BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods X-ray Instrumentation Part 2.

BME 560Medical Imaging: X-ray, CT, and

Nuclear Methods

X-ray Instrumentation Part 2

Today

• Anti-scatter devices

• X-ray screen-film systems

• Other methods of X-ray detection

X-ray System

Source

Restrictor (Collimator)

FilterSubject

Anti-scatter Detector

Produces X-rays from electrical energy

Determines size and shape of beam

Tailors X-ray spectrum

Selectively removes scattered photons

Converts X-rays to light and records

Total linear attenuation coefficient

Nx N0e x

Rayleigh photoelectric compton pair productin

Soft Tissue

At X-ray energies, most photons that interact in the patient are Compton-scattered.

X-ray Scatter

Object

Grid

Detector

Tube Scattered radiation comes into detector from all directions.

Result is a relatively uniform background “fog” that reduces dynamic range of the detector available to image true signal.

Would like some way to reduce scattered radiation without blocking much direct radiation.

Anti-scatter Strategies

• Collimation of the beam at the front end

• Air gaps

• Grids

• Scanning Slits

Air Gap

Moving the patient away from the detector reduces the scatter reaching the detector.

Square-law

Solid angle

What price do we pay for this?

Anti-scatter Grids

Construct a device to collimate photons after they leave the patient.

Thin lead strips must be precisely aligned.

Performance depends on the grid ratio

What is the price paid for a high grid ratio?

Typical grid ratios: 5:1 to 16:1 (lower for mammography)

Anti-scatter Grids

• A stationary grid will leave line artifacts in the image.

• A Potter-Bucky diaphragm is a movable grid that basically blurs the grid lines during exposure.

• The grid also blocks some primary radiation in the system.

Anti-scatter Grids

Tradeoff between scatter penetrating the grid and primary radiation detected

Anti-scatter Grids

• Thickness of strips determines the likelihood of penetration.– Less scatter penetration =

less primary radiation

• Low-angle scatter may still get through.

• Multiply-scattered photons may get through.

Anti-scatter Grids

• Additional exposure is needed to maintain same detector exposure level when using grid.

• Grid Conversion Factor

• Typically 3 < GCF < 8

• May avoid grid for small body parts or low energies.

mAs with grid for exposure E

mAs without grid for exposure EGCF

Scanning SlitsMoving source and collimator

Moving slit

Stationary detector

Move source, collimator, and slit together.

Only takes one part of image at a time.

Very high scatter reduction

Slow

X-ray Detectors

• Film-Screen

• Image Intensifiers

• Panel Detectors

Film-Screen Detectors

• Roentgen’s first X-rays exposed a photographic plate directly.– But photographic film has very low stopping

power (a couple of percent).– To expose the film to its full dynamic range

(contrast) would require high dose and most would be wasted.

• Augment this with an intensifying screen that converts X-ray photons to visible light.

Screen-film System

• Double emulsion film sandwiched between pair of intensifying screens

• Phosphor particles (high Z) covert X-ray into light photons

• Screen enhances contrast but lowers resolution

• Engineering tradeoff: Phosphor thickness

Film

CoatingPhosphor

Reflective Layer

Base

Screen-film System

• Reflective layer reflects light back into the film

• Base for mechanical support• Phosphor layer material choice:

– More fluorescent than phosphorescent

– High linear attenuation coefficient = stopping power

• Conversion efficiency: total light energy per unit incident X-ray energy (usually 5 – 20%)– Energy dependent

Film

CoatingPhosphor

Reflective Layer

Base

Film

• Very similar to photographic film; must be developed to fix the image

• Two components:– Base: Plastic sheet, dimensionally stable (size and

shape do not change under environmental and processing conditions)

– Emulsion: Crystals of silver bromide suspended in gelatin substance; on one side (single-emulsion), or both sides (double-emulsion) of base.

• Image is formed in the silver bromide crystals.

Screen-film System

• The screen-film combination usually has a speed quoted– More sensitive (= fewer X-

ray photons to result in a given image density) pairs have higher speed

– At a particular energy!

SpeedSensitivity

(mR)

1200 0.1

800 0.16

400 0.32

200 0.64

100 1.28

50 2.56

25 5.0

12 10.0

Exposure to get a standard level of film density

From Sprawls

Radiographic Cassette

• Ensures firm and uniform contact between intensifying screens and film sandwiched in between

• Optical mirrors located outside screens to direct light towards film, maximize light conversion efficiency

• Contains ID card and loaded only one way into X-ray machine

Image source: The Essential Physics of Medical Imaging

Film Density

• Density describes the overall blackness of the radiograph

Image source: http://www.nurseslearning.com/courses/fice/fde0030/Imaging_terms.htm

Screen-film System

Thicker screens result in higher sensitivity but increase image blur

Other Detection Schemes

• Detection is a result of radiation interaction with matter. Radiation interaction results in emission of by products, e.g. electrons, electromagnetic radiation, that can be sensed by instrumentation and recorded by data acquisition systems– Gas-Filled Detectors

– Scintillation Detectors

– Flat-panel detectors

– PSP plates

– Solid State Detectors

Gas Filled Detectors

• Radiation ionizes the gas. Charges freed by ionization produce a current.

Positive Power Supply

Radiation Ammeter Negative Resistance R Gas Chamber

Gas Filled Detectors• Radiation interacts with

gas and ionizes its atoms• Freed electrons interact

with gas and ionize more atoms - amplification

– Ionization chamber: No amplification

– Proportional counter: Amplification up to 106

times– Geiger-Muller counter:

Very strong avalanche

Ionization Proportional Geiger Mueller Chambers Counters Counters

Collected Charge

Voltage

Spatial sensitivity is lacking – Not used for imaging

Scintillation Detectors

• Interaction of X-rays with some materials (CsI, cadmium zinc telluride - CZT) produces ‘scintillation’ or “flash of light”.

scintillator

photomulitplierX-ray

Visible light Electrical pulse

The pulse can tell you about the energy of the incident photon.

Not capable of handling high photon flux.

Photomultiplier Tube

• Photomultiplier tube (PMT) converts light into electric current by photoelectric effect

Dynodes Photocathode grid Anode photons

Flat-panel Detectors

Varian Medical Systems

Scintillator

Light coupling

Light-sensitive digital detector (CCD array)

Photostimulable Phosphor Plates

• PSP plates

• X-rays excite electrons which are trapped in the material lattice.

• Later, the plate is scanned by a laser in a “plate reader” which frees the electrons locally and digitizes the image.

• The plate can be reused.

• Plugs in to the film-screen cassette slot.

Solid State Detectors

• They are compact semiconductors. Electrical conductivity of semiconductor is sensitive to impurities. The depletion layer is sensitive to radiation and electric current flow through, thus the measured current is a measure of radiation.

+ _ n-type depletion layer p-type - Radiation or incident particles

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

X-ray Image Detection

• Screen-film: Still in use• PSP Plates: Displacing screen-film in many

applications• Flat-panel: Increasing use but expensive• Solid state: Still in development for X-ray• Scintillation detectors: Not fast enough for X-ray

imaging, but still important research tools.– SPECT imaging

• Gas counter: Not useful for imaging but used for active measurement of patient exposure.