Medical Image Analysis · 2020. 4. 27. · DTU Compute 13 DTU Compute, Technical University of...

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Medical Image Analysis

Slides adapted from Jens E. Wilhjelms lectures

Rasmus R. PaulsenDTU Compute

[email protected]

http://www.compute.dtu.dk/courses/02512

mailto:[email protected]

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2020Medical Image Analysis2 DTU Compute, Technical University of Denmark

Lecture 12 – X-ray imaging and CT scanning

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What can you do after today? Describe the basic use of X-rays Describe the X-ray as an electromagnetic wave Explain the basic technique in an X-ray tube Estimate how simple materials will look on an X-ray Use Lambert-Beers law to compute material attenuation Compute the attenuation of simple non-homogenous

materials Describe the concept of tomographic reconstruction Describe the relation between Hounsfield units and tissue

types Compute a CT slice using a simple and idealised setup

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X-ray imaging The most used form of

medical imaging Simple Cheap Fast Radiation

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The history of X-ray Discovered by chance

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Wilhelm Conrad Röntgen German physics professor Experimented with a Crookes

tube Discovered that an unknown ray

could be captured on photographic plates

Named them X-rays– Other call them Röntgen-rays

Had no idea they were dangerous

Made an X-ray of his wife's hand– First medical X-ray

Wilhelm Röntgen's first medical X-ray, of his wife's hand, taken on 22 December 1895

Crookes tube

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Quick popularity X-ray became popular

extremely fast– Shoe fitting– Examine your bones in coin

machines– Wedding pictures

X-ray clinics in small normal apartments

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Dangers People started to realise that

exposure to X-rays could be dangerous

Hands of X-ray pioneer Mihran Kassabian

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Brief history of X-ray and CT 1895 Wilhelm Conrad Röntgen discovered X-ray 1896 Within a week X-ray was not known worlwide 1896 GE and Siemens begin selling X-ray equipment 1904 Dangers of radiation are described 1956 First reconstruction algorithm is described 1958 First prototype CT scanner without computer 1968 Hounsfield’s method for CT patented 1972 Hounsfield’s method for CT demonstrated in the US 1989 First spiral CT scanner enters the market

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X-rays as electromagnetic radiation It has a frequency f

– (v is used for frequency in notes)– Measured in Hertz [Hz]

It has a wavelength λ (lambda)– Measured in meters [m]

It has a speed– “The speed of light” c

http://upload.wikimedia.org/wikipedia/en/1/1a/Wavelength.svg

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Wavelength– 10 pm < λ < 10 nm

pm = picometer = 1×10−12 m nm = nanometer = 1×10−9 m

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Production of X-rays Electrons are accelerated using a cathode Some hit the anode (heavy metal target) Slowed down in the anode material

– Generating heat– A small part of the energy is transformed

to X-rays The electron comes very close to the

nucleus – Electromagnetic interaction causes a

deviation of the trajectory– The electron looses energy and an X-ray

photon is emitted.

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Electron volts 1 eV is the energy increase that an electron

experiences, when accelerated over a potential difference of 1 V.

In medical imaging– 20 keV < E < 150 keV

keV = kilo-electron-volts

Cathode Anode

- +100 kV

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X-ray tube

The anode rotates to avoid over heating

http://upload.wikimedia.org/wikipedia/en/9/9b/Xraytubeinhousing.png

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X-ray tube

Jackson X-ray tube, 1896. Modern rotating anode tube

http://upload.wikimedia.org/wikipedia/commons/5/51/Rontgenbuis-draaianode.jpg

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Full X-ray system

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X-ray film Until recently “real film was

used” Being replaced by digital

radiography The first step was to use

phosphor plates that was later “scanned”

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Contrast in X-ray images Some materials absorb more

X-rays than others We see the X-rays that “got

through” Dark area – high radiation

– Air– Soft-tissue– Fat

Bright area – low radiation– Metals– Bone

Scanned X-ray film

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X-ray attenuation (dæmpning) X-rays hits an object and

travels through it is the intensity at the

entrance is what is left on the

other side – after a length of x

The rest disappears in several different ways

Computed using Lambert-Beer’s law

I (x)I 0

I 0

I (x)

x

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Lambert-Beer’s law Materials can be described

using a linear attenuation coefficient– How much do the material

dampen X-rays High coefficient

– Metal and bone Low coefficient

– Soft tissue and fat

μ: Linear attenuation coefficient

0 1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Distance in object (cm)

exp(µ

x)

High µLow µ

I (x)I 0

x

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Non-homogeneous material Non-homogeneous – not the same everywhere

x1

x2 x5x3

x4

I 0

I 0

I 0

I 0I 1

film

I 2

I 3?

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X-rays in practise Several parameters to tune

– KV– Exposure time– Etc

Settings dependent on body part Radiologists are trained for this Still a lot of human knowledge in

taking good X-rays

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Uhomogentmateriale A) 1

B) 2C) 3D) 4E) 5F) 6

Chart1

A

B

C

D

E

F

-

-

-

-

-

-

1

1

1

1

1

1

Sheet1

A1

B1

C1

D1

E1

F1

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CT scanning

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Invention of Computed Tomography 1972: G.N. Hounsfield, scientist in Middlesex, England

– Announced computed axial transverse scanning– Presented cross-sectional images of the head showing tissues

inside the brain as separate structures of gray matter, white matter, cerebrospinal fluid, and bone

– Pathologic processes such as blood clots, tumors, and infarcts could be easily seen

Dr. Hounsfield's discovery completely revolutionized the practice of medicine: Structures inside the human body that had never been imaged before, could now be visualized.

1979 Nobel Prize in Medicine

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Development of CT Early 70’s : Several minutes to acquire single slice Today: Less than a minute for a full body scan

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Basic principle of CT – Super Sudoku It is assumed that the object

consists of small cubes of homogeneous material

We want to find the linear attenuation coefficient in each cube

I 0

I 0

I 0 I 0

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Quiz? What is A, B, C, D? Can be formulated as a

system of equations– 4 unknowns– 4 equations

A

C

B

D

A+B = 13

C+D = 10

B+D = 11A+C = 12 DISCLAIMER: just an illustration – not solvable…

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Basic principle of CT – Super Sudoku It is assumed that the

object consists of small cubes of homogeneous material

We want to find the linear attenuation coefficient in each cube

We measure the radiation that comes through

I 0

I 0

I 0 I 0

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Basic principle of CT – Super Sudoku What is ?

I 0

I 0

I 0 I 0

x

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Basic principle of CT – Super Sudoku Four equations Four unknowns Can be solved directly

I 0

I 0

I 0 I 0

x

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Real CT More than 4 pixels per slice 512 x 512 pixels Many projections Enormous system of

equations Not solvable by direct

methods

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Hounsfield Units Instead of using a linear

attenuation coefficient Dimensionless Calibrated

– Air: -1000– Water: 0

Linear attenuation coefficient in voxel

Different equation used in book (eq (1))

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Hounsfield Units

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Real CT scanning

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Real CT scanning Many projections Advanced reconstruction

algorithms– Filtered back projection– Radon transform

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Different versions of CT machines

Rotating detectors

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Rotating source – stationary detectors

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Modern CT

High Speed Advantage GE Spiral CT

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Multi-slice CT

128 slice scanner

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What can be seen on the CT image

Photograph of cryosectioned head CT scan

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Uhomogent MaterialeA) 𝜇𝜇1B) 𝜇𝜇2C) 𝜇𝜇3D) 𝜇𝜇4E) 𝜇𝜇7

0

11

3

6

0

A B C D E

Chart1

A

B

C

D

E

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6

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Sheet1

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Medical Image AnalysisLecture 12 – X-ray imaging and CT scanningWhat can you do after today?X-ray imagingThe history of X-rayWilhelm Conrad RöntgenQuick popularityDangersBrief history of X-ray and CTX-rays as electromagnetic radiationSlide Number 11Production of X-raysElectron voltsX-ray tubeX-ray tubeFull X-ray systemX-ray filmContrast in X-ray imagesX-ray attenuation (dæmpning)Lambert-Beer’s lawNon-homogeneous materialX-rays in practiseUhomogent�materialeCT scanningInvention of Computed TomographyDevelopment of CTBasic principle of CT – Super SudokuQuiz?Basic principle of CT – Super SudokuBasic principle of CT – Super SudokuBasic principle of CT – Super SudokuReal CTHounsfield UnitsHounsfield UnitsHounsfield UnitsReal CT scanningReal CT scanningDifferent versions of CT machinesRotating source – stationary detectorsModern CTMulti-slice CTWhat can be seen on the CT imageUhomogent Materiale

Medical Image Analysis · 2020. 4. 27. · DTU Compute 13 DTU Compute, Technical University of...

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