Exam Flashcards
CT
Computed tomography uses a computer to process information collected from the passage of x-ray beams through the area of anatomy. The radiation will hit a detector after passing through a patient. Image reconstruction from projections.
Each CT slice represents a specific plane in the patient’s body
The thickness of the plane is referred to as the Z-axis. The Z-axis determines the thickness of the slice.
Slip ring technology
Before helical scanning systems, there was high tension cable that had to be wound and unwound as images were being taken. Slip rings fix this problem.
Data gathering system using a continuous rotation of the x-ray source.
With a slip ring, the x-ray source can reach much higher speeds and decrease the acquisition time.
Multi-slice CT (MSCT)
Uses multiple rows of detectors in conjunction with widening the x-ray beam in the Z direction. May contain between 4 and 64 rows.
Allows for data to be collected from more than once slice at a time
Certain rows of the detectors can be selected to change the slice thickness along with the collimator.
Advantages:
- faster scanning due to wider total active detector width
- better dynamic imaging due to faster scanning times
- thinner slices
- 3D imaging which is enabled by thin slices
- simultaneous acquisition of more than one slice
Detector no longer collects the penumbra only the central beam
Grayscale
There are more than 2000 Hounsfield units.
The monitor can display 256 of those.
The human eye can only see about 40.
Imaging system
Produces x-rays Shapes & filters (hardens) beam - collimates - attenuates for uniformity Provides digital intensity data to the computer
Filter purpose
Protects patient from low-energy photons.
Provides beam closer to mono-energetic
The purpose of filtration is to provide a more uniform beam
Collimation purpose
To ensure constant beam width at the detector
Determines the thickness of the slice
Types of detectors
Scintillation Detector (solid-state): photodiode is attached to the crystals and transforms the light energy into electrical (analog) energy Gas ionization detector: these ions are accelerated by the high voltage on the detector plate
Data acquisition system (DAS)
Detector & electronics
Includes analog to digital converter, provides digital information to the reconstruction computer
Positioned within the gantry near the detectors.
Reads each arriving ray and measures how much beam is attenuated.
Attenuation coefficient
Reflects the degree to which the x-ray intensity is reduced by the material it passes through.
Reconstruction
The process of summing up all the attenuation data from thousands of angles.
When raw data are manipulated to create pixels that are used to create an image.
Hounsfield units
Air = -1000 HU
Water = 0HU
Bone = 1000 HU
Bone appears white, gasses and liquids are black and tissue is grey.
Tomographic images
Is a picture of a slice of the patient’s anatomy.
The 2D array of pixels in the CT images corresponds to an equal number of 3D voxels in the patient.
Each pixel on the CT image displays the average x-ray attenuation properties of the tissue in the corresponding voxel.
Ray
Single transmission measurement through the patient made by a single detector at a given moment in time.
Projection or view
A series of rays that pass through the patient at the same orientation
Two projection geometries
Parallel beam geometry with all rays in a projection parallel to one another
Fan beam geometry: in which rays at a given projection angle diverge.
Localizer scan (scout)
Most CT studies begin with one or more localizer scans.
Used to prescribe location of cross-sectional slices. Determined by the area being scanned with anatomical landmarks that can be readily identified.
Also need to select the optimal field of view and image center.
Similar to images captured with conventional radiographic projection technique.
- with slightly poorer image quality and deliver the same amount of dose.
The position of the tube determines the orientation of the patient.
- tube positioned above patient an AP image will be acquired
- tube positioned to side a lateral image will be acquired
The optimal localizer scan includes all areas to be scanned.
- anatomy to be images must be placed within the scannable range (z-direction)
- the patient must also be centred in the x and y-direction
Step and shoot scanning (conventional)
Scan method in which the CT table moves to the desired location and remains stationary while the x-ray tube rotates within the gantry collecting data.
Scans produced with a step-and-shoot method result in images that are perpendicular to the z-axis and parallel to every other slice.
Main disadvantages:
- interscan delay: the cumulative effect of the pauses between each data acquisition adds to total exam time.
- slice to slice misregistrations
Helical scanning
Produces slices in which the beginning point and the end point are not in the same Z axis
Key aspects:
- continually rotating x-ray tube
- constant x-ray output
- uninterrupted table movement
Eliminates the interscan delay
Advantages:
-ability to optimize iodinated contrast agent administration
- reduces respiratory misregistration
- reduces motion artifacts from organs
- pitch > 1 can be used to reduce scan time and/or radiation dose and still cover the same volume
Single detector row system
Each detector element is quite wide in the z-direction. Opening and closing the collimator controls the slice thickness by controlling the portion of the width of the detector that is exposed.
Detector collects both the primary beam and the penumbra zone radiation.
X-ray tube
The focal spot is the area of the anode surface which receives the beam of electrons from the cathode.
Modulation of focal spot (FS) size:
- small FS: thinner slices and increased resolution
- larger FS: increased power, larger volume scans with increased soft-tissue contrast
Bowtie filters
The purpose is to taper the radiation profile - fewer x-rays are emitted in regions where the body part is thinner.
Scatter grids
Created due to MDCT have increased scatter due to large beam widths. Scatter grids reduce scatter and preserve the contrast to noise ratio. Placed in front of the detector and focused on optimal alignment with the x-ray primary beam.
Helical pitch (SDCT)
The parameter that describes the CT table movement during helical acquisition.
The travel distance of the CT scan table per 360 degrees rotation of the x-ray tube, divided by the collimation width.
- table feed = colimation the pitch = 1
- table feed < beam collimation the pitch < 1 and scan overlaps occurs.
- table feed > beam collimation the pitch > 1 and scan gaps occur
As the pitch increases so does the slice angle. More interpolation is required to straighten the image.
Increasing the pitch will result in a scan covering more anatomy lengthwise, it will reduce the radiation dose to the patient.
Decreasing the pitch slows down the table speed and decreases the anatomy covered, this will increase the radiation dose to the patient.
