16- Digital X-Ray Imaging

Front 1

What are the 4 components of image quality

Back 1

spatial resolution
signal
noise
contrast

Measure of flow and spatial resolution

Front 2

The sensitivity ("speed") of a film is based on what inherent feature of the silver halide crystal?

Is a high speed (more, less) grainy

Back 2

size. The larger the crystal, the less light is required to produce the same opitical density therefore is more "sensitive"
more

Front 3

What two "particles" can activate a silver halide crystal?

Which of the two is more effective?

Back 3

photon (light)
xray

xray

Front 4

What is the role of a phosphor

A thicker phosphor layer (increases, decreases) the probability of xray interaction?
The thicker the phosphor layer, the (less, more) lateral dispersion of photons?

Back 4

convert incident xrays into visible light photons
increases
more

Front 5

(Film, digital) detector have a narrower exposure curve and are therefor more prone to exposure errors

Back 5

Film

Front 6

What are discreet sampling and intensity resolution?

Back 6

spatial resolution in terms of pixel pitch (spacing between pixel element centers) ie pixels/mm

bit depth, the maximum number of grayscale pixel values to describe exposure

Front 7

How does indirect digital imaging work?

Back 7

xrays activate a phosphor or scintillator which in turn emits visible light which is digitally measured and stored on a sensor. Image readout/display occurs as a separate process.

Front 8

How does CR work?

Back 8

An xray stimulates a storage phosphor. This excites the phosphor atom to a metastable state which can exist for several days. The excited phosphor will emit blue green light if it interacts with a visible red light photon. This occurs in the reader and the emitted blue green light will be digitally measured and stored.

Front 9

why is CR considered a digital technique?

Back 9

because it produces digital data prior to image display

Front 10

How does a flatpanel scintillator-CCD work?

Back 10

incident xrays strike a sintillator and the emitted light is "focused" by fiberoptic filaments onto a charge coupled device which is usually much smaller than the field of view.

this requires a demagnification step and results in loss of image quality

Front 11

How does a flatpanel scintillator-TFT device work?

Back 11

The TFT array is a 2D charge measuring device. Visible light induced charge separation occurs in a photodiode layer formed in the alpha Si above the TFT array

Front 12

Fundamentally, indirect imaging is based on converting xrays to __________________

Back 12

visible light

Front 13

What's the difference between direct and indirect aSe-TFT detectors?

which is more efficient, produces less scatter and can therefore reduce patient dose?

Back 13

indirect will store/measure charge based on emitted light
direct will store/measure charge based on incident xrays

DIrect sensors are more efficient

Front 14

How does a photon counting detector work

Back 14

Linear array of detector cells which continuously measure charge separation from absorbed xrays which produce a voltage spike (photon counting) relative to background noise.

this improves efficiency, minimizes scatter (1D setup) and sig reduces dose.

"micro-dose mammography"

Front 15

What is more efficient? Storage phosphors or scintillators?

Back 15

Scintillators

Front 16

Light dispersion (increases, decreases) with phosphor/scintillator thickness and results in (gain, loss) of spatial resolution)

Absorption Efficiency is directly/indirectly proportional to phosphor/scintillator thickness?

Back 16

increases, loss

Directly

Therefor: inverse relationship between absorption efficiency and spatial resolution

Front 17

What are two ways to overcome lateral dispersion?

Back 17

phosphor/scintillator sandwhich
"/" columation

Front 18

Charge separation is an important step in:

Direct
Indirect
both

Back 18

both

Front 19

What type of detector performs continuous measurement of charge separation?

Back 19

photon counter. All others provide a summed charge during the entire exposure

Front 20

Fill factor in TFT arrays are an issue in direct/indirect systems

Back 20

indirect: a large portion (up to 60%) is occupied by a transistor which does not detect xrays and therefor results in significant loss of detection efficiency.

Direct systems have curving electric field lines which eliminate this problem

Front 21

photon counting detectors have least noise and offer potential for mltiple energy imaging in a single exposure

Back 21

...

Front 22

What is MTF?

Back 22

Modulation transfer function: a measure of loss of contrast as a function of spatial resolution

ie. spatial resolution performance

Front 23

What is DQE?

Back 23

Detective Quantum Efficiency: Not yet standardized was of assessing detector performance with respect to noise AND spatial resolution

Front 24

All digital imaging methods are based on measuring charge separation.

whether it requires generation of visible light determines if it is direct or indirect

Back 24

...

Front 25

Approximately 30% of absorbed xray energy is stored in the metastable form of a storage phosphor

Back 25

...

Front 26

Visible light dispersion is the greatest source of spatial resolution loss

Back 26

...

Front 27

Amorphous selenium is able to produce charge seaparation as a result of xray absorption

Back 27

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