1_Introduction to Medical Imaging (1)

Introduction to Medical Imaging – Chapter 1

Radiation and the Atom – Chapter 2

17-Feb-20



a copy of this lecture may be found at:

http://courses.washington.edu/radxphys/PhysicsCourse.html

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Chapters 1 & 2 Lecture Objectives

Intro to Medical Imaging – what are we after technically?

Spatial Resolution

Contrast

Generally describe what processes are involved in the

diagnostic radiology imaging chain

Describe the basic characteristics of electromagnetic

(EM) radiation and how they are mathematically related

Describe how atomic electronic structure determines the

characteristics of emitted EM radiation

Particulate radiation and the atomic nucleus – what’s the

matter?

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What a Nobel Path you Tread

Roentgen (1901, physics): discovery of x-radiation

Rabi (1944, physics): nuclear magnetic resonance

(NMR) methodology

Bloch and Purcell (1952, physics): NMR precision

measurements

Cormack and Hounsfield (1979, medicine): computed

assisted tomography (CT)

Ernst (1991, chemistry): high-resolution NMR

spectroscopy

Laterbur and Mansfield (2003, medicine): discoveries

concerning magnetic resonance imaging (MRI)

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Introduction to Medical Imaging

Medical imaging requires some form of radiation capable

of penetrating tissues

This radiation must interact with the body’s various

tissues in some differential manner to provide contrast

The diagnostic utility of a medical image relates to both

technical image quality and acquisition conditions

Image quality results from many trade-offs

Patient safety – levels of radiation utilized (ALARA)

Spatial resolution

Temporal resolution

Noise properties

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Transparency of Human Body to EM Radiation

MRI

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c.f. Macovski, A. Medical Imaging Systems, p. 3.

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X-rays – the Basic Radiological Tool

Roentgen’s experimental apparatus (Crookes

tube) that led to the discovery of the new

radiation on 8 Nov. 1895 – he demonstrated

that the radiation was not due to charged

particles, but due to an as yet unknown

source, hence “x” radiation or “x-rays”

Known as “the radiograph of

Bera Roentgen’s hand” taken

22 Dec. 1895

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Radiography - Fluoroscopy

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Mammography

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X-ray Computed Tomography

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Computed Tomography

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NMR T1 for Tumor and Normal Tissue

c.f. Damadian, R, et al. PNAS 1974; 71: 1471-3.

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Magnetic Resonance Imaging

c.f. Bushberg, et al. The Essential Physics of

Medical Imaging, 2 nd ed., pp. 426, 429 & 461.

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Magnetic Resonance Imaging (MRI)

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Ultrasound

c.f. Bushberg, et al. The Essential Physics

of Medical Imaging, 2 nd ed., p. 501.

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c.f. http://www.cs.adelaide.edu.au/~evan/

project/prog1.htm

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Ultrasound Imaging

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Nuclear medicine – Gamma Camera

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Nuclear Medicine/Positron Emission Tomography

c.f. http://www.griffwason.com/gw_images/

MRI_scanner/glw-pet_scanner1.jpg

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c.f. http://www.medscape.com/content/2003/

00/45/79/457982/art-ar457982.fig10.jpg

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Single Photon Emission Computed Tomography (SPECT)

Positron Emission Tomography (PET) - SPECT/CT - PET/CT

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A Systematic Approach to Medical Imaging

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Contrast – What does it depend on?

Radiation must interact with the body’s various tissues in

some differential manner to provide contrast

X-ray/CT: differences in e - density (e - /cm 3 = r ∙ e - /gr)

Ultrasound: differences in acoustic impedance (Z = r·c)

MRI: endogenous and exogenous differences

endogenous: T1, T2, r H , flow, perfusion, diffusion

exogenous: TR, TE, and TI

NM: concentration (r) of radionuclide or b + emitter

Contrast agents exaggerate natural contrast levels

Iodinated (x-ray/CT)

Paramagnetic (MRI)

Microspheres (US)

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Radiation and the Physics of Medical Imaging

“Without radiation, life itself would

be impossible” – Prof. Stewart

“Radiation is all around us. From

natural sources like the Sun to

man made sources that provide

life saving medical benefits,

smoke detectors, etc...”

- nuclearactive.com

“You’re soaking in it” – Madge,

Palmolive spokeswoman

“10 mGy/day keeps the Dr. away”

"It’s not the volts that’ll get ya, it’s

the amps.“ – Billy Crystal,

Running Scared

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Radiation

The propagation of energy through:

Space

Matter

Can be thought of as either:

Corpuscular (particles, e.g., electron)

Electromagnetic (EM)

Acoustic

Acoustic radiation awaits the ultrasound sessions later

on in the course

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Characterization of Waves

Amplitude: intensity of the wave

Wavelength (l): distance between identical points on adjacent

cycles [m, nm] (1 nm = 10 -9 m)

Period (t): time required to complete one cycle (l) of a wave [sec]

Frequency (n): number of periods per second = (1/t) [Hz or sec -1 ]

Speed of radiation: c = l ∙ n [m/sec]


of Medical Imaging, 2 nd ed., p.18.

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Electromagnetic (EM) Radiation

EM radiation consists of the transport of energy through

space as a combination of an electric (E) and magnetic

(M) field, both of which vary sinusoidally as a function of

space and time, e.g., E(t) = E 0 sin(2ct/l), where l is the

wavelength of oscillation and c is the speed of light



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The Electromagnetic (EM) Spectrum

Physical manifestations are classified in the EM spectrum based on

energy (E) and wavelength (l) and comprise the following general

categories:

Radiant heat, radio waves, microwaves

“Light” – infrared, visible and ultraviolet

X-rays and gamma-rays (high energy EM emitted from the nucleus)

c.f. http://www.uic.com.au/ral.htm

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EM Radiation Share the Following

Velocity in vacuum (c) = 3 x 10 8 m/sec

Highly directional travel, esp. for shorter l

Interaction with matter via either absorption or scattering

Unaffected by external E or M fields

Characterized by l, frequency (n), and energy (E)

So-called wave-particle duality, the manifestation

depending on E and relative dimensions of the detector

to l. All EM radiation has zero mass.

*X-rays are ionizing radiation, removing bound electrons

- can cause either immediate or latent biological damage

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EM Wave and Particle Characteristics

Wave characteristics – used to explain interference and

diffraction phenomena: c [m/sec] = l [m] ∙ n [1/sec]

As c is essentially constant, then n 1/l (inversely proportional)

Wavelength often measured in nanometers (nm = 10 -9 m)

Frequency measured in Hertz (Hz): Hz = 1/sec or sec -1



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EM Wave and Particle Characteristics

Particle characteristics – when interacting with matter,

high energy EM radiation act as energy quanta: photons

E [Joule] = hn = hc/l, where h = Planck’s constant

(6.62x10 -34 Joule-sec = 4.13x10 -18 keV-sec)

If E expressed in keV and l in nm: E [keV] = 1.24/l [nm]



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Transparency of Human Body to EM Radiation


of Medical Imaging, 2 nd ed., p.18. c.f. Macovski, A. Medical Imaging Systems, p. 3.

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Raphex 2000 Question: EM Radiation

G46. Regarding electromagnetic radiation:

A. Wavelength is directly proportional to frequency.

B. Velocity is directly proportional to frequency.

C. Energy is directly proportional to frequency.

D. Energy is directly proportional to wavelength.

E. Energy is inversely proportional to frequency.

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G51. Which of the following has the highest photon

energy?

A. Radio waves

B. Visible light

C. Ultrasound

D. X-rays

E. Ultraviolet

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G52. Which of the following has the longest wavelength?

A. Radio waves

B. Visible light

C. Ultraviolet

D. X-rays

E. Gamma rays

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G51. Visible light has a wavelength of about 6 x 10 -7 m.

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Co gammas have a wavelength of 10 -12 m and an

energy of 1.2 MeV. The approximate energy of visible

light is:

A. 720 MeV

B. 72 keV

C. 2 eV

D. 7.2 x 10 -4 eV

E. 2 x 10 -6 eV

E 1 = hc/l 1 and E 2 = hc/l 2 , so E 1 l 1 = hc = E 2 l 2

E 2 = E 1 l 1 /l 2 = (12 x 10 5 eV)(10 -12 m)/(6 x 10 -7 m) = 2 eV

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Particulate Radiation

Corpuscular radiations The most significant particulate

mass-energy relationship: of energy (E) and momentum

E = m 0 c 2 are comprised of moving radiations of interest are:

particles of matter the

energy of which is based

on the mass and velocity

of the particles

Alpha particles

Electrons

Positron

Negatrons

α 2+

e -

β +

β -

Protons p

Kinetic energy (KE)

= ½ m 0 v 2 Neutrons n

(for nonrelativistic

velocities)

Interactions with matter are

Simplified Einstein

collisional in nature and are

governed by the conservation

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c.f. http://www.ktf-split.hr/periodni/en/index.html

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Electronic Structure – Electron Orbits

Pauli exclusion principle

No two electrons in an atom may

have identical quantum numbers

→ max. 2n 2 electrons per shell

Quantum Numbers

n: principal q.n. – which e - shell

l: azimuthal – angular momentum

q.n. (l = 0, 1, ... , n-1)

m l : magnetic q.n. – orientation of

the e - magnetic moment in a

magnetic field (m l = -l, -l+1, ..., 0,

... l-1, l)

m s : spin q.n. – direction of the e -

spin (m s = +½ or -½)

For a more detailed discussion, see - http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/eleorb.html



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Electronic Structure – Electron Orbits (2)

s, p, d, f, g, h, …

c.f. Hendee, et al. Medical

Imaging Physics, 2 nd ed., p.4.

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c.f. Hendee, et al. Medical

Imaging Physics, 4 th ed., p.13.

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Electronic Structure – Electron Binding Energy

E b Z 2

Highly suggested, very nice detailed description - http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html

c.f. http://astro.u-strasbg.fr/~koppen/discharge/

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Radiation from Electron Transitions

Characteristic X-rays

Auger Electrons and Fluorescent Yield (w K ):

(characteristic x-rays/total)

Preference for Auger e - at low Z



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c.f. Sorenson, et al. Physics in

Nuclear Medicine, 1 st ed., p.8.

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The Atomic Nucleus

Covered in Nuclear Medicine course (May 2009)

Composition of the Nucleus

Protons and Neutron

Number of protons = Z

Number of neutrons = N

Mass number = A = Z + N

Chemical symbol = X

Isotopes: same Z, but different A

Notation: A ZX N , but A X uniquely defines an isotope (also written

as X-A) as X → Z and N = A - Z

For example 131 I or I-131, rather than 131 53X 78

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Raphex 2000 Question: Atomic Structure

G10-G14. Give the charge carried by each of the following:

A. +4

B. +2

C. +1

D. 0

E. -1

G10. Alpha particle

G11. Neutron

G12. Electron

G13. Positron

G14. Photon

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G17. Tungsten has a K-shell binding energy of 69.5 keV.

Which of the following is true?

A. The L-shell has a higher binding energy.

B. Carbon has a higher K-shell binding energy.

C. Two successive 35 keV photons could remove an electron

from the K-shell.

D. A 69 keV photon could not remove the K-shell electron, but

could remove an L-shell electron.

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G18. How many of the following elements have 8

electrons in their outer shell?

Element: Sulphur Chlorine Argon Potassium

Z: 16 17 18 19

A. None

B. 1

C. 2

D. 3

E. 4

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G18. B The n th shell can contain a maximum

of 2n 2 electrons, but no shell can contain more than 8 if it

is the outer shell. The shell filling is as follows:

Z K shell L shell M shell N shell

Sulphur 16 2 8 6 0

Chlorine 17 2 8 7 0

Argon 18 2 8 8 0

Potassium 19 2 8 8 1

For interactive answer, see - http://www.webelements.com/webelements/elements/text/Ar/econ.html

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G15. 226 88Ra contains 88 __________ .

A. Electrons

B. Neutrons

C. Nucleons

D. Protons and neutrons

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