Digital Radiography

Question 1

Advantages of Digital Radiography

Answer 1

wide latitude

ability to window image to exploit this dynamic range

ability to process and store the images on the computer

Question 2

the thin film transistor array (TFT) is common to both indirect and direct DR systems. True or False

Answer 2

True

Question 3

what does the detector element consist of?

Answer 3

TFT
charge collection electrode
charge collection capacitor

Question 4

what connects the TFT and the capacitor

Answer 4

the gate and drain lines connect the TFT and the capacitor

Question 5

the TFT is ____ during exposure, allowing the charge that is produced to be stored in the ____

Answer 5

the TFT is closed during exposure, allowing the charge that is produced to be stored in the capacitor

Question 6

when the X-Ray exposure is terminated, one gate line at a time is set to a ___ _____ which activates the ___ along that row

Answer 6

when the x-ray exposure is terminated, one gate line at a time is set to a high voltage which activates the TFTs along that row

Question 7

charge flows from the ____ to the transistors and down the ____ in parallel

Answer 7

charge flows from the capacitors to the transistors and down the drain lines in parallel

Question 8

what is the charge met with at the column of each matrix

Answer 8

output charge amplifiers

Question 9

what effect does deactivating the gate line have?

Answer 9

deactivating the gate line will reset the TFTs for the next exposure

Question 10

what is the fill factor

Answer 10

the percentage of the detector element that collects the charge from the energy deposited by the photons in the converter material
- basically the area of the detector that can take X-Rays

Question 11

name some dead areas of the detector

Answer 11

the TFT
the capacitor
the gate and drain connections

Question 12

what is the effect of a smaller detector element on fill factor and DQE

Answer 12

as detector element gets smaller, the fill factor and DQE gets smaller

Question 13

Typical range for a fill factor

Answer 13

0.5-0.8

Question 14

what is used as the detector material in direct conversion systems

Answer 14

Amorphous selenium

Question 15

in direct systems, where in relation to the TFT layer is the photoconductor

Answer 15

it is on top of the TFT

Question 16

what does the large voltage placed between the two electrodes (between selenium) have

Answer 16

this reduces the spread of ion pairs and prevents recombination

Question 17

Advantages of direct conversion systems

Answer 17

simpler TFT structure with no photodiode
high intrinsic spatial resolution due to high voltage ion spread
selenium plates are easy to manufacture

Question 18

disadvantages of direct conversion systems

Answer 18

charge trapping in thick selenium layer reducing absorption efficiency, increases signal retention and causes greater lag
can damage TFT due to over charging caused by high x-ray exposures

Question 19

difference between direct and indirect DR systems

Answer 19

indirect DR systems have the addition of an amorphous silicon photodiode and a scintillator made of CsI on top of the TFT

Question 20

what does the photodiode and scintillator do

Answer 20

converts x-ray photons to light photons proportional to the incident x-ray intensity

Question 21

what happens to the light photons in indirect DR

Answer 21

The light photons are converted to an electric charge by the amorphous silicon photodiode

Question 22

how is the light spread in the scintillator reduced

Answer 22

the light spread in the scintillator is reduced by structuring the scintillator CsI into crystal needles

Question 23

spatial resolution is determined by ___ in DR systems

Answer 23

pixel size

Question 24

what determines pixel size in DR systems

Answer 24

by the size of teh detector element

Question 25

DR vs CR

Answer 25

More rapid image production in DR, higher DQE, similar spatial resolution