COMPUTED TOMOGRAPHY Flashcards
CT scan parameters can be altered or optimized in order to
– reduce patient radiation dose
– improve image quality
This is the size of the FIELD in the gantry aperture.
SFOV Scan Field of View
■ Tip: The field size is NOT the entire visible area you see within the aperture.
■ The size of the scan parameter tells the computer which data to use and which data to ignore.
■ The__ determines the number of detectors required to collect data for a particular procedure.
■ The __ should always be larger than the area of interest.
SFOV Scan Field of View
When the anatomical area of the patient is OUTSIDE the SFOV, out-of field artifacts will occur. These artifacts can present as:
■ Streaking
■ Shading
■ Miss-assignment of CT numbers
Too Large a SFOV may also produce artifacts such as:
■ Shading
■ Streaking at the skin surface
Also known as the Reconstructed field of view (RFOV)
■ __ plus matrix size determine the limitations of perceived detail
■ __ should be equal to or smaller than the SFOV NEVER LARGER
■ The ___ also impacts image noise and resolution.
■ Wider __ increase the quantity of the photons from which data is retrieved.
■ Image noise is reduced, however, at the expense
of resolution
DFOV Display Field of View
Determines the size of the image viewed on the monitor
DFOV Display Field of View
is normally set to include other structures or pathology that may also be located in the scan plane.
■ ___ - best for imaging tissue types that carry greatly
■ ___ - tissues with similar densities
Window width; Wide window width (500- 2000 HU); Narrow window width (50- 500 HU)
determines the center of the gray scale and is generally set at the average tissue density of the structures within the anatomy being scanned.
Window level
Basically, the __ shows the range of densities you will be seeing, and the __ depicts the average density you will be seeing on the final reconstructed image.
width; level
The manipulation of window width and window level to optimize image contrast is referred to as
windowing
Width and Level: Head examinations
Posterior Fossa: 150-40
Brain: 100-30
Temporal bone: 2,800-600
Width and Level: Neck examinations
Neck: 250-30
Width and Level: Chest examinations
Mediastinum: 350-50
Lungs: 1,500- -600
Width and Level: Abdominal examinations
Soft tissues: 350-50
Liver (high contrast): 150-30
Width and Level: Pelvis examinations
Soft tissues: 400-50
Bones: 1,800 - 400
Width and Level: Spine examinations
Soft tissues: 250-50
Bones: 1,800-400
■ Primarily responsible for ensuring that the scanned anatomy is accurately represented
■ Secondarily responsible for enhancing the spatial or contrast resolution of the final image
Reconstruction Filters
provide definitive borders and edges, used for high contrast areas, musculoskeletal (increases image noise)
High-pass (sharp) filters
do not define borders and edges to the same extent as High-pass filters. Used for low contrast areas such as brain, abd. etc.
Low-pass (soft) filters
■Primarily responsible for accurate anatomy representation on the final image
■ Secondary responsibility for enhancing the spatial resolution
■ High pass filters
■ Low pass filters
Reconstruction Filters
■ Represent segment of the anatomy scanned
■ Contain numerical information based on tissue density
■ Two-dimensional representation
■ Slice thickness turns pixels into voxels (volume elements)
■ Expressed in __
Pixels; Hounsfield units
Hounsfield units are __ to the attenuation coefficient of the tissue
directly proportional
512 x 512 matrix has __ than a 1024 x 1024 matrix
fewer pixels
= faster reconstruction = decreased information to be archived
Fewer pixel
the __ the size of the pixel the less detail is presented on the image. __ the pixel the greater the detail.
larger; Smaller
In general, __ slices produce sharper images because to create an image the system must flatten the scan thickness (a volume) into two dimensions (a flat image). The __ the slice, the more flattening is necessary
thinner; thicker
To reduce image noise more __ is required.
mAs
The slice thickness is determined by the:
■ Beam-width (BW)
■ Pitch
■ Shape and width of the reconstruction filter in the Z-axis
Defined as the distance of couch top travel, per one revolution of the x-ray tube, divided by the slice thickness or the beam collimation.
Pitch
A __ pitch, with increased overlap of anatomy and increased sampling at each location, results in an __
smaller ; increased radiation dose.
__ suggests overlap between adjacent acquisitions, __ implies gaps between adjacent acquisitions, and __ suggests that acquisitions are contiguous, with neither overlap nor gaps
Pitch <1; pitch >1; pitch of 1
Radiation dose is affected by the pitch of the __
helical scan.
___ is associated with less image noise, fewer artifacts, and improved signal-to-noise and contrast-to noise ratios
Low-pitch technique
■ 1 : 1 changed to 1.5 : 1 results in a __ in patient exposure.
■ May be significant when scanning pediatric patients.
33% reduction
■ Newer scanners have __
■ The scanner detects the remnant radiation and will
reduce the mA accordingly.
“smart scan” or “smart mA”
__ result in improved image quality, decreased image noise, and increased patient dose
Increase in tube current or the product of tube current and scan time (mAs)
Is directly proportional to the mA (tube current).
Patient Dose
have the ability to produce more than one set of reconstructed images using the same Raw Data
Scanners
Remember not enough sampling causes artifact called __
aliasing
■ The rate at which a CT machine samples data is selectable at purchase.
■ This rate does affect the scan time select ability.
Sampling and scan time
Only half of the otherwise available data are available to reconstruct the image with partial scans; therefore, they are inferior to standard 360° scans.
180° (Partial scan)
The most common selection in rotation arc
360°
Allows some overlap of data from the first and last tube positions and reduces motion artifacts.
400° (Overscan)
the __ cause more geometric unsharpness in the image and reduce spatial resolution.
larger focal spots
