Lecture 7 Other directions in X-ray images, current research.

Lecture 7

Other directions in X-ray images, current research.

Image series demonstrates high-energy image (left), low-energy image (center), and soft-tissue image (right) produced by processing both images. Appears to do a better job than one energy technique.

Recent research on dual-energy digital radiography imagingAim - good images without large exposure ( << computer tomography) and done in generic not CT facilities. In dual-energy imaging, two exposures are collected with the x-ray system's tube set to different energy levels; the resulting images highlight either bone or soft tissue, or they can be merged into a composite image. A filtration wheel on the x-ray tube enabled to easily select different kVp (maximal potential between anode and cathode), energy levels and attenuation. The researchers also adapted the system with a flat-panel DR detector. A cardiac trigger enabled to coordinate the acquisition times of the two dual-energy studies (few seconds apart) so they occurred during the same period of the cardiac cycle. Typical settings: low-energy V=60 kV with 2.5-mm aluminum total filtration, while the standard high-energy setting was V=120 kV with 4.5-mm aluminum + 0.6-mm silver total filtration.

Lower x-ray levels are produced continuously and many images must be presented almost immediately

GI (gastroenterological ) tract

Angiography

Real time imaging or fluoroscopy

Barium GI fluroscopy

barium strongly absorbs x-raysdark image on the screen.

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Few other examples of using digital techniques.

High pass Filter

Suppose we were looking for a small coronary artery passing over a large contrast filled ventricle or over the top of the diaphragm. We can use the fact that the spatial variation of the signal from the artery is much stronger than the larger objects. Mathematical technique is based the Fourier analysis which allows to enhance high spatial frequencies - so called high pass filter.

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The effect if a high pass filter on a canine coronary angiogram is used

Original High Passed

Dual Energy High Passed Dual Energy

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Computed radiography lends itself well to applications in which quantitative computations are required. One example is dual energy chest radiography where correction for scattered photons and for increase of the average photon energy (beam hardening) must be done as well image combination and noise reduction computations.

An example of such an application is shown schematically on the next slide where a cassette consisting of four phosphor plates is used to acquire, in a single exposure, low and high energy images from the front and back plates, respectively, with the two intermediate plates acting as a filter to separate the energy spectra.

A gadolinium filter is used to provide some initial separation of the incident spectrum into a low and high energy portions which are further filtered by the phosphor plates.

•

Phosphor plates

Gadolinium filter

Inputspectrum

Absorbed inFront plate

Absorbed in Rear plate

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Images obtained with this detector system are illustrated below (F. Zink, Ph. D Thesis, U. Of Wisconsin) which shows a low energy image (A) and the tissue (B) and bone (C) images derived from the front and back plate images following several processing steps incorporating the corrections mentioned above.

Conventional Tissue Bone

It would be better to have sources with monochromatic beams of variable

energies.8

After W. C. Roentgen has discovered his X-rays the benefit for medical diagnostics has been perceived very early. Since then a lot of improvements have been achieved, but modern radiographic systems use still the same principle: the energy of the photons hitting a picture element are integrated over the exposure time. This means that the information about the individual photon energy is lost.

In medicine it is important to get as much information as possible by the dose being applied. A so-called quantum imaging system is capable of discerning and processing each single X-ray quantum and is therefore very attractive for medical application. A first step in this direction are photon counting detectors, where each pixel determines the number of the incident photons.

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Recent developments have lead to a hybrid pixel detector concept which is a very promising approach for quantum imaging. These detectors are sandwich structures consisting of two flip chipped layers - an X-ray absorbing semiconductor and a read out electronic chip. Both layers are connected by bump bonding technology. The Medipix2 detector is a very powerful realization of such a photon counting system. The Medipix2 has an active area of 1.4 by 1.4 cm2 consisting of 65000 parallel working photon counting pixel cells with a size of 50 μm. The Medipix2 readout chip can be combined with different absorbing materials like Silicon, Gallium Arsenide or Cadmium Telluride. The challenge in hybrid pixel detectors for medical application is the realization of large area detectors. But there are already proposals on how to overcome the smallness of a single detector. Photon counting detectors like Medipix2 might be just the beginning of a new exciting evolution, the aim could be a spectroscopic pixel detector which is capable of measuring the energy of each single photon. Monte Carlo simulations show that such a detector would lead to a big benefit in X-ray imaging in future.

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The Medipix2 CMOS ASIC is the successor of the

Medipix1 (or PCC) photon counting chip. It benefits from the quick progress of

CMOS (Complementary metal–oxide–semiconductor) technology which

allows enhanced functionality of the pixel cell at the same time as providing a

significant reduction in pixel size.

• The square pixel size of 55 µm side length overcomes one of the limitations

of the Medipix1 chip and makes the Medipix2 chip competitive with most of the

existing imaging devices in terms of spatial resolution.

• Direct X-ray conversion in a semiconductor sensor minimizes image

blurring and avoids an extra conversion stage from X-rays into visible light.

• The chip is designed to accept either positive or negative charge input

in order not to restrict the choice of the sensor material (Si, GaAs, CdZnTe,...).

Detector leakage current gets compensated pixelwise at the input.

• With the Medipix2 chip it is possible to select a window in energy. Upper

and lower threshold can be adjusted pixelwise with 3 bits for uniform performance

of the whole pixel matrix and will open new measurement perspectives. The

photon counting principle in contrast to systems based on charge integration

suppresses noise and leads to superior SNR properties.11

http://medipix.web.cern.ch/MEDIPIX/Medipix1/medipix1.html

• Exposure times can be chosen arbitrarily. Data is accumulated in a 13-bit counter per pixel. Each pixel can handle count rates of about 1 MHz. Read-out is performed after exposure to avoid dead time.•Parallel and serial read-out will be realized.•The Medipix2 chip has an active area of about 2 cm². 256 x 256 pixels form the pixel matrix.•Larger area coverage is a big concern of the collaboration. Medipix2 will therefore be 3-side bootable.

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The medipix chip

The Medipix All Resolution System (MARS) scanner – a type of CT scanner – makes use of technologies first developed for particle physics research at CERN. The imaging chip behind the scanner – the Medipix chip – allows x-rays to be seen in color, as opposed to the more usual black and white.

http://wiki.canterbury.ac.nz/display/MARSCT/Home

X-ray image of froglegs. This image was taken using a Mo X-ray tube + 30 μm Mo filter.

MARS Micro-CT Scanner With its unique Medipix detectors, the state-of-the-art true-color micro-CT scanner –Medipix All Resolution System (MARS)– has been designed and produced

by MARS Bioimaging Ltd, in collaboration with the Christchurch School of Medicine and Health Sciences at the University of Otago and the University of Canterbury in New Zealand. The

gantry of this system allows a full-scan with an x-ray source (SourceRay – SB80-1k 80kV) and spectral camera assembly under precise control around a specimen or animal up to 100 mm

in diameter and 300 mm in length. The customized camera has a MARS gigabit readout accommodating up to six Medipix2 or Medipix3 detectors.The MARS-CT imaging technology is

based on counting the number of photons at multiple narrow energy bands. This is often referred to as spectral or spectroscopic imaging. The Medipix chip is made up of a specially fabricated CMOS semiconductor chip with a detector material bonded to it. Each chip has 65,536 pixels. Every pixel accurately counts the number of photons that reach it within

specified energy bands. Black and white CT cannot distinguish different density tissue as they absorb similar amounts of photons. The MARS-CT advances this as body tissue of similar

density interacts differently with certain energies. The Medipix chip can detect these different interactions. In addition the energy of a photon can change based on interactions with the

electron shells of the atoms in the body tissues it encounters. These effects mean that information about both the density and the atomic make-up of a tissue can be imaged and

can be assigned to colors on an image.

The first product released by MARS is the MARS Micro-CT scanner for use in biomedical research. This is a small, self-standing machine consisting of an x-ray source and camera system in a fully shielded shell, together with computer hardware, electronics and customized image reconstruction and viewing software.

The detection systemThe detection system

Detctor characteristics

Si or GaAs

pixel: 170 x 170 μ m 2

64 64 x pixels

1.18 area cm 2

Photon Counting ( )Chip PCC

CERN EP Microelectronics Group

MEDIPIX Collaboration :

,CERN ’Universita , ed INFN Pisa

Napoli e Cagliari

’Universita di Glasgow e Friburgo

charcteristics :

1 SACMOS mm technology

: 170 170 pixel x μ m 2

64 64 x channels

1.7 area cm 2

threshold adjust 3 - bit

15 - bit counter

detector

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In blue is the spectrum observed by a 55 mm square silicon pixel detector which is uniformly exposed to 10 keV photons. In red is the spectrum seen by a pixel operating in charge summing mode where the output of 4 pixels are added. Other material and energies have also been simulated.

Medipix 3

SCINTILLATING GEMS

Inelastic processes that lead to the avalanche growth in proportional counters result also in copious emission of photons (fluorescence). This light can be exploited for detection. Devices incorporating one or more GEM and optimized for their light yield are being developed by several groups. On irradiation with an ionizing source (charged or neutral) electrons released in the sensitive volume are drifted into the multiplier(s). The flashes of light emitted in the avalanches are detected with a low noise solid state camera. Because of the linearity of response up to very high fluxes, and of the simplicity of the read-out hardware, imaging GEM detectors can find applications in high-rate medical diagnostics (portal imaging)

Development at LIP-Coimbra:

F.A.F. Fraga, L.M.S. Margato, S.T.G. Fetal, R. Ferreira Marques and A.J.P.L. Policarpo, IEEE Nucl. Sc. Symposium Lyon 2000

Schematics of the scintillating GEM detector:19

9 keV absorption radiography using GEM

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“My very modest contribution to physics has been in the art of weaving in space thin wire detecting the whisper of nearby flying charged particles produced in high-energy nuclear collisions. It is easy for computers to transform these whispers into a symphony understandable to physicists.

But the whispers can also be produced by radiations widely used in biology or in medicine,such aselectrons from radioactive elements or X-rays. In this last case it is possible to reduce,by a large factor, the doses of radiations inflicted on the patients.”

Georges Charpak, Banquet speech, Nobel Academy (1992)

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Innovative flat emitter X-ray source can help to identify small vessels up to 70 percent better than conventional X-ray tube technology. A new crystalline silicon detector allows more effective amplification of the signal and allows to use ultra-low doses of radiation.

Very recent new product: Simens Artis Q angiographic system

Dosimetry Units

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Digression: Units used in radiology.

Before going further here are a few definitions.

X -Ray fluence = # X-Rays per unit area = N

Energy fluence (intensity)

I

X-Ray flux (fluence rate) = dΝ / dt (t= time)

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X-ray dosimetry IX-ray dosimetry IQuickTime™ and aGraphics decompressorare needed to see this picture.

X-ray exposure X is quantified by measuring the number of free chargecarriers (positive ions + electrons) generated in air at standard conditions.This leads to the traditional unit for exposure, the Roentgen [R]

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The radiation absorbed dose D [ Rad ] is defined as

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Energy it takes to produce one ion pair in air: 33.97 eV (= 33.97 J/C)

⇒ 1 :energy absorbed in air for R

™ QuickTime and a Graphics decompressor .are needed to see this picture

:Conversion from exposure to dose

1 = 2.58 R × 10

-4 /C kg

2.58 × 10-4 / C kg× 33.97 / J C = 0.00876 / J kg = 87.6 / ergs g= 0.876 Rad

1 Rad = 100 / = 0.001 / = 0.001 [erg g J kg GrayGy]

Dair [Rad] = 0.876 × X [ ]R SI units

1 R ~ 2 * 1010

x-rays / cm2 at 35 keV

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The number of x-rays per R depends on energy is given by

# x-rays / cm2 per R ~ 1 / µk

where µk is the attenuation coefficient associated with energy deposition in the

patient. This coefficient, which we discussed before, is large at low energies and decreases with energy. The net effect is that the number of x-rays required per Roentgen of exposure increase with energy because of the smaller probability of

interaction ( smaller µk ) and then decreases again as the amount of energy

deposited per x-ray increases and becomes the dominant term in the denominator.This is because electrons produced in the Compton scattering carry significant fraction of the photon energy and they stop in the media close to the point where photon interacted leading to local ionization. THIS IS COUNTED IN THE DEFINITION.

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Plot of dependence of number of photons per Roengen

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Biological effects of ionizing radiationBiological effects of ionizing radiationQuickTime™ and aGraphics decompressorare needed to see this picture.

Damage depends on deposited (= absorbed) energy (intensity × ) time per tissue volume


: , Threshold No minimum level is known below which damage occurs


: , Exposure time Because of recovery a given dose is less harmful if divided


: Exposed area The larger the exposed area the greater the damage

( , !)collimators shields


/ : 50/30 ( 50% Variation in Species Individuals LD lethal for of a population

30 , ~450 over days humans

rads / )whole body irradiation


: Variation in cell sensitivity Most sensitive are nonspecialized , rapidly dividing

( : , , . cells Most sensitive White blood cells red blood cells epithelial cells Less

: , )sensitive Muscle nerve cells


/ : (> 100 Short long term effects Short term effects for unusually large

rad )

( , , , , ); doses nausea vomiting fever shock death long term effects

( / ) carcinogenic genetic effects even for diagnostic levels ⇒ maximum

5 / 0.2 / [ . allowable dose R yr and R working day Nat Counc . on Rad . . Prot and

.]Meas

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Chest film 20 mR

Dental 650 mR

Mammogram ≈ 1R surface exposure per view 2 views/breast (300 mrads

CT 2R

Background ( excluding Radon ) 125 mR / yr

Some Typical Diagnostic Exposures

average glandular dose)

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It took a heck of a lot of x-rays but we finally discovered what is wrong with you. You are suffering from

excessive exposure to radiation.

Lecture 7 Other directions in X-ray images, current research.

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Transcript of Lecture 7 Other directions in X-ray images, current research.