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CT Premidterm

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sihirlifil's version from 2017-10-31 18:15

CT

Question Answer
The ___ houses the x-ray tubeGantry
Size limit?Must fit through the gantry
What does the CT machine look like?X-ray tube that rotates around on a wheel, table moves in & out of the gantry
What does the detector do?Records incident radiation (on other side of patient)
How does the CT acquire a volume of data?Take images all the way around the patient, advance (maybe a few mm, depends on setting), repeat
CT scanner typesPencil beam
Multiple pencil beams
Fan beam
Stationary ring of detectors
Axial scanner: how does it work?Single rotation of x-ray tube around patient, table moves forward, repeats. Data collected slice by slice
Downside of axial scanner?No useful reconstruction possible
Helical scanner: how does it work?Continual rotation of x-ray tube (x-ray machine turns while table moves patient into gantry). Volume of data collected
Benefits of helical scanner?Faster
Greater flexibility of reconstruction
How is the image created?Beam passes through patient, detector picks up how much of the primary beam has been attenuated (absorbed) by the patient. Different body tissues absorb to a different degree
The computer converts attenuation values viaMathematical algorithms
Grayscale images on the computer screen formed byFiltered back-projection (done pixel by pixel)
CT images are described how?Using attenuation (signal void, hypo, iso, hyper)
What do CT images each represent?Thin slices through the patient body (cross-sectional)
Two benefits of CT over radiographs?Cross-sectional imaging overcomes problem of superimposition in radiography
Very high contrast resolution i.e. can differentiate between types of ST (muscle vs. fluid, brain vs. CSF, blood vs. pus)
How is it that serous fluid can be differentiated from pus on CT?Pus is more cellular, more protein; has different attenuation
Other advantages of CT?Option of multiplanar and 3D reconstruction (can virtually remove soft tissue, useful for screw placement, contouring, etc for surgery)
Excellent imaging of bone/skeletal structures, lungs, complex structures (skull & nose, articulations)
Contrast studies w/ IV iodine allow imaging of vascular structures, urinary system, lesions that accumulate contrast (inflammation, neoplasia)
Basic image planesSagittal (top left), transverse (bottom left), dorsal (right)
Disadvantages of CTIncreased radiation dose (takes longer to take CT image, more exposure)
Expensive
Need general anesthesia
Not readily available
Reading images requires experience/specialist training
Whole body scans don’t automatically provide diagnosis
What is a Hounsfield unit?Expression of attenuation (‘attenuation coefficient’)
Water (baseline) = _________ HU0 HU
Bone = _________ HU>1000 HU
Air = _________ HU-1000 HU
Soft tissue = _________ HU+0-100 HU
What is the image reconstruction algorithm for?CT machine takes images, computer creates different versions from raw data (soft tissue vs. bone)
Soft tissue algorithm looks like? Spatial frequency?Low spatial frequency
Bone algorithm looks like? Spatial frequency?High spatial frequency
What is windowing?Postprocessing the images that have been acquired using different types of algorithms for reviewing clinical diagnosis (aka adjust shades of gray for high/low inherent contrast)
Tissues with low inherent contrast (all soft tissue) need to be viewed with a ___ window widthNarrow
Tissues with high inherent contrast (lungs, nasal cavity) need to be viewed with a ___ window widthWide
Window width corresponds to?Contrast of a monitor display (how many shades of gray)
Needs to match object contrast latitude (width) (= if tissue is high/low inherent contrast)
Window level corresponds to?Brightness of monitor display
Needs to be set according to the density level of the organ imaged
ST window: what’s it look like?WL between 0-100; WW narrow (300)
Bone window: what’s it look like? (what’s wrong with the cat?)WL 600; WW wide (4593) to include bone, air, soft tissue (encompass whole HU unit scale)
(Middle ear disease- can see fluid accumulation in tympanic bullae)
In the bone window, what does the soft tissue look like?Contrast is lost, all ST appears similar
Basis of images are thin slices through the body, consisting of what?Matrix of voxels (volume element). Height x width x slice thickness
How are voxels displayed? What then?Translated to pixels (makes 3D image into 2D), then assigned a shade of gray according to window setting
A voxel is a volume- so how can it be expressed in 2 dimensions?Mean value is expressed on the image and displayed as a gray level = Partial Volume Averaging
What’s the problem with partial volume averaging?May cause reduced contrast and spatial resolution (artifacts)
How is partial volume averaging reduced?Use thin slice thickness
Clinical applications of CT: options for horses?Sedate for standing CT of skull, moving table/gantry
Horses hung up (under GA), moves horse in and out of gantry
How is slice position planned?Scout views where vertical lines represent individual slices, decide from where to where you want to scan
How can you look at CT images?Slice mode or as 3D reconstruction
What can you see w/ thoracic CT?
Anatomy without superimposition (lungs, pulmonary vessels, bronchi shape/diameter/wall thickness/position, esophagus, how the mediastinum wraps around everything…)
Good lung detail
Hilar lymph nodes
Thoracic CT is great diagnostic tool for?Sensitive for detection of pulmonary metastases & pulmonary/extrapulmonary/pleural masses
Interventional: take CT where you want to insert needle, check if it’s in the right place (can be very accurate)
How does slice thickness potentially create an artifact? Problem with this?If too thick, ‘step formation’ can form which causes blurring in reconstructed images (original acquisition looks clear)
Blurring of contours can mask/mimic small subchondral lesions
What’s going on with this image? Slice thickness artifact
What is beam hardening?“Streak” formation, beam travels through really thick parts of bone. Lower energy part of beam absorbed in bone, so only strong part of beam remains
Where does beam hardening usually occur?Base of skull, extended parts of bone
What is noise?Unwanted information created by scattered radiation
How do you reduce noise?Increase amount of data collected to increase Signal-to-noise ratio
Increase slice width, tube current (mAs), window width (WW)
What’s going on with this image? Noise artifact
What can CT contrast studies do?Allow imaging of vascular structures/anomalies (portocaval shunts), neoplastic lesions, disruption of BBB, abscesses
What is this showing? Contrast study before & after iodinated contrast medium IV (Meningioma in cat)
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