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Medical Physics 2nd Semester > Radiography (Diagnostic Radiology) > Flashcards

Flashcards in Radiography (Diagnostic Radiology) Deck (27)
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1

Equation for magnification

SID / SOD

2

What color resembles greater optical density, white or black?

What about greater relative exposure?

Black for both

3

Equation for transmittance

T = I / I0

4

Equation relating optical density to transmittance

OD = -log10T

or

T = 10-OD

5

What does the reciprocity law state and when is it not true?

Relationship between exposure and OD should remain constant regardless of exposure rate.

 

Fails at long and short exposure times. OD is not constant

6

On the Hunter and Driffield curve, what represents the film contrast?

Steepness (average gradient) of the linear region.

Makes sense, for a change of exposure, there should be a greater change in OD

7

What regions of the H&D curve have poor contract (small slopes on the curve)

Toe and Shoulder region

8

What is the usuable range of the H&D curve? What is the range of x-ray exposures within this usuable range?

Usuable range: Area where exposure and OD are linearly related

 

Latitude gives the exposure range

9

Fast films require __________ exposure to achieve a given OD

 

(more or less)

Less exposure

10

Faster SF systems result in lower patient dose, but in general exhibit more ____________

Quantum noise

11

By how much does using film-sceen versus film only reduce the radiation dose to the patient?

By up to 50x!

 

Much more x-rays are detected. Resolution is degraded though

12

What is the conversion efficiency of a phosphor?

Fraction of absorbed energy emitted as UV or visible light

13

Difference between Conversion efficiency (CE) and absorprtion efficiency (QDE)

CE - ability of screen to convert energy deposited by absorbed x-rays into film darkening

 

QDE - how efficiency the screen detects the x-ray photons incident upon it

14

Main difference between direct and indirect digital film

Direct: signal comes straight from the beam x-rays

 

Indirect: there is an intermediate conversion step

15

Which of these are untrue about digital

Wider dynamic range than screen film

Reduces repeat rates

Higher patient throughput but lower dose efficiency

No consumables

16

How many bits are in a byte?

8

17

Equation for bit depth

2n 

where n is the number of bits

18

What is the difference between the H&D curve of digital detectors vs analog?

Digital holds the linearity much longer than analog

19

Remember, resolution is always defined by the bigger dimension.

Example, you have 1000 lp

for a 24 x 30 cm plate

resolution = 1000 lp/ 300 mm = 3.3 lp/mm

20

What is the process in which a laser scans a imaging plate and PSP processes data?

Computed Radiography

21

Which of the following is not a posisble cause of blurring at the borders of high contrast objects?

Large focal spot

Patient motion

High magnification

Trick question, they all are

22

What is the major concern of optical coupling?

Optical coupling: occurs from a gap between scintillator and focusing lens to detector

Large loss of light!

23

What is the fill factor of a thin-film transistor

Light sensitive area / detector element area

24

Which gives better absorption, thick or thin screen?

What about better resolution?

Better absorption - Thick screen

Better resolution - Thin screen

25

If 2 mGy of dose is delivered to the surface, approximately how much is delivered to center of chest? What about posterior end?

0.22 mGy to center

0.02 mGy to other side

26

What is the relationship between output kerma and typical x-ray tube voltages?

Roughly linear. Maybe slightly exponential

27

Dose at depth d in a patient given entrance skin dose, SSD and attenuation coefficient

Dd = ESD e-µd (SSD/(SSD+d))2