Chapter 3 Rad Safety Flashcards by Shelby Owens

Describe the process of photoelectric absorption

Incident ray comes in and hits inner shell electron and becomes absorbed by the atom

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Describe the process of Compton scatter

Incident ray comes in and hits outer shell electron. Incident ray loses energy and then changes direction when leaving the atom

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Attenuation of an Xray beam depends on 2 things what are they

beam quality

2. tissue density

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TREND

patient-to- detector distance decreases, then the dose will…..

increase

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What is the difference between Focus-to- isocenter distance and Focus-to- detector distance

Focus-to- isocenter distance describes difference from Ct tube to center of gantry

Focus-to- detector distance describes distance from Ct tube to detector array

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What happens when you position the patient off center relative to the gantry

unwanted variation of dose and exposure

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What material lines the inner CT x-

ray tube and promotes filtration of unwanted beams

Aluminum. 6-9 mm

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Describe inherent absorption efficiency

the detectors ability to capture x-ray quanta and provide a response

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Describe geometric efficiency

The spatial arrangement of everything in

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Describe the process of over beaming? How does it impact the image ?

When the detectors are exposed to undesirable “penumbra”.

It does not contribute to the formation of the CT image

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What technique do some Ct manufactures employ to control the position of the x ray beam on detectors?

focal spot tracking

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To expose the widened detector arrays to xray beams equally MDCTs utilize what shape of beam?

What beam shape was employed in the traditional single slice scanners?

cone

fan

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At what point does overranging occur?

When radiation dose is applied before and after the acquisition volume. (half rotaion before and after spiral)

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What is used during the reconstruction process to reduce noise in a CT image?

Adaptive filters ( noise reduction algorithms)

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What are 4 reasons radiation dose is usually higher in MDCT scanners vs SSCT

Decreased focus to detector distance
Cone beam utilization
More phases in acquisition
Thinner sections

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Why is it necessary to optimize scanning parameters

follows ALARA principles to minimize patient radiation dose

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The relationships between milliampere (mA) setting, scan time, and radiation dose is inversely or directly proportional?

directly

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The relationships between kvp setting and radiation dose is inversely or directly proportional?

directly and EXPONETIAL

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For which scans can reducing kvp not have a negative effect on image quality ?

Pediatric CT, and CT angiography

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What does the ‘Dose Profile” reference in Ct ?

the section of the body exposed to radiation

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Thinner slices lead to more or less noise?

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When you increase the collimation on a camera are slices getting thinner or thicker?

thinner

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With MDCT scanners does more over beaming occur when you have wider or thinner collimation?

Wider collimation increases/ decreases dose efficency?

thinner

increases

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Decreasing patient does increases/ decreases image noise ?

What are two solutions used in image reconstruction to create a less noisy image

increases

using the correct reconstruction kernel
using the correct window setting

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Decrease in patient size increases/ decreases image noise?

decrease

During SSCT acquisition what range in pitch will allow you to take the picture in shorter time

pitch > 1

How does increasing pitch in MDCT scanners affect image noise?

increases noise

Identify whether x, y, or z references the angular or longitudinal axis

x and y axis = Angular z= Longitudinal

current is measured in what units

milliampere (mAs)

Describe angular tube current modulation

mAs is adjusted based on the difference and thickness of the part as the tube rotates

How do CT scanners maximize radiation dose reduction

Employ both angular and longitudinal current modulation techniques

What complex material can be used as a selective shield to reduce dose to sensitive areas? ( hint: one is impregnated into the other)

Rubber with bismuth inside

What is the disadvantage of bismuth shielding? | What can you use to reduce this scattered streaking artifact ?

- streaking artifact | - Foam spacer between patient and bismuth shield

Room shielding requirements should account for what factors?

1. daily work load 2. scanner position 3. window and door construction

If a family member must absolutely be in the room with the patient during the scan what must they wear ?

Appropriate lead shielding

What is CTDI? Is it calculated for all slices in a CT scan ?

A measure of the dose received in a single CT slice. No. It is calculated for only the central slice

What equipment is used to measure CTDI? How do they work together to collect the CTDI index

Dosimeter Head and body phantom you place the dosimeter which is usually a small ionization chamber inside of the phantoms and then scan them

CTDI is calculated for the central slice or all of the slices?

central

Whats the difference between the three CTDI's: CTDI 100 CTDI vol CTDI w

CTDI-100: uses a pencil shape dosimeter 100 mm in length CTDI-w: Weighted dose index that accounts for variance in dose distribution. CTDI-vol: accounts for the dose measured in each section during a helical scab

What is the equation for CTDI-w? CTDI-vol?

CTDI-w: 2/3( periphery exposure)- 1/3(central exposure) CTDI-vol: CTDI-w/ pitch

True/ False. In the acquired field of view of a CT image the radiation dose varies depending on which area you are looking at

True

Between CTDI-w and CTDI-vol which uses step-and-shoot and which uses helical scanning to obtain radiation measuremnets

CTDI- vol: helical scanning CTDI-w : Step and shoot

Between CTDI-w and CTDI-vol which axes are utilized to approximate radiation measurements in an acquired image

CTDI-w : uses the x/y axis CTDI-vol: uses z-axis

What does MSAD stand for? What does it measure?

Multi slice average dose Average collective dose to each slice in the center of a scan

What are some equation to calculate MSAD ?

MSAD= (T/I) x CTDI T* is slice thickness I* is image spacing MSAD= CTDI/ pitch

Multi slice average dose (MSAD) accounts for the effects of what during axial scanning?

Image spacing

Overlapping scanning increase/decrease the patient radiation dose.

increase

How does slice thickness and image spacing effect the multi slice average dose ( MSAD)

Slice thickness > image spacing = increase in MSAD Slice thickness < image spacing = decrease in MSAD Slice thickness= image spacing --> MSAD= CTDI

What is another word that refers to 'Image spacing '

bed index

During helical or spiral scanning what factor controls the MSAD

pitch

True/ False. MSAD measurement is more accurate at either end of the acquisition rather than in the center of the scan

false

What does DLP stand for?

Dose length product

What are some equations DLP?

DLP= (MSAD) x (slice width) x (# of slices) slice*= pre-patient collimator setting ONLY. This applies for CTDI- vol as well. DLP= (CTDI-vol)x (scan length)

What do the following stand for: DICOM RDSR

Digital imaging and communications in medicine Radiation dose structured report

What is the RDSR

it is an archive of all the patinet exposure information . Includes: voltage, scan time , pitch , width , acquisition length

Does the radiation dose report (RDSR) only keep archived records of quality scans or do they save EVERYTHING including those with low quality or other issues? True/ False . Industrial standards requires this data to be automatically archived to patients chart after scan.

saves everything true

A dose check system employs 2 types of alerts what are they? What's the purpose of a dose check system?

Dose notification Dose alert Notify tech if a patient is anticipated to receive over the recommended amount of exposure

What's the difference between dose notification and dose alert

" Notification" refers to the technical setting " Notification" requires tech to confirm settings before acquisition starts ( ie... date/time, dose value, comments, etc.) "Alert" refers to the impending acquisition at that time "Alert" requires tech to adjust settings to confirm dose OR confirm setting and enter initials to proceed. "Alert" sounds when the CTDI-vol exceeds that which is recommended for s prescribed CT with multiple acquisitions

industry standards require CT scanners to employ the following to keep patient dose to a minimum:

1. Automated dose check 2. Adult and Ped reference protocols 3. DICOM RDSR's 4. Automatic exposure control

True/ False. across the field of view of an image there is a dose gradient

true

Dose patient size affect the dose gradient exhibited on a CT image? How?

Yes Larger patients have more massive gradients where the difference in absorbed dose is greater. Pediatric patients don't have a gradient since they are so small

Why do pediatric patients don't normally have a dose gradient?

Radiation intensity upon patient entry and exit is equal to each other and therefore the distribution along the field of view is uniform

True/ False. When all technical parameters remain constant , absorbed dose is typically higher in smaller patients

true

What are 5 ways recommended to reduce pediatric dosage in CT?

1. get rid of CT orders with improper indications 2. reduce multiphase scanning 3. Lower KVP 4. Lower mAs 5. Increase pitch

``` True/ False . Protocol optimization ( adjusting mAs, kVP , and pitch) should be done based on age not weight ```

false

Who developed the 'Image Gently' campaign ? What is its purpose?

Alliance for radiation safety in pediatric imaging develops guidelines to reduce radiation exposure to pediatric patients

What are the 3 guidelines listed in the 'image gently' campaign?

1. implement "child size" mA's and kVp doses 2. need only one acquisition phase 3. Image only the area that is needed

What is the advantage of using iterative reconstruction techniques over filtered back projection

reduces noise and therefore lowers patient exposure