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