographs compress everything into 2 dimensions for one image, CT produces many slice images which allow for a representation of full spacial seperation.
Helical Scan
Rotation of gantry around a single consistent movement of treatment bed through boar.
Pitch and its effects
Speed at which treatment couch moves through boar. Increase pitch = faster couch movement = reduced time of radiation exposure = reduced dose. faster measurements taken = less data = reduced image quality.
Cone beam vs Fan beam CT
Cone beam (LA) = cone of x-rays. They don’t require as many rotations but data set isn’t as extensive therefore has worse image quality.
Fan beam (CT) = Fan of x-rays. requires more rotations but data set is more extensive and therefore has better image quality.
Windowing and it’s benefits
Setting the range/window width of the Hounsfield displayed with gray scale. Takes the same anatomy and presents it in different ways. Allows us to see detail in certain structures we wouldn’t with default settings.
Contrast and it’s benefit’s (include common types)
Contrast = Not all tissues image well. Contrast is used to artificially alter RED of a tissue and therefore distinguish between tissue types. Contrast helps us to highlight a specific structure so that we can see it in more detail on an image.
Barium and iodine are most commonly used. Barium = inserted into body openings, iodine = inserted intravenously by attaching itself to a drug compound, this takes the iodine directly to a desired organ.
Why is CT and not MRI used in RT
MRI is based on proton not electron density like CT is, therefore we cannot make dosimetry predictions. Its also a much longer process and more expensive technology but does produce better quality images.
Radiolucent vs Radiopaque
Radiolucent = Low RED, material doesn’t attenuate beam and therefore does not appear solid on the image.
Radiopaque = High RED, material attenuates beam and therefore appears solid on the image.
PACS and DICOM
PACS = picture archiving and communication system. Stores images.
DICOM = digital imaging and communications in medicine. A file format for images.
Image slice vs acquisition slice
Image slice = how much of an individuals anatomy is represented on each image, the thinner the image slice thickness, the greater the number of individual images making up the overall dataset.
Acquisition slice = how many individual elements make up the detector array, if you have more slices, you can collect more data points on each rotation of the gantry.
