CT Test 2 Flashcards
Name the components housed within the gantry.
slip rings, generators, cooling system, x-ray source, filtration, collimation & detectors, tube, filters, detector array, microphone
Why is tungsten commonly used for the x-ray tubes used in computed tomography
Due to its high atomic number because it has to be able to handle long exposures daily and tubes must dissipate during x-ray production.
High melting point
- Identify the type of generator used in computed tomography to create images. Why is this type of generator used?
High frequency generators because they produce a high voltage and transmit it to the xray tube
- Identify the scanning mode the advancement of the slip ring technology has allowed for computed tomography.
Helical scan modes
- What is the purpose of beam filtration?
It reduces the range of x-ray the reaches the patient by removing the long wavelength xrays. It also improves image quality as well as reduces radiation dose to the patient.
The first filter was the bow tie introduced in the second generation
- State the range of kV that is commonly used in computed tomography.
120-140 kV with high KV lower mA can be used!!!!
What is the affects of incorporating high kV in computed tomography on the following factors: . Heat load – . Intensity of the Beam- . Penetrating ability - . Patient Dose –
Heat load – reduces heat load which allows for lower mA settings to be used
. Intensity of the Beam- increases
. Penetrating ability - increases
. Patient Dose –decreases
- What are the common focal spot sizes used in computed tomography?
0.5 &1.0 mm
- Name an advantage and a disadvantage of using a small focal spot size?
An advantage is better recorded detail, but a disadvantage is that is constantly heating one small spot and you’ll end up having the replace the tube faster.
Pro-sharper image
Con- small hear spot- easy overload
- Differentiate between the location of the pre-patient collimator and post-patient collimator in the gantry?
Pre- located near the x-ray source and limits the amount of x-ray emerging into thin ribbons.
Post- located below the patient and above the detector array. This collimator shapes the beam after it has passed through the patient.
- Explain the purpose of collimation in computed tomography?
It restricts the beam to a certain area of interest only and improves image quality.
Controls the slice thickness by narrowing and widening the beam.
- The detectors serve what purpose in computed tomography image production?
Detectors capture x-ray photons and convert them to a usable signal. The more detectors there are the more information that will be obtained.
- Why is it importance for a detector to be efficient during the data acquisition process in computed tomography?
Because it is what captures the photons and converts them to a usable signal
Why are solid state detectors more common to in new CT scanners as opposed to the xenon gas detectors
Due to their high photon absorption. They absorb nearly 100% of photons that reach them compared to xenon gas scanners who only ansorb 60-87% of the photons that reach them. Also Xenon systems are not capable of using the MDCT system.
- Why is it important for detectors to be positioned so close together?
Detector spacing affects the amount of scatter recorded. When detectors are placed closer together it results in less scatter on the images. More detectors also increase capture efficiency.
It captures more information and less scatter
- Explain the importance of not over working the computed tomography table.
If a patient is over the weight limit scanning is still possible but it affects the scanning increments drastically throwing your scan off.
When Overworked the slices or increments may not come out accurate
- Differentiate between raw data and image data for image reconstruction process.
Raw data are bits of data that are acquired by the system with each scan and the data is waiting to be made into an image, while image data is the data when the computer processes the raw data by assigning it a Hounsfield unit value for every pixel and is displayed on the monitor.
Raw is what we actually scan
- Which data requires more computer space, raw or image data?
Raw Data
- Why is the Image display so important for CT imaging?
It is important because it contains all of the system components necessary to convert the digital data created from the reconstruction process to electrical signals needed by the CT display monitor.
- Explain the image reconstruction process steps: (Ray, Ray Sum ect) (Chptr 3)
The x-ray tube travels along its circular path and continuous energy is being generated. The path the x-ray beam takes from the tube to the detector is referred to as a ray. The DAS reads each ray reaching the detector and how much of the beam is attenuated. This is known as ray sum. A complete set of ray sums is a view. Multiple views are needed to create a CT image.
Use analogy of dog
- Provide an example in which a technologist would utilize each of the following methods of data acquisition in which scanner acquires data: (Chptr 5)During what part of a CT examination procedure)
. Localizer, or Preliminary:
. Axial Scanning:
. Helical or Spiral Scanning:
Localizer, or Preliminary: scout images/scanograms
. Axial Scanning: headwork and prescanning. Also can be used when shooting an infant or uncooperative patient because you can shoot and then pause to fix their position if they moved… Delayed Bladder
. Helical or Spiral Scanning: contrast scans
- Identify an advantage and a disadvantage of using Step – and Shoot Scanning.
Advantage would be high image quality and repeat scans are easier to do. A disadvantage would be it adds some time to the total exam time.
- Identify an advantage and a disadvantage of using Helical Scanning.
An advantage would be its ability to optimize iodinated contrast media and it reduces motion artifacts from organ. A disadvantage is that patient motion highly effects quality of the scan. Limits data . Increases patient dose. On for an extended amount of time
