MEDRADSC 3K03 - Radiation Dose

Dose per xray exam

Approx 1mGy

Dose per CT exam

Approx 50 mGy

Dose per fluoroscopy exam

40 mGy

Exit dose in gen rad is typically _% of entrance dose

1%

Ideal radiation profile if scatter radiation did not exist

Radiation profile since scatter is always present

Standard phantom used to measure dose

CTDI phantom with TLD

The cylinder device is long enough to cover ___ contiguous ___mm slices

14, 7

CTDI phantoms are made of

Plexiglass or water equivalent plastic

Typical shape of CTDI phantoms

Cylindrical

Typical diameter of CTDI phantoms

16cm and 32cm

MSAD

Multiple scan avg dose

DLP

Dose length product

CTDI

CT dose index

CT dose descriptors

1. MSAD

2. DLP

3. CTDI

MSAD is a means to define ___

Scatter radiation

How is MSAD calculated?

Series of scan slices performed on phantom → measurements taken at centre of slice as well as several around periphery to account for scatter radiation coming from adjacent slice → each slice delivers bell shaped dose curve

What does this graph show?

MSAD

Tails add an additional __% dose to an entire study

25-40%

Amount of scatter is defined by

• Pt size/make up

• kVp

How is MSAD calculated?

Mathematically sampling the peaks and valleys of the multiple scan dose curve

MSAD formula

MSAD = CTDI x slice width / bed index or slice spacing

CTDI was developed by who and why

FDA, to inform users and allow for comparison btwn machines

CTDI represents

Mean absorbed dose in scanned object volume

How can you increase the area under the curve?

• Raising the height

• Widening curve

How can the height of the curve be increased?

Increase intensity of radiation

How can the curve be widened?

Increase pre pt collimation

What happens when slice width = bed index/slice spacing?

MSAD = CTDI

How can the requirements for CTDI_FDA be met?

Slices must be contiguous

How to CTDI_FDA be decreased?

Increase slice spacing

CTDI_FDA

Axial CT scanners

CTDI_W

Weighted CTDI

CTDI_W accounts for

Avg dose in XY direction instead of z direction

CTDI_VOLUME

For spiral/helical CT in Z direction

CTDI_VOLUME represents

Exposure per slice

How is CTDI_VOLUME and CTDI_W related?

CTDI_VOLUME = CTDI_W / pitch ratio

Pitch = 1

CTDI_V = CTDI_W

DLP

Total amount of dose for a series of scan

DLP formula

DLP = CTDI_VOLUME x Scan length

Why do manufacturers prefer CTDI_VOLUME?

DLP is specific to pt and does not allow for an accurate comparison of systems

How can we reduce pt dose?

• ALARA

• Challenging necessity of CT exams

• Reduce vol of anatomy being scanned

• Choose optimal technical factors

General strategies for peds pts

• Scan only when clinically indicated

• Customize CT scan

• Adjust techinical factors

• Shielding

General strategies for pts

• Automatic tube current modulation

• Pitch

• Limit thin slices

• Limit repeat scans

ODM stands for

Organ Dose Modulation

What is ODM?

Reduces tube current when xray tube is in the ant position relative to pt