CT Test 2
State the range of kV that is commonly used in computed tomography
120-140 kVp
Why are scintillation-type solid state detectors coupled with a photodiode more common in new CT scanners as opposed to the xenon gas detectors
99% efficient, but slight afterglow
In order to create a CT image, the process can be described or subdivided into three main processes or phases; data acquisition, image reconstruction and image display. During these three main processes or phases involved in creating a CT image, what happens to the "data" during each phase?
Date Acquisition- acquiring data that results from the attenuation of the x-ray beam as it passes through the patient to strike the detector, the mechanisms housed within the gantry and the patient table are the components necessary for data acquisition -Image Reconstruction-refers to the process whereby a computer manipulates data collected from the detectors to create a CT image, raw data include all attenuation measurements obtained from the detector array, some raw data used in creation of the image, computer manipulates the raw data throughput the image reconstruction process -Image Display-final step in creating the CT image, analog monitors display the CT image
Why is it important for a detector to be efficient during the data acquisition process in CT
because that is what collects the information
Why is the Image Display so important for CT imaging
because the image is created to get a report
Location of pre-detector(post) collimators
below the patient and above the detector array, right before detectors
Disadvantage of small focal spot size
cannot tolerate as much heat
Advantage of Helical scanning
continually rotating X-ray tube, constant x-ray output, and uninterrupted table movement, decreased respiratory and cardiac motion artifact
Incorporating high kV affects patient dose by(if reducing mA)
decreasing patient dose
Purpose of pre-patient(source) collimator
dose profile, limits the amount of x-ray emerging to thin ribbons, acts on the beam before entering the patient
During what part of a CT exam procedure would the helical or spiral scanning occur
dynamic motion, contrast studies
During what part of a CT exam procedure would the conventional step and shoot or axial scanning occur
head, patient moves can stop and fix
Identify the scanning mode the advancement of the slip ring technology has allowed to be performed in computed tomography
helical
Why is Tungsten commonly used for the X-ray tubes used in computed tomography
high atomic number, high melting point
Identify an advantage of using Conventional or Step - and Shoot Scanning
highest image quality, CT table moves to desired location and remains stationary while the X-ray tube rotates within the gantry
Incorporating high kV affects intensity of the beam by
increasing intensity
Incorporating high kV affects penetrating ability by
increasing penetration
Why is it important for detectors in a detector array to be positioned so close together
information will get lost if there is space in between, allows the beam to be highly collimated
Explain the importance of not over working the computed tomography table or couch
it will wear out faster and need to be replaced more often
The detector array serves what purpose in CT
measures the intensity of transmitted x-ray radiation along a beam projected from the x-ray source to that particular detector element
Components housed within the gantry
microphone, speakers, laser light to position patient
Does raw data or image data require more computer space for archiving and storage
raw dara
Raw data for image reconstruction process
raw data- data sitting in the computer waiting to be made into an image, may be used to generate a new image, requires more space
Incorporating high kV affects heat load by
reducing load
Location of pre-patient(source) collimators
right before patient, near x-ray source
During what part of a CT exam procedure would the localizer, scanogram, or preliminary scanning occur
scout image, to get approx dose report
Disadvantage of Conventional or Step - and Shoot Scanning
slight pause in scanning between data acquisitions, only a single scan can be made each time patient holds breath
Why are high frequency generators used in computed tomography to create images
small enough so that they can be located within the gantry, CT needs high voltage for high energy
What are the common focal spot sizes used in computed tomography
small focal spot, 0.5 and 1.0mm most used
Advantage of small focal spot size
smaller size creates sharper image because of reduced penumbra
Image Data for image reconstruction process
the data averaged so that each pixel has an associated number, so an image can be formed, requires 1/5 of the storage raw data needs
Purpose of post-patient collimator
this detector shapes the beam after it has passed through the patient
What is the purpose of beam filtration
uniform intensity, patient has to be isocenter, reduce radiation dose and minimize image artifact