MIPR Lecture 6 Copyright Oleh Tretiak, 2004 1 Medical Imaging and Pattern Recognition Lecture 6...

MIPR Lecture 6Copyright Oleh Tretiak, 2004

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Medical Imaging and Pattern Recognition

Lecture 6 X-ray ImagingOleh Tretiak


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Wilhelm Conrad Roentgen

• Roentgen discovered penetrating radiation on 8 November 1895.

• The famous radiograph made by Roentgen on 22 December 1895, and sent to physicist Franz Exner in Vienna. This is traditionally known as "the first X-ray picture" and "the radiograph of Mrs. Roentgen's hand. "

• Roentgen received the first Nobel prize in physics in 1901

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.


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X-rays at Present• Superior definition• Clear images of bones• Some indication of

tissue• No tissue detail

(tendon, muscle, skin)• Negative image: bone

is white, air is black


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Talk Outline

• Examples of X-ray imaging procedures

• Physics: X-ray attenuation, transmission, and contrast

• X-ray recording systems• Summary and new developments


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Chest X-ray

• Clear images of bone– ribs, vertebra,

clavicles

• Soft tissue: shoulder muscles, hart, abdomen

• Pattern of passages in lungs


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Abdominal X-ray

• Visible: Bony structures– Vertebra, pelvic bones, legs,

ribs

• Soft tissues– liver, stomach, leg muscles

• Confusing image of intestines– Intestinal gas, walls

• Cannot see:– Details of liver, back muscles,

kidneys


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Abdomen - more

• Abdomen after Barium contrast enema

• Large intestine easily visible


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Another Abdomen

• Contrast medium in aorta (angiography)

• Visible: – descending aorta, – renal arteries, – iliac arteries


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Pelvic X-Ray

• Can see– Fracture in

pelvis– Femur

• Cannot see– Soft tissues


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Skull

• Can see bones, scalp

• Cannot see ventricles, blood vessels


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Skull: Subtraction Angiography


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Summary

• X-ray imaging is a successful modality

• Limitations: Cannot distinguish among soft tissues

• Limitations can be overcome under some conditions with contrast media


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X-Ray

• Schematic of x-ray imaging


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What are X-rays?• X-rays (Roentgen rays) are electromagnetic, like

radio waves and light• There are three ways to measure the “quality” of

electromagnetic waves– Wavelength– Frequency– Photon energy

• f - frequency, Hertz (Hz)• - wavelength, meters (m)• E - photon energy, electron volts (Ev)• c - speed of light, 3x1010 m/sec• h - Planck’s constant, 4.1x10-15 Ev/Hz

€

=c / fE = hf


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Examples

FrequencyWavelengt

hPhoton Energy

Radio1e6 Hz

1 Mega Hz300 m

4e-9 Ev4 nanoV

Green light 5.45e14 Hz0.55e-6 m0.55 m

2.2 Ev

X-ray 7.3e18 Hz 4.1e-11 m3e4 Ev30 kEv


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Generation of X-rays

• X-rays are generated when electron hit a target

Cathode(electron source)

Anode(X-ray source)


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X-ray Spectrum

• An X-ray tube produces a broad spectrum of energies


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X-ray Attenuation

• For medical imaging, we can assume that X-rays travel along straight lines (rays).

• In the presence of matter, X-rays are removed from from a beam. This process is called attenuation.

• For homogeneous material and X-rays, attenuation

follows an exponential law. €

€

I1 = I0e−μtI I

- linear attenuation coefficient, in cm-1


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Attenuation Coefficient Values

• Tables of X-ray attenuation and absorption coefficients can be found on the web - for example, http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html

http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html

http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html


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KEVFat, ρ = 0.916

, Muscle ρ = 1.04

, Bone ρ = 1.65

10 2.764 5.339 8.81015 0.924 1.668 2.75220 0.488 0.809 1.33530 0.271 0.380 0.62740 0.216 0.274 0.45250 0.193 0.233 0.38460 0.180 0.212 0.34980 0.164 0.189 0.312

100 0.154 0.176 0.290150 0.137 0.155 0.256200 0.125 0.141 0.233

Linear Attenuation

, [ ]Coefficient cm -1

Attenuation coefficient

0.100

1.000

10.000

10 100

KEV

µ (cm-1)

Fat, r = 0.916

Muscle, r = 1.04

Bone, r = 1.65


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Examples

• Conclusion: High voltage photons are needed to penetrate thick objects.

20 kev 0.65 9.07E-08 0.2750 kev 0.21 4.90E-03 0.65

Abdomen t = 25 cm

Hand (palm) t = 2 cm

Values of transmission,T = exp(-t)


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Contrast

t

t

m

m

z

( )I z

I(z)

z

I I

€

C =I0 − I1I0

If |(0–1)t1| is small,

€

C ≈ (μ0 −μ1)t1


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Contrast and Photon Energy

• Contrast is increased if the difference in attenuation coefficients between tissues is larger

• At 20 kev, muscle - fat = 0.320

• At 50 kev, muscle – fat = 0.040• To increase contrast, use lower

voltage!


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Recording X-rays

• Direct film recording (like Roentgen)– Very low efficiency: film is thin, most X-

rays pass through the film emulsion• Screen-film combination

– Fluorescent screen captures X-rays and produces light

– Film exposed by light– Much more sensitivity than with film

alone


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Recording X-rays

• Fluoroscope: Television camera observes fluorescent screen– Useful for real-time viewing– Lower image quality than screen-film

recording

• Computed radiography: use imaging plate instead of film to record image. – The plate is scanned with a laser and a

digital image is obtained


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Recording X-rays

• Digital radiography– Digital recording system (like digital

camera, but as large as an X-ray film) produces electrical signals that are digitized

– Can be used for fluoroscopy


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Comparison• SF ~ screen-film recording, CR ~

computed radiography, DR ~ digital radiography– Image quality: SF is best– Initial cost: SF is lowest– Operating cost: DR is lowest, film is highest– Sensitivity (patient exposure): DR and and

CR are better– Operating convenience: DR is best

• Conclusion: Each system has a use– Digital recording is displacing film


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Big Picture

• Types of imaging procedures– Screening: detect disease when there are

no symptoms– Diagnosis: a disease is probably present,

identify the type of disease– Staging: we know the disease, what type of

treatment?– Treatment monitoring.

• Would like to screen, but there are few diseases that warrant it


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Breast Cancer Screening

• Breast cancer screening requires high resolution and contrast

• Mostly done with screen-film at low voltage


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Computer Interpretation• Reason for computer interpretation:

– Better accuracy than human?– Less expensive than human?– Human expert not available?

• Much research, many claims– In the US, a system must be tested and

approved by the Federal Drug Administration (FDA)

– There is an FDA approved system for mammography interpretation

– At present, used as adjunct for human doctors.


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Other X-ray Applications

• Image from X-ray telescope

• Nebula left by exploding star

• X-ray telescopes are on satellites because X-rays do not penetrate the atmosphere


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Summary

• X-rays are 100 years old• Created a revolution in medicine• Useful for many diagnostic tasks

– Limitation: cannot distinguish between soft tissues

– Contrast radiography helps


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Developments in X-rays

• Digital recording systems are replacing film– Decrease in image quality– Improvement in sensitivity– More convenient

• Computer interpretation of X-rays is here– Now assisting mammography. May become

better.– I expect that procedures for cardiography are

next.

MIPR Lecture 6 Copyright Oleh Tretiak, 2004 1 Medical Imaging and Pattern Recognition Lecture 6...

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