Fluoroscopy in Specials: Flat Detector vs. Image

Intensification

William R. Geisler M.S. DABRDepartment of Radiology/Medical Physics

Fluoroscopy in Specials: Flat Detector vs. Image

Intensification• Acknowledgements:

– Perry Juel, SIEMENS– Dean Rindlisbach, PHILIPS– John Rowlands, Univ. Toronto– Phil Rauch, Henry Ford Health System

Fluoroscopy in Specials: Digital Flat Detector vs. Image

Intensification• Differences and Similarities with image

intensification (II) and flat detector (FD)– Brief review II technology– Overview FD technology

• Artifacts associated with II and FD

Comparison of Image Intensifier and Digital Flat Panel Detector

Review II Tube• Vacuum tube• Curvature at input window

required to allow focusing of e- and minimize stress from vacuum

• Thin (~1 mm) layer of Aluminum at input housing– Attenuates x-rays striking

input phosphor• Output phosphor coupled

to camera or CCD

CsI Input Phosphor/ZnCdS Output PhosphorX-ray Visible Photon e- Visible Photon

• X-ray exits patient and interacts with CsI (200 - 400 μm thick for II)

• X-ray converted to visible light• light travels along CsI and

interacts with photocathode• photocathode emits e-

• 25 – 35 kV accelerates e-s to output phosphor (ZnCdS, < 10 μm) to increase light output

Bushberg 2nd ed fig. 9-3

Bushberg 2nd ed fig. 9-6

II Gain Factors• Flux Gain (FG)

– Increase no. light photons emitted from output phosphor compared to input phosphor; 50 - 100 typical

• Minification Gain (MG)– (dia. Input/dia. Output)2

– e.g., 23 cm to 2.5 cm => 85• Brightness Gain

– = FG × MG– Typical overall gain 5,000:1– BIG advantage of II over FD is

brightness gain factor– FD uses pixel binning and

image frame averaging to reduce noise

Carlton, Adler 4th ed fig 40-2

Flat Detector Design• FD also uses CsI to convert x-rays to

visible light– Use of scintillator only similarity of FD

to II– CsI used due to Si detector being far more

sensitive to visible light than to x-rays– FD does not have e- acceleration or

minification gain to reduce noise• However can combine adjacent pixels

(called pixel binning) to reduce noise• Binning also permits fast frame rate

imaging– Additionally FD does not have Al housing

so less attenuation of x-rays vs II– Additionally, CsI thicker for FD compared

to II (more x-rays absorbed for FD vs II)• Light sensor (photodiode) behind CsI

adjusts technique

(Cesium Iodide)

Flat Detector Design

FD Reading Image Info• Image “stored” in pixels on

detector as electrical charge• Each line of data read

sequentially• Detector cannot acquire

another image until all data is read

• Time needed to read data part of reason for need to bin pixels for fast frame rate– Combine two rows of pixels

into one and read as single line of data

– Binning pixels also results in less noise

• HOWEVER, not all of the information/charge is removed during read process– If not removed will see “ghost”

image– Bright light removes residual

charge (every 30 msec for Philips)

FD Data Collection

For higher frame rate imaging require combining adjacent pixels; less information required to be read by electronics

II Magnification• II magnification done by moving out

focal point of e-s inside II– Now central region of e-s strike output

phosphor and outer region e-s miss output phosphor

– Results in larger image and increased resolution

– Fewer e-s striking output also results in dimmer image

– Increase technique required to maintain same amount of light output

– That is, technique increased to keep magnification noise level same as unmagnified noise level

– Technique increases with area (diameter2)

FD magnification• FD magnification done one

of two ways, depending on design of unit1. Electronically zoom image;

no actual increase in resolution

– mag imaging will still be binned (if binning available)

– No true increase in resolution, image displayed larger electronically, similar to enlarging digital picture

2. Unbin pixels and have true increased resolution

Resolution Comparison II vs FD

3.415 cm3.113 cm3.420 cm2.817 cm

3.425 cm (diagonal)

2.223 cmLP/mmModeLP/mmMode

Allura FD 10 (25 cm)Allura 9 (23 cm II)

***Philips Allura FD 10 does not bin pixels***

Effect of Pixel Binning

No binning 4x4 binning

This is a portion of a Mars Observer Camera image of gullies cut into a crater wall. The portion shown is about 1 km across. The left hand image has not been binned. The right hand image has been binned 4 x 4. Note that binning reduces the spatial resolution and the finest details can no longer been seen.

Specifications Philips FD10, FD20 and Siemens dBA

Philips FD10/20• FD10

– 1024 x 1024– No binning of pixels– Mag modes uses fewer pixels

but no change in resolution• FD20

– 2480 x 1920, 48 cm in diagonal direction

– Bins pixels until max mag– Max mag provides highest

resolution (= 22 cm FOV)– 0.5 – 6 f/sec 2K x 2K– 15 and 30 fps 1Kx1K

Siemens Axiom Artis dBA• 2480 x 1920, 48 cm in

diagonal direction• Bins pixels until max mag

(zoom 3 = 22 cm FOV)– Overview, zoom 1, zoom 2

0.5 – 7.5 f/sec 2K x 2K– All Zoom Modes: 0.5-7.5 f/sec – 10/15/30 f/sec with High Speed

Acquisition in 1024 x 1024 matrix

Magnification and Dose• For image intensifier dose increases with area (or

diameter2)• For digital flat panel, how magnification affects dose

slightly more complicated– If magnification unbins pixels then dose increases to maintain

same S/N per pixel– If magnification is actually electronic mag with no actual increase

in resolution, dose will increase—based on linear distance (diagonal), not area [not all vendors may follow this convention]

– Why? Both unmagnified and magnified image will have same S/N but apparent noise (noise/mm2 as viewed on monitor) will increase

Electronic Magnification and Dose

Identical images, second image electronically magnified. Both images have same SNR (largest disc SNR = 10) but different apparent noise characteristics

DQE• For radiographic applications the DQE of digital flat panels is typically

greater than for screen-film• For fluoroscopic applications DQE is more difficult to compare

– Pixels have varying sensitivity; radiographic FP will adjust for pixel response

– With fluoroscopic imaging ability to correct for varying pixel sensitivity depends on frame rate

• Low frame rate FD DQE > II DQE• High frame rate FD DQE ? II DQE

– At high frame rates it is possible that for FD the DQE per pixel is lower—however since high frame rate required binning pixels, this would off-set decreased DQE

• Additionally, DF rooms typically use greater filtration at x-ray tube as compared to II rooms, resulting in the possibility of greater radiation exposure reaching detector with concurrent reduction in patient dose

Dynamic Range

• Dynamic range of flat panel is greater than an image intensifier

• Issues such as burnout (blooming) and blackout (saturation into black) regions in image is not as significant an issue with FD as it is with II

Images taken from Jack Cusma AAPM 2006

Image Artifacts for II and FD

• II image distortion (pincushion, vignetting and S distortion) caused by– curvature of II surface– e-s repel each other– stray magnetic fields affecting

e- path• FD does not suffer from

these types of artifacts• Does this mean FD images

are artifact free?

• No. FD also has inherent artifacts

FD Artifacts

• All large area digital detectors have defects that results in artifacts– Raw image unsuitable

for clinical display

• Multiple causes– Different pixels have

different sensitivities to radiation

1 mR

FD Artifacts

• Can correct for different pixel sensitivities– Referred to as pixel gain– Expose FD to uniform

radiation and measure response of each pixel

– Correct pixel sensitivity during patient imaging

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275200

FD Image Artifacts Correction• All large area digital detectors

suffer from artifacts– Pixels can have different

sensitivity to radiation (gain factors)

– Pixels can be unresponsive– Data line drop off– Clusters of adjacent pixels– ….

• These artifacts can be corrected by software

• Illustration of artifacts and corrections based on 1st

generation experimental FD

FD Image Artifacts Correction

FD Image Artifacts Correction

FD Image Artifacts Correction