Image Display and Radiation Dosimetry in CT

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What is included in image display?

Includes all of the system components necessary to convert the digital data created from the reconstruction process to electrical signals needed by the CT display monitor

Includes the ability to display patient information and scan protocol data

Provides many graphic aides designed to assist in image interpolation

What is the cathode ray tube (CRT)?

Basically, a standard television set with some modifications that improve image resolution

What is the newer flat panel type display monitors called?

TFT LCD (thin-film transistor, liquid crystal display)

LED (light emitting diode)

What advantages do the newer display monitors have over the older type

Produce higher luminescence (brightness)

Higher spatial resolution

What is the HU for bone

+1000

What is the HU for water

0

What is the HU for air

-1000

How many shades of gray can be displayed on a medical monitor

1024 shades of gray

What is the contrast difference that can be appreciated by medical monitors

Can appreciate contrast differences of less than 1%

How many shades of gray can be seen by the human eye

40 shades of gray

What contrast difference can be appreciated by the human eye

Cannot appreciate contrast differences of less than about 10%

What is Artificial Intelligence in Radiology and how does it use radiomics

A CT image contains information that goes beyond what we can appreciate visually so AI uses radiomics to extract quantitative image features

It can also learn image patterns directly

What does radiomics mean

Extracting quantitative image features

What are some examples of radiomics

Size

Volume

Density

Shape

Texture

What is window width?

Determines the quantity of Hounsfield units represented as shades of gray on a specific image

All values higher than the selected range appear WHITE

All values lower than the selected range appear BLACK

Increasing the window width (widening the range) = more numbers are assigned to a shade of gray

What is window level?

Selects which Hounsfield values are displayed as shades of gray

Determines the center pixel value in the gray scale

How do you calculate the range of HU’s

Divide the window width by 2

Add that number to the window level to get the top end of the range

Subtract that number from the window level to get the bottom end of the range

What are window settings?

Manufacturer software will have present values programmed in when new equipment is installed

Radiologists and Technologists work together when a facility gets new equipment to make sure settings are what they want them to be and can make changes if desired

If changes are made, they are saved to an external drive so they can easily be reuploaded if needed (major power outage)

What should the window level be set at relative to the tissue

The window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest

Use a wide window width if the goal is to see many different shades of gray

What is the ROI tool

Region is defined by the operator/technologist

The tool is most often circular but can be other shapes

What 2 things is the ROI tool used for

Used to obtain an average Hounsfield unit of an area

Used to acquire the standard deviation

What is standard deviation

The amount of CT number variants within the ROI

If the ROI measurement is 20 and the standard deviation is 0, then every pixel value in the ROI is 20

If the ROI measurement is 20 and the standard deviation is 10, then every pixel in that region is NOT 20

The higher the standard deviation, the greater the variation among pixels within the region

What factors produce high standard deviation

mixed attenuation tissue within the ROI

An ROI that includes streak artifacts

An ROI that is not inside the margins of the object being measured

What are distance measurements?

A tool that is a line the user can put from one area to another

Measurements are given in cm or mm*

Important tool to place a biopsy needle or drainage apparatus

What is image annotation?

Software that allows the operator to annotate specific images with words, phrases, arrows, or other markers

What is a reference image?

Displays the slice lines in corresponding locations on the scout image

What is image magnification?

Just makes the existing image appear larger

Uses only image data

Does not change the spatial resolution

Not the same as making changes to the DFOV

What can magnifying the image help the technologist with

Helps to measure a tiny structure like a kidney stone

Helps to correctly place the ROI in the region you want to measure

What is a histogram

A graphical display showing how frequently a range of CT numbers occurs within the ROI

What is multiplanar reformatting (MPR)

Two-dimensional reformation done to show anatomy in various planes

• Coronal

• Sagittal

• Oblique

What type of data is used to create multiplanar reformats

Image data

What kVp range is used in CT

80-140 kVp

What is the average energy of the x-ray beam

70 keV

What is the SI unit of measurement for absorbed energy

Gray

Gy

What is the SI unit of measurement for deposited energy

Sievert (Sv)

What does quality factors mean

Accounts for the different biological effects produced from different types of ionizing radiation

For diagnostic x-rays used in the CT the quality factor is 1

What does effective dose mean

A measurement that attempts to account for the effects particular to the tissue that has absorbed the radiation dose

What data source does effective dose use to determine risk

Japanese atomic bomb survivors

What is KERMA?

Kinetic energy released in matter

The amount of energy transferred from the incident x-ray photons to charged particles per unit mass in the medium

What is the difference from x-ray to CT dose geometry

CT gives a more uniform dose

The beam is heavily filtered as it exits the X-ray tube, which means fewer low-energy (soft) photons remain. Because the beam is harder than that used in radiography, a lower percentage of the beam will be absorbed or scattered as it passes through patient tissue

The exposure in CT comes from all directions

What two things cause dose uniformity to decrease in CT

Increased SFOV

Increased patient thickness

What is radiation profile in CT

Variations along the length, or z axis of the patient

What is the AKA for radiation profile

Z-axis dose distribution

How does scatter affect the neighboring slice and what is it called

It attributes to the dose of 1 slice due to the scatter from adjacent slices

“tail”

What is multiple scan average dose

MSAD

Dose calculated from multiple scans

What is the multiple scan average dose used to figure out

Total dose

The central slice radiation dose plus the scatter overlap or tails

What is computed tomography dose index

Allows an estimate of the MSAD to be accomplished with a single scan

Who is the computed tomography dose index reported to

FDA

When does MSAD equal CDTI

When there is no overlap or gap between slices

What 2 things are needed for CTDI testing

CTDI phantoms

CTDI 100

What is pitch

Relation of table speed to slice thickness

Pitch = 1

The table moves a distance equal to the slice thickness

Slices are contiguous with no gaps, providing optimal image quality

Pitch <1

The table moves less than the slice thickness, causing overlapping slices

This improves image resolution but increases radiation dose to the patient

Pitch >1

The table moves more than the slice thickness, creating gaps between slices

This will decrease radiation dose and scan time but may slightly decrease image quality

What is the most common pitch settings used in CT

1-1.5

Most common is 1.2

What is CTDIw

Adjusts for variation across the scan field of view by providing a weighted average of measurements at the center and the peripheral slice locations ( the x and y dimesions of the slice)

How are the CTDIw doses calculated

2/3 CTDI 100 (periphery) + 1/3 CTDI 100 (center)

What is CTDI100

Result when the CTDI is measured using a pencil ionization chamber

What is CTDIvol

A measure of exposure per slice and is independent of scan length

Takes into account the exposure variation in the z direction

It is the preferred expression of radiation dose in CT dosimetry

How are CTDI doses calculated?

CTDIvol= CTDIw/pitch factor

What is DLP

Dose-length product

the DLP more closely reflects the radiation dose for a specific Ct examination, its value is affected by variances in patient anatomy

How are DLP doses calculated?

DLP=CTDIvol x scan length

Which CTDI calculation is the preferred expression for measuring dose in CT

CTDIvol

Know about pediatric patients in CT

Children are much more radiosensitive than adults

What are specific ways to reduce pediatric dose

Use CT only when clinically indicated

Customize the CT examination

Decrease mAs

Why is patient shielding less beneficial in CT

Because of the narrow collimation, radiation to areas outside of that selected scan area is minimal usually attributable to the internal scattering of photons that are unaffected by surface shielding

Is worker shielding still beneficial in CT

Yes it is still important to shield yourself if you must be in the room watching an injection, the patient is vomiting, or for biopsy procedures

What are the 7 strategies for reducing radiation dose

Reduce mAs (quantity of x-ray)

Use ATCM (automated tube current modulation)

Avoid increasing kVp (affects quanity)

Increase the pitch

Limit the use of thin slices

Limit repeat scans

Use iterative and deep learning reconstruction methods

What is ACTM

Automated tube current modulation

A software that allows the computer to estimate the dose a patient needs based off of their scout images

Reduces the amount of radiation b7 35 to 60%

How does increasing the pitch affect reducing radiation dose

From 1 to 1.5 decreases the dose by 33% without loss of information

How does limiting the use of thin slices reduce radiation dose

Using a large number of thin adjacent slices resulting in 30-50% more radiation dose to the patient than using fewer thicker slices to scan the same anatomy

How does the use of iterative and deep learning reconstruction methods reduce radiation dose

Reduces the dose by as much as 50%