Week 5: CT Dose and QC Flashcards
(33 cards)
Dose Uniformity
Decreases as SFOV and patient thickness increase.
Multiple Scan Average Dose (MSAD)
- average dose patient gets for exam (sequential or helical)
- includes central slice dose reading and “tails”
- dose for entire examination
- tissue dose, not just skin dose
- Grays or mGys
CT Dose Index (CTDI)
- estimate of MSAD manufacturers use
- dose measurements are taken on a phantom then programmed into the machine so that a tech knows about how much dose will be received when certain parameters are set.
- used for protocol planning
- done with contiguous slices, no gaps, no overlaps
CTDI 100
- CTDI created using 100mm pencil ionization chamber (to cover and therefore evaluate multiple slices at one) is used as the baseline
- must take readings in different locations in the phantom
- must take reading at each selectable condition that caries the rate or duration of irradiation
CTDI w
- weighted CT dose index
- weighted or average CTDI 100 across entire single slice/section
CTDI Vol
- CTDI vol = CTDI w/pitch
- take into account exposure in the z-axis
- measure of exposure per slice independent of scan length
- the CTDI vol is “fixed” and independent of patient size and scan length thus the CTDI volume does not quantify how much radiation any specific patient receives but simply indicates the intensity of the radiation being directed at the patient.
Effective Dose
- with DLP we know how much dose has been given but not how it affects individual tissues, effective dose considered tissue sensitivity
WT = 0.12: stomach, colon, lung, red bone marrow, breast, remainder tissues
WT = 0.08: gonads
WT = 0.04: urinary bladder, esophagus, liver, thyroid
WT = 0.01: bone surface, skin, brain, salivary glands.
DLP
- the total amount of radiation incident on the patient
- the product of the CTDI vol and scan length (in cm)
- the DLP is the second dose metric that is easily accessible to the radiologist and account for both radiation intensity (CTDI vol) and scan length of the CT exam
- mGy x cm
mA: Balance QM and Dose
Increasing mA will increase dose so use the lowest mA and time that will give acceptable SNR
kV and Dose
kV changes have a massive effect on dose, 15% reduction is folate results in 35% dose reduction. Smaller patients and children must rescanned at lower kV.
Rotation Time affects Total Time
When it takes longer for the tube to make a rotation, dose is increased. When the tube rotates slower, spacial resolution is increased. Rotation time is inversely related to rotation speed.
Pitch and Dose
As pitch increases, dose decreases but so does spacial resolution. We must minimize overlap but be wary of pitches greater than one because data will be skipped and missed.
Thin Slices and Dose
Thin slices are good for detail but increase dose. We have to think about how thick we can acquire our slices and still get a good detail in reconstructed MPRs. Slice thickness increases = dose, noise and spacial resolution decreases.
Air Calibrations
Usually initiated with hr warm up cycle. HUs are real attenuation measurements so it must be assured every day that the detectors are returning accurate readings. Takes exposure at various settings (kV, SFOV, slice thickness, helical mode etc) and evaluates the CT numbers to see if they all return an HU appropriate for air. Corrects for detector drift/calibration errors.
CT Tube Warmup
Performed daily by the first tech on duty. Unit is turned on, unit generally initiates a warm up upon start up and takes many exposures to bring focal spot to appropriate heat.
CT HU Linearity
SC35 recommends this be done monthly, Toshiba recommends daily using one kV. Determines the CT numbers for certain know material densities. Uses phantom containing a variety of know densities.
How to perform HU Linearity Test
- place the phantom on the couch with the holder
- scan the phantom
- place an ROI over each section on the resultant image
- record average HU
- scanner HU tolerances are used
CT Number Accuracy
Determines accuracy of CT number for water. SC35 requires this dest be done weekly.
How to perform CT Number Accuracy
- scan homogenous/uniform water phantom
- place an ROI near centre of image
- record the output HU
- must be within +/- 4 HU of zero
CT Noise
Determines the standard deviation of CT numbers across an image. SC35 requires this be done weekly.
How to perform CT Noise Test
- scan the uniform water phantom
- read ROI in one area and evaluate standard deviation within area
- must be within 10% or 0.2 HU
CT Uniformity
Determines whether CT numbers are the same regardless of area of the image in a uniform phantom. SC35 requires this test be done weekly.
How to perform CT Uniformity Test
- scan homogenous water phantom
- drop a ROI central and four peripheral areas
- must be within +/- 2 HU
- baseline values must be within +/- 5 HU from manufacturer
Calibration of CT Number
Determines the mean CT number and standard deviation for water and air on a reconstructed image. SC35 requires this be done monthly. Uses water filled phantom, image and measure CT numbers for air/water with ROIs. CT number for water must be 0 +/- 4 HU. CT number for air must be -1000 +/- 10 HU.
