CT image production
During an axial scan, the total amount of anatomical coverage per tube rotation is dependent on which of these variables? - The number of active detector rows2 - The thickness of active detector rows3 - - the pitch
- The number of active detector rows2 - The thickness of active detector rows3 -
During an axial scan, the total amount of anatomical coverage per tube rotation is dependent on which of these variables? 1 - The number of active detector rows2 - The thickness of active detector rows3 - The pitch
1 and 2 aka collimation
Common DFOV for c spine is
120 mm (12 cm)
KVP should never exceed
140
After performing a CT of the chest, an ROI placed in the air outside of the patient yields of an average CT number of -1,223. This error represents a problem with
Accuracy
What does undersampling cause
Alaising
Iterative reconstruction
Basically you can lower the dose and allegedly maintain imaging quality with advanced reconstruction
What artifact occurs when the x-ray beam passes through dense material like bone metal or contrast
Beam hardening
out of field artifacts is caused by patient
Being outside SFOV
Which algorithms increase spatial resolution?
Bone Lung
Dense materials such as _______ and _______attenuate _______
Bone Metal More
Consider a CT image that was initially displayed with a 300 window width and a 30 window level. DECREASING the window level from 30 to 0 will have what effect on the image?
Brighter
Reconstruction of raw data into image data occurs within which component of the CT system?
COmputer system
window width refers to the range of
CT numbers that will be expressed in the image as some shades of gray
Smoothing algorithms like standard are better for _________but not for _________
Contrast resolution Spatial resolution
What is the principle function of the data acquisition system (DAS) in CT?
Converts electrical signals into raw data
The purpose of the detector array in CT is to:
Converts photons to electrical signals
What data process is used to smooth or sharpen image data for better visualization of the desired anatomy?
Convolution
The physical dimensions of the matrix and pixels are determined by
DFOV
Compared to a pitch of 1, a pitch of 1.137 will have: Decrease scan time Decrease patient dose decrease spatial resolution
Decrease patient dose Decrease spatial resolution
What factors increase temporal resolution
Decrease rotation time Increase slice thickness Increase pitch
Increasing the window level of a CT image will result in:
Decreased brightness
What 6 factors increase spatial resolution?
Decreased detector size Decreased pitch increased rotation time Decreased DFOV Decreased slices Decreased noise Increased matrix size Bone algorithm
Using a larger reconstruction slice thickness results in which of the following?
Decreased image noise Decreased spatial resolution
As noise increases, contrast ___________
Decreases
Increasing mA and kVp _______noise and _________contrast
Decreases Increases
WIDER window width ________contrast and has a ____________ gray scale
Decreases Wider
increasing slice thickness _______total scan time which __________ temporal resolution
Decreases increases
Adjacent areas with significant density differences can cause an artifact called
Edge gradient
Alaising is caused by
Fast scans or increased pitch
High pitch results in a __________scan with ________resolution images
Faster Lower
If pitch is greater than one this results in
Faster scan speed Lower Dose Less spatial resolution
Window level is usually set similar to the
HU
The most common mode of scanning is
Helical
Spatial resolution is tested using QC and line pairs. The more line pairs the _________the spatial resolution
Higher
What uses an ROI and provides a frequency of HU for the area
Histogram analysis
Which of these quantitative analysis methods in CT can be used to identify the maximum Hounsfield value in an area of concern?
Histogram analysis
Other methods for improving temporal resolution
Image gating Half scan algorithms
What is the most important factor influencing contrast resolution
Image noise
Consider a PACS network in which imaging studies are distributed directly to specific reading stations and to the PACS archive server. What is the main advantage of this distribution system?
Images are duplicated in the PACS archive server and at the reading workstation If the archiver server goes down, images can still be sent to specific reading stations
What factors increase contrast resolution?
Increase technical factors Decrease patient size Increase thickness Lower standard deviation
Contrast is _________with thicker slices
Increased
During a routine CT scan of a pediatric chest, the radiologist requests that the technologist reconstruct the image data with 1.25 mm slices rather than 2.5 mm slices. One negative effect of this change will be:
Increased image noise
As window level _______the contrast gets _______
Increases Darker
Decreasing pixel size and FOV
Increases spatial resolution
What is the mostly likely cause of the blurry appearance of this coronal reformation of the abdomen?
It was reconstructed from a thick slice
During CT angiography, images may be reconstructed using only the greatest density encountered along each ray. This type of specialized CT image is called a(n):
MIP
Window level represents the
Midpoint of CT numbers that will be viewed
Motion artifact causes _________ of the patients anatomy
Misregistration
What do the numbers in a matrix represent
Number of pixels
Which of the following is used for long-term storage of digital image data?
PACS
Different tissues being averaged into one slice is
Partial volume averaging
Increasing DFOV decreases________but _________spatial resolution
Pixel size Increases
What portion of the patient can be visualized through the process of retrospective reconstruction?
Portions in the SFOV
A single "view" acquired during a CT scan is also called a
Projection
What puts specific numbers to observations seen on a CT (the exact size of an aneurysm)
Quantitative analysis
A measurement tool used in quantitive analysis to acquire various calculations for an area of interest
ROI
The process of assigning a specific level of edge-enhancement to the CT image data is controlled by mathematical filters called:
Reconstruction algorithms
Concerning personal health records, federal laws does NOT allow a patient to:
Remove their records
What cannot be changed retrospectively
SFOV Acquisition thickness
What type of 3D rendering just shows the surface?
Shade surface display
How does cone beam artifact affect the appearance of the CT image?
Shape distortion of anatomy
Select all parameters that CAN be changed retrospectively.
Slice interval Slice thickness Reconstruction algorithm DFOV Start and end
Bc MPR images are created in different planes the technologist must select new
Slice thickness and intervals
A detector array is arranged into rows called ________and having more of them makes the beam_____
Slices Wider
In MPR and reconstruction slice thickness affects
Spatial resolution Noise
What algorithm must be used with volume rendering to avoid noise
Standard (smoothing)
What does filtered back production remove
Star like artifacts
Retrospective reconstruction
Start and end locations Slice thickness and interval DFOV Reconstruction algorithm Window width and level
EMR is different than EHR bc it just involves patient information from
That certain doctors office
Uniform array
The detector rows are equal size
When using a CT scanner with non-uniform detector rows, which of the following statements is true:
The inner rows have the highest spatial resolution
1024 x 1024 matrix refers to
The number of pixels in a matrix
Acquisition thickness refers to:
The thickness of active detector rows
Changing the thickness of a voxel may also change
The tissue types in the voxel
Kvp should only be changed with the _______of patient
Thickness
Because of loss of information MPR reformats must be taken from ________slices
Thin
What quality control test evaluates the ability of the CT scanner to produce the same CT number for the same material regardless of where the ROI is placed?
Uniformity
When scanning a water phantom for quality control, you notice that an ROI in the center of the phantom reads 0 HU but an ROI on the edge of the phantom reads 4 HU. This is a problem with:
Uniformity
Digital process of projecting the entire data set
Volume rendering
Linearity
When the attenuation of an object increases so should the CT number
Numbers above the window width are _______and numbers below are ______
White Black
The gantry houses
X-ray tube detectors collimators high voltage generator ADC
Changing the slice thickness of an axial reconstruction changes the dimensions of the image data in what axis?
Z axis
Reconstruction algorithm (convolution)
an add sharpness to the images (bone and edge-enhancing kernels) or they can smooth the images for viewing soft tissues (standard kernels)
MPR reformats axial images to
be viewed in other planes (sagittal, coronal, obliques)
Unlike axial who acquires one section at a time, helical
captures one large spiral of data while the tube and table are in constant motion
Detectors are responsible for
collecting photons that have passed through the patient and converting them into a measurable electrical signal
A wide beam might cause a ______artifact
cone beam artifact
Increasing pitch ________ total scan time which _______ temporal resolution
decreases
Pitch only applies to helical scanning and is a ratio describing
distance the table moves during scanning compared to the beam width .
Kvp controls
energy of beam (quality) but also influences intensity
If a radiologist needs to know the ratio of fluid to blood in a cerebral lesion, what quantitative tool can help to estimate this ratio?
histogram analysis
MPR images are created from what kind of data
image data
What is interpolation
images must be viewed in axial sections (not a spiral), the computer must reconstruct the data into perfect planar slices from the volume of data
What affects noise the most?
kVP
DAS does 3 things
measures the transmitted radiation from the detectors converts electrical signal to a digital signal transmits the digital signal to the main computer
as voxel/slice thickness increases what happens
more anatomy is shown
Temporal resolution is the ability to see
moving parts
A higher standard deviation causes more _________
noise
mA is
quantity & intensity of beam and the exposure to the arrays
reconstruction images come from _______data and only come from _____plane but can use any_______
raw axial Algorithm
SFOV
refers to the entire area inside the scanner that is actually being scanned .
When a detector fails it causes a ________artifact
ring
Contrast resolution is the ability to
see parts with similar densities
Spatial resolution is the ability to
see small parts
The capacity of the machine to differentiate between objects of similar densities
sensitivity of the system
Prospective reconstruction
setting up reconstruction before the scan
Axial scanning is also called
slice by slice or step and shoot
voxel thickness equals
slice thickness
Smaller detector size results in better:
spatial resolution
Ejection fraction is the
the percentage of blood that is pumped out of the heart with each contraction (above 55 are good)
Convolution
the process through which scan data is mathematically filtered to better visualize specific tissue types.
The larger the matrix size
the smaller the pixels, the better spatial resolution
Viewing blood vessels and bone should be viewed using what slice thickness
thin
a uniform array can produce ______ slices and shows ________detail but has a ________dose
thin more higher
What else happens when pitch is increased
this results in a faster scan and less dose to the patient. Think of increased pitch as stretching out the spiral. More anatomy is scanned with each rotation of the tube. Less time is needed, and less dose is used. Unfortunately, the spatial resolution will decrease.
Which of these images is displayed with a widow width of 2500 and window level of 600
which image is most like the window level
