Introduction to Biomedical Imaging

Transcription

1 Alejandro Frangi, PhD Computational Imaging Lab Department of Information & Communication Technology Pompeu Fabra University

2 Basic Ideas & Image Quality Nice resource: P. Sprawls

3 Intelligent interpretation of medical images requires understanding: A collaborative paradigm The interaction of the basic unit of imaging in a biological environment The process of formation of a quantifiable signal Physiology and Current Understanding Applications and Intervention Physics of Imaging Instrumentation and Image Acquisition Detection and acquisition of the signal of interest Computer Processing, Analysis and Modeling Appropriate image reconstruction In general, in doing medical image analysis domain knowledge on the type of images helps in devising more effective analysis techniques

4 Biomedical Imaging Several classifications of medical imaging modalities exist According to the energy of the radiation source

5 Biomedical Imaging Several classifications of medical imaging modalities exist According to the energy of the radiation source

6 According to the location of the radiation source Biomedical Imaging

7 Characteristics and quality factors in medical images P. Sprawls

8

9

10

11 A medical image is a window to the body But there is no perfect window

12

13

14

15

16

17

18 Energy (kv)

19

20

21

22 Blurring limits visibility of detail There is some blurring in all medical images

23

24

25

26

27

28

29 Blurring There 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 is determined 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.

30

31 Noise The effect of noise is to reduce the visibility of low contrast structures

32 Noise affects structures with low contrast Blur affects structures of small size Blur versus noise Most small anatomical objects also have relatively low contrast and their visibility is reduced by both noise and blurring

33 Types of views in medical imaging 2D projection views Tomographic views

34 Types of views in medical imaging Volumetric reconstructions

35

36 Types of distortion Images not always depict the true spatial and geometrical characteristics Aspects that can be distorted are: Relative size Shape Position within the body

37 Position distortion in radiography Relative size and position are distorted in projection imaging

38 Position distortion in radiography High FRD to minimize the variation in magnification of different parts of the body

39

40 Physics of Energy Matter Interaction

41 Physics of radiography Ionizing radiation: radiation capable of ejecting electrons from an atom. Other radiation types: particulate radiation and gamma rays The physics of high-frequency electromagnetic waves will underpin all imaging modalities that use ionizing radiation: projection radiography, computed tomography, emission computed tomography, among others.

42 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

43 Energy-matter interactions Classical (Rayleigh or elastic) scattering Excitation of the total complement of atomic electrons occurs as a result of interaction with the incident photon No ionization takes place The photon is scattered (re- emitted) in a range of different directions, but close to that of the incident photon No loss of E Relatively infrequent probability 5%

44 Comptom scattering 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 E o >> binding energy, E b of the e- ) The encounter results in ionization of the atom and probabilistic distribution of the incident photon E o to that of the scattered photon and the ejected e- A probabilistic distribution determines the angle of deflection Energy-matter interactions

45 Energy-matter interactions Comptom scattering

46 Comptom scattering Compton interaction probability is dependent on the total no. of e- in the absorber vol. (e-/cm3 = e-/g density) With the exception of 1 H, 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: E = E + E E = 0 sc e SC Energy-matter interactions E0 E 1 + (1 cos θ ) mc 0 2 e

47 Energy-matter interactions Photoelectric effect Interaction of incident photon with inner shell e- All E transferred to e- (ejected photoelectron) as kinetic energy (E e ) less the binding energy: E e = E 0 E b Empty shell immediately filled with e- from outer orbitals resulting in the emission of characteristic x-rays (Eγ = differences in E b of orbitals), For example, Iodine: E K = 34 kev, E L = 5 kev, E M = 0.6 kev

48 Energy-matter interactions Photoelectric effect

49 Photoelectric effect Energy-matter interactions Difference in binding energy released as either characteristic x-rays or auger electrons Probability of photoe- absorption Z 3 /E 3 (Z = atomic no.) Due to the absorption of the incident x-ray without scatter, maximum subject contrast arises with a photoe- effect interaction Explains why contrast as higher energy x-rays are used in the imaging process Increased probability of photoe- absorption just above the E b of the inner shells cause discontinuities in the attenuation profiles (e.g., K-edge)

50 Photoelectric absorption versus Compton scattering Energy-matter interactions 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

51 Energy-matter interactions Pair production Conversion of mass to E occurs upon the interaction of a high E photon (> 1.02 MeV; ; rest mass of e- e = 511 kev) ) in the vicinity of a heavy nucleus Creates a negatron (β-)( - positron (β+)( pair The β+ + annihilates with an e-e to create two 511 kev photons separated at an of 180º

52 Energy-matter interactions Recap on energy-matter interactions Photoelectric absoption Thomsom scattering Comptom scattering Photodisintegration Pair production

53 Attenuation Total attenuation coefficient Is the combined effect undergone by energy when passing through a medium due to absorption and scatteing Attenuation = Absorption + Scattering

54 Attenuation Linear attenuation Attenuation is the removal of photons from a beam of x-rays or gamma rays as it passes through matter The 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

55 Attenuation Linear attenuation An exponential relationship between the incident radiation intensity (I0) and the transmitted intensity (I) with respect to thickness: I(E) = I 0 (E) e -μ(e) x μ total (E) = μ PE (E) + μ CS (E) + μ RS (E) + μ PP (E) At low x-ray E: μ PE (E) dominates and μ(e) Z 3 /E 3 At high x-ray E: μ CS (E) dominates and μ(e) ρ Only at very-high E (> 1MeV) does μ PP (E) contribute The value of μ(e) is dependent on the density of material: μ water vapor << μ ice < μ water

56 Attenuation Linear attenuation

57 Photon Intensity Tomography X- and gamma-rays 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.

58 Medical Imaging Techniques Images can be structural or functional depending whether they represent anatomy or physiological/chemical processes MRI EIT X-ray CT CT/PET SPECT US