Introduction to Biomedical Imaging

Alejandro Frangi, PhDComputational Imaging Lab

Department of Information & Communication TechnologyPompeu Fabra University

www.cilab.upf.edu

Introduction to Biomedical Imaging

Basic Ideas & Image Quality

Nice resource: P. Sprawls http://www.sprawls.org

Introduction to Biomedical Imaging

A collaborative paradigm

Physiology and CurrentUnderstanding

Physics of Imaging

Instrumentationand Image Acquisition

Computer Processing,Analysis and Modeling

Applications andIntervention

Intelligent interpretation of medical images requires understanding:

The interaction of the basic unit of imaging in a biological environment

The process of formation of a quantifiable signal

Detection and acquisition of the signal of interest

Appropriate image reconstruction

In general, in doing medical image analysis domain knowledge on the type of images helps in devising more effective analysis techniques

Introduction to Biomedical Imaging

Biomedical Imaging

Several classifications of medical imaging modalities exist

According to the energy of the radiation source …

Introduction to Biomedical Imaging

Biomedical Imaging

Several classifications of medical imaging modalities exist

According to the energy of the radiation source …

Introduction to Biomedical Imaging

According to the location of the radiation source …

Biomedical Imaging

Introduction to Biomedical Imaging

Characteristics and quality factors in medical images

P. Sprawls http://www.sprawls.org

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

A medical image is a window to the body

But there is no perfect window…

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Energy(kV)

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Blurring limits visibility of detail

There is some blurring in all medical images

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

BlurringThere are three specific effects of blurring in medical imaging:

Reduced visibility of detail Image unsharpness Reduced spatial resolution

The amount (size) of blurring in a specific imaging procedure isdetermined by:

Design characteristics of the imaging equipment Technique and protocol operating factors

In later modules we will see that blurs has different shapes, depending on the source of the blurring.

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Noise

The effect of noise is to reduce the visibility of low contraststructures

Introduction to Biomedical Imaging

Blur versus noiseNoise affects structures with low contrastBlur affects structures of small size

Most small anatomical objects also

have relatively low contrast and their visibility is reduced by both

noise and blurring

Introduction to Biomedical Imaging

Types of views in medical imaging

2D projection viewsTomographic views

Introduction to Biomedical Imaging

Types of views in medical imaging

Volumetric reconstructions

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Types of distortion

Images not alwaysdepict the true spatialand geometricalcharacteristics

Aspects that can be distorted are:

Relative sizeShapePosition within thebody

Introduction to Biomedical Imaging

Relative sizeand positionare distorted in projectionimaging

Position distortion in radiography

Introduction to Biomedical Imaging

High FRD tominimize thevariation in magnificationof differentparts of thebody

Position distortion in radiography

Introduction to Biomedical Imaging

Introduction to Biomedical Imaging

Physics of Energy Matter Interaction

Introduction to Biomedical Imaging

Physics of radiography

Ionizing radiation: radiation capable of ejectingelectrons from an atom.Other radiation types: particulate radiation andgamma raysThe physics of high-frequency electromagneticwaves will underpin all imaging modalities thatuse ionizing radiation: projection radiography, computed tomography, emission computedtomography, among others.

Introduction to Biomedical Imaging

Energy-matter interactions

There are several means of x-rays and gamma rays being absorbed or scattered by matter Four major interactions are of importance to diagnostic radiology and nuclear medicine, each characterized by a probability of interaction

Classical (Rayleigh or elastic) scattering Compton scattering Photoelectric effect Pair production

Introduction to Biomedical Imaging

Energy-matter interactions

Classical (Rayleigh or elastic) Classical (Rayleigh or elastic) scatteringscattering

Excitation of the total complement Excitation of the total complement of atomic electrons occurs as a of atomic electrons occurs as a result of interaction with the result of interaction with the incident photon incident photon No ionization takes place No ionization takes place The photon is scattered (reThe photon is scattered (re--emitted) in a range of different emitted) in a range of different directions, but close to that of the directions, but close to that of the incident photon incident photon No loss of E No loss of E Relatively infrequent probability Relatively infrequent probability ≈≈5%5%

Introduction to Biomedical Imaging

Energy-matter interactionsComptomComptom scatteringscattering

Dominant interaction of x-rays with soft tissue in the diagnostic range and beyond (approx. 30 keV- 30MeV) Occurs between the photon and a “free” e- (outer shell e- considered free when Eo >> binding energy, Ebof the e- ) The encounter results in ionization of the atom and probabilistic distribution of the incident photon Eoto that of the scattered photon and the ejected e-A probabilistic distribution determines the angle of deflection

Introduction to Biomedical Imaging

Energy-matter interactions

ComptomComptom scatteringscattering

http://www.bh.rmit.edu.au/mrs/subject/mr100/interact.htm

Introduction to Biomedical Imaging

Energy-matter interactionsComptomComptom scatteringscattering

Compton interaction probability is dependent on the total no. of e- in the absorber vol. (e-/cm3 = e-/g ·density) With the exception of 1H, e-/g is fairly constant for organic materials (Z/A ≅ 0.5), thus the probability of Compton interaction proportional to material density (ρ)Conservation of energy and momentum yield the following equations:

00

021 (1 cos )

sc SCe

e

EE E E E Em c

θ−= + =

+ −

Introduction to Biomedical Imaging

Energy-matter interactionsPhotoelectric effectPhotoelectric effect

Interaction of incident photon with inner shell e-All E transferred to e- (ejected photoelectron) as kinetic energy (Ee) less the binding energy: Ee = E0 – Eb

Empty shell immediately filled with e- from outer orbitalsresulting in the emission of characteristic x-rays (Eγ = differences in Eb of orbitals),

For example, Iodine: EK = 34 keV, EL = 5 keV, EM = 0.6 keV

Introduction to Biomedical Imaging

Photoelectric effectPhotoelectric effect

Energy-matter interactions

http://www.bh.rmit.edu.au/mrs/subject/mr100/interact.htm

Introduction to Biomedical Imaging

Energy-matter interactions

Photoelectric effectDifference in binding energy released as either characteristic x-rays or auger electrons Probability of photoe- absorption ∝ Z3/E3 (Z = atomic no.) Due to the absorption of the incident x-ray without scatter, maximum subject contrast arises with a photoe- effect interactionExplains why contrast ↓ as higher energy x-rays are used in the imaging processIncreased probability of photoe- absorption just above the Ebof the inner shells cause discontinuities in the attenuation profiles (e.g., K-edge)

Introduction to Biomedical Imaging

Energy-matter interactions

Photoelectric absorption versus Compton scattering

Photoe- absorption is primary mode of interaction of diagnostic x-rays with screen phosphors, contrast materials and bone Compton scattering will predominate at most diagnostic energies for low Z material such as tissue and air

Introduction to Biomedical Imaging

Energy-matter interactions

Pair production Conversion of mass to E occurs upon the interaction of a Conversion of mass to E occurs upon the interaction of a high E photon (> 1.02 high E photon (> 1.02 MeVMeV; rest mass of e; rest mass of e-- = 511 = 511 keVkeV) in ) in the vicinity of a heavy nucleusthe vicinity of a heavy nucleusCreates a negatron (Creates a negatron (ββ--) ) -- positron (positron (ββ+) pair +) pair The The ββ+ annihilates with an e+ annihilates with an e-- to create two 511 to create two 511 keVkeVphotons separated at an photons separated at an ∠∠ of 180of 180ºº

Introduction to Biomedical Imaging

Recap on energy-matter interactions

Energy-matter interactions

Photoelectric absoption

Comptom scattering

Pair production

Thomsom scattering

Photodisintegration

Introduction to Biomedical Imaging

Total attenuation coefficient

Is the combined effect undergone by energy when passing through a medium due to absorption and scatteing

Attenuation

Attenuation = Absorption + Scattering

Introduction to Biomedical Imaging

Attenuation

Linear attenuation

Attenuation is the removal of photons from a beam of x-rays or gamma rays as it passes through matterThe fraction of photons removed from a beam of x-ray and gamma rays per unit thickness of material is μ

μ(E) ↓ as E ↑ except at attenuation edges, e.g., for soft tissue

μ(30 keV) = 0.35 cm-1 and μ(100 keV) = 0.16 cm-1

μ(E) = fractional number of photons removed (attenuated) from the beam by absorption or scattering

Introduction to Biomedical Imaging

Attenuation

Linear attenuation

An exponential relationship between the incident radiation intensity (I0) and the transmitted intensity (I) with respect to thickness: I(E) = I0(E) e-μ(E)·x

μtotal(E) = μPE(E) + μCS(E) + μRS(E) + μPP(E) At low x-ray E: μPE(E) dominates and μ(E) ∝ Z3/E3

At high x-ray E: μCS(E) dominates and μ(E) ∝ ρOnly at very-high E (> 1MeV) does μPP(E) contribute The value of μ(E) is dependent on the density of material: μwater vapor << μice < μwater

Introduction to Biomedical Imaging

Attenuation

Linear attenuation

Introduction to Biomedical Imaging

X- and gamma-rays

Photon Intensity Tomography

X-rays with a wavelength longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays overlap the range of long-wavelength (low energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.

Introduction to Biomedical Imaging

Medical Imaging TechniquesImages can be structural or functional depending whether they represent anatomy or physiological/chemical processes

X-ray CT

SPECT

CT/PET

US

MRI

EIT