Basic CT Physics Flashcards
What is kVp?
The maximum voltage applied
What controls quantity, energy and intensity?
kVp accelerates electrons ____________ to _____________.
cathode; anode
An increase to kVp does what to contrast?
decreases contrast
What controls the quantity of x-rays?
mAs (…over a set amount of time)
This determines how hot the filament gets.
mAs
What does the heat of the filament control?
The number of electrons, mAs.
Define pitch
The ratio of table travel per rotation in millimeters divided by beam collimation.
What type of pitch minimizes helical artifact?
Interleaved
Increasing pitch from 1.0 to 1.375 decreases dose by a factor of about ____%.
27.
What is interleaved?
A pitch of <1.
Interleaved has a(n) _____________ in resolution and dose.
increase.
Interspaced has a(n) _____________ in resolution and dose.
decrease
What is interspaced?
a pitch of >1.
Focal spot size is a key factor in ______________.
resolution
What is a key factor in resolution?
Focal spot size
What is the focal spot?
The area of the anode target that is hit by the electron beam from the cathode.
What determines the size of the focal spot?
The size/shape of the electron beam.
A lower pitch will result in a ________ exposure time and ________ mA to be delivered.
longer; more
______________ helices have more interpolated data and _____________ helical artifact.
Interspaced; increased
What are the two common pitches used in HyperDrive?
1.375 and 1.531
To minimize motion artifacts one should use a ____________ rotation time.
faster
What are the rotation times listed in the Apex User Manual?
- 0.28
- 0.35
- 0.5
- 0.6
- 0.7
- 0.8
- 0.9
- 1.0
When is focal spot deflection used?
During High Resolution scanning (High Definition)
How does the system select focal spot sizes S, L and XL?
Based on kV, mA and exposure time. (kV and mAs)
What may be adjusted to indirectly select the focal spot size?
The mA value.
An 80 mm detector has ___________ detector rows in the Z direction and __________ slices.
128; 256.
What size are the detector rows in the Gemstone Clarity Detector?
0.625 mm
The 160 mm detector has ___________ detector rows in the Z direction and ___________ slices.
256; 512.
What are the beam collimation/detector coverage for the 160 mm detector (256 rows, 512 slices)?
- 5
- 40
- 80
- 100
- 120
- 140
- 160
What are the beam collimation/detector coverage for the 80 mm detector (128 rows, 256 slices)?
- 5
- 40
- 80
What is the CT detector?
Wide coverage cone beam detector with multiple detector rows along the longitudinal (Z axis) plane.
The detector consists of a scintillator that converts _______ to _______.
x-rays; light
What factor is utilizing the bowtie filter?
SFOV
How many cm does a small SFOV support?
Up to 32 cm DFOV
List the three small SFOV and what is the max DFOV?
- Ped Head
- Small Head
- Ped Body
- 32 cm DFOV limitation
List features of the Ped Head SFOV.
- Uses IBO processing to correct for beam hardening effects.
- Useful for infants up to 18 months of age.
When should the small head SFOV be used?
For patients from 1.5 to 10 years old.
Why center the anatomy of interest in the center of the gantry?
- To take advantage of the increased X-ray intensity.
- Essential when using SmartmA to reduce unnecessary radiation exposure.
What SFOV’s utilize a 16 cm phantom to calculate CTDI?
Ped Head
Small Head
Head
What SFOV’s utilize a 32 cm phantom?
Ped Body
Small Body
Medium Body
Large Body
Cardiac Small
Cardiac Medium
Cardiac Large
What is DFOV?
The particular anatomy selected within the SFOV that is reconstructed into an image.
Define WWWL
WW is the range of grayscale values displayed controlling the contrast and WL represents the midpoint of that range controlling the brightness.
A narrow WW provides ________ contrast with ________ shades of gray.
high; fewer
A wide WW shows _________ contrast with ________ shades of gray.
low; more
WWWL of abdomen
400 40
WWWL Head
100 35
WWWL Lung
1500 -600
WWWL mediastinum
350 40
WWWL Spine
300 35
WWWL Vertebrae
2000 350
What is isotropic?
Voxels with the same dimensions and spatial resolution along all three axes.
Define temporal resolution.
The ability to capture images in a short amount of time. Important for reducing motion artifacts and improving image quality.
What are the three factors that affect temporal resolution?
Gantry rotation time
Helical pitch factor
Collimation width
A higher pitch will have a __________ temporal resolution.
higher
What is the goal of temporal resolution?
Create motion free images.
Define cardiac gating.
Image reconstruction is centered to a given phase percentage along the R-R interval as well as being centered in the Z-axis.
Achieved by attaching patient to ECG monitor which is attached to the CT gantry. The waveform is collected during image acquisition.
What is slice thickness?
The thickness of tissue represented in each image slice.
What does slice thickness affect in CT?
- Spatial resolution - thinner slices better spatial resolution to better differentiate small changes in the image.
- Image noise - Thinner slices have more image noise
- Scan time - Thicker slices have shorter scan times
- Signal to noise ratio - Thicker slices have a better SNR because more photons are available.
What does selecting a smaller detector size provide?
Higher spatial resolution which is crucial for detailed imaging of fine detail of small structures.
What does selecting a larger detector size provide?
A wider FOV for scanning larger body regions.
What is CTDI (Computed Tomography Dose Index)?
-A metric that measures the average radiation dose from a single CT scan rotation
-Measured in grays (Gy)
What is DLP (Dose Length Product)?
-Used to calculate the effective dose, an estimate of the radiation dose to the whole body.
-Multiply the volume CTDI by the scan length (mGy.cm)
