Upgrade to remove ads
Biomedical Imaging Lecture 1
Terms in this set (54)
Distinguishing properties that cause contrast?
2. Reflectance, refraction, scattering
3. Fluorescence, phosphorescence, luminescence.
Density, phase delay, amount of vessels, time for a dye to wash in, temperature, molecular environment, deformability, conduction
Types of Radiation
1. Sub-atomic particles (alpha, gamma, beta, positron nuclear imaging)
2. X-rays (x ray, x ray CT)
3. Light/infrared (optics/thermal)
4. B fields/RF waves (MRI)
5. Acoustic waves (US-pressure wave)
Can have harmful effects on tissue. True for x-rays, gamma-rays, subatomic particles in nuclear imaging.
How does radiation affect imaging?
Wavelength of imaging determines size of objects that can be imaged
Reflects interactions a type of radiation has with tissue.
'Types' of Contrast
Intrinsic: bones in x-rays, structures formed by different tissue types in MRI and US, blood flow changes in fMRI.
Contrast agents: isotopes in nuclear med, I/Ba in x-ray and CT, fluroophores in optics and microscopy, gadolinium in MRI, microbubbles in US
Radioisotopes: unstable nuclei and dissipate excess energy by spontaneously emitting radiation in form of alpha (He nucleus), beta (electrons), gamma rays (photons).
Alpha particles can't really penetrate tissue so they are not used for imaging.
High energy electrons that come from the nucleus through conversion of a neutron to a proton. Beta particles penetrate more than alpha, but not really useful either. Rapidly div cells are particularly sensitive to radiation damage. Beta emitters can be implanted into tumors where radiation has limited range, but can kill cancer cells.
N12 decays into C12, a positron, and an antineutrino. Positrons are created in the nucleus, and share their energy with neutrinos. Positron decay ass with gamma ray emission.
Once emitted, positron often annihilated as a result of a collision with an electron within 1 ns, producing a pair of photons of 0.511 MeV moving in opp dir. A minimum transition energy of 1.022 MeV is req for any positron decay. Photons emitted in the process (B+).
E = me*c^2
Absorption of materials with high atomic # compared to tissue. X-rays are used as radiation, and you look for x-ray absorption of structures.
Ionizing radiation: dangerous.
Detection of radiation from injected radioactive isotopes of oxygen, glucose, etc which act as contrast agents.
Ionizing radiation: dangerous.
IE PET scan, SPECT.
FDG = common PET tracer, glucose analog, where O is replaced by F. Body thinks FDG = glucose, and phosphorylates it using hexokinase (mitochondrial form elevated in malignant tumors).
O req for next step, so FDG trapped in cell that takes it up, accumulating over time until it decay.
Fluorescence of fluorescent proteins expressed in model organisms.
Structural images report on the local environment of H atoms in water.
Mostly back-scattering of acoustic waves from impedance mismatches.
How to choose an imaging system
1. Sensitivity of imaging technique to disease state/problem
2. Availability (PET req radioactive isotopes and shielding; MRI req huge magnetically shielded machines)
3. Size/shape of body part/person (each tech has limits on geometry of subject; field of view, resolution)
4. Time to collect image
5. Safety (no metal in tattoos for MRI)
Provides contrast in imaging techniques where external radiation is used.
I = I₀*e^-μax
I₀ = incident intensity
I = transmitted intensity
μa = linear absorption coeff (mm⁻¹) = prob of absorption per unit dist
x = path length (mm)
μa = ∑εn*Cn
μa = linear absorption coeff (mm⁻¹)
ε = molar extinction coeff (mm⁻¹M⁻¹)
c = concentration (M)
Scatter, reflection, refraction
-Radiation can also change direction (either maintain energy or lose some)
-Types of structures that scatter/reflect/refract depend on the type of radiation
-Redirection can provide contrast, but also affect image-forming abilities of the system
n1sinθ1 = n2sinθ2
n = refractive index of medium
Mismatch in impedance, which equates to changes in speed of the wave
Once a photon is absorbed, it's gone unless the energy is re-emitted
X rays and gamma rays scatter.
Scattering causes image blurring and reduced contrast, as photons end up in the wrong detector.
Collimation describes trying to make radiation co-linear (parallel).
Nuc/XR collimator = honeycomb of tubes of lead that only let photons travel in one dir.
F = Iabs n
F = fluoresence
μa = σ = absorption cross section
n = quantum yield
x = pathlength
Signal to noise
Dynamic range = # of levels captured by imaging system.
Perception and display
Sensitivity = funct of contrast mechanism; often equates to how many moles of change can be detected within dynamic range. Also to describe disease detectability by an observer.
signal to noise = contrast to noise.
Fourier Transform; Inverse Fourier Transform; Euler's
FT expresses signal over time/space in terms of freq components. X(ω): complex valued function of cont freq ω.
X(ω) can be transformed back to time domain.
E = hv = hc/𝛌
E of each photon (not tot amt of photons emitted from the tube) for tissue penetration and contrast. Gamma rays = highest E, so pass out of bod v well.
Radioisotopes need to have a reasonably short half life so they're harmless within days of the procedure.
Technetium-99m. 6 hr ½ life. Isomeric: emits gamma rays and low E electrons. Low energy gamma rays emitted easily escape body.
Particle accelerator that allows a wide range of isotopes to be made for medical imaging. Works by proton bombardment of 18O enriched water. 18F has ½ life under 2 hrs.
Planar imaging (gamma camera).
Single-photon emission computed tomography. Collects radiation coming out in all directions using multiple gamma cameras. Performs image reconstruction to find out where radiation came from. Superseded by PET
Gamma ray detectors all around pt. Two events detected at same time assumed to come from 1 annihilation whose position is along the chord joining these two dectors, which record the time the photons arrive. Difference in time indicated where along chord event occurred.
PET Spatial Resolution
Spatial resolution det by timing accuracy of detectors, # photons collected, and distance traveled by positron following production and before annihilation.
Biggest risk of nuc med is exposure of pt. Image quality is compromise bw dose and resolution/sensitivity and signal to noise.
Soln: better, more sensitive, efficient detectors, and ways to collimate w/o discarding photons (XR optics?)
Need smaller infrastructure for isotope production/handling and same contrast w/o radiation dose.
XR are produced via e- bombardment of a metal anode.
Target = tungsten, molybdenum or Cu. Bombardments accelerates other e-, ions, nuclei, within anode mat. About 1% of E gen is emitted or radiated, usually perp to path of e- beam as XR. Rest of E released as heat. E spectrum of XR produced dep on anode mat and V acc the e-.
Lower energy of XR, more interactions. Bones: higher E photons; don't want soft tissue absorp. CT and mammo req lower E to interact with these tissues too. Lower E req higher XR dose.
Each breast compressed horizontally, obliquely, and XR taken of each pos.
Breast almost entirely fatty; scattered areas of fibroglandular density (degree of XR attenuation of tissue); heterogeneously dense which may obscure small masses; dense tissue which lowers mammo sensitivity.
Dual Energy X-Ray
Use fastest image R compatible with diagnostic task. Collimation of beam to size of R when poss. Proper film exposure and processing. Use of protective aprons and thyroid collars. Limit # of images obtained to min needed.
Film VS Digital
Some digital XR are less sensitive than film, and can lead to higher doses. Digital images can be checked during acquisition and edited or enhanced to avoid retakes. Tomographic dental XR provide fairly high exposure.
Digital XR Benefits
Faster scanning/results; increased pt throughput; better image quality (sometimes); lower overall cost of ownership; no chemicals or film processing; multiple storage options; portable; no film; online analysis, visualization and enhancement
Extend planar XR to 3D; cross section views of body. Req computational reconstruction. Accurate spatial sacling.
Back projection tech used to reconstruct PET. Each photon pair sorted based on angle of chord. Set of meas eq to XR proj along that dir, so image can be reconstructed. do not want attenuation of photons (just location of origin). Simultaneous CT scan may be used to correct PET data for absorption.
Optical imaging is best at providing imaging at level of single cells in living tissues (<500nm).
Used widely in-vitro and in-vivo.
d = 𝛌/2NA
Benefits of Optical Imaging
Intrinsic absorption contrast: Hg (oxy/deoxy) and scattering props (Rayleigh/Raman)
Intrinsic fluorescence/SHG/lifetime (NADH, FAD, collagen, keratin, elastin, etc)
Optical Contrast: In-Vivo Dyes
Targeted dyes, activatable dyes (FRET), quantum dots and other nanoparticles, reporter dyes (pH, Ca, V-sensitive)
Optical Contrast: Transgenic Techniques
GFP, and multicolor derivatives good for genetically targeted in-vivo labelling.
Active (Ca sensitive fluorescent proteins and switchable version make it poss to watch cellular funct)
Optogenetics places genetically-targeted light-activatable channels into cells so they can be switched on/off using light. Also bioluminescence using luciferase.
Other Optical Contrasts
Size dep fluorescence of CdSe quantum dots, GFP, Texas dextran red, fluorescence lifetime, phosphorescence, Mie scattering, Raman scattering, bioluminescence
Energy of light on order of e- transitions and vibrational E for many common molecules.
Light interacts and these interactions either lead to conform/env changes (Lots of intrinsic contrast, targeted contrast, active probes (V and Ca2+ snesitive dyes, fluor pro, FRET-based active fluor pros) , or mediate changes in molecules (imaging combined with light-activatable molecules like caged glutamate, optogen, etc)
Biggest issue with light?
Scattering. Props dep on size, and also scatterer shape.
Sets with similar terms
Chapter 3: Image Formation and Radiographic Quality
physics and imaging Chapter 8
Chapter 3: Image Formation and Radiograp…
Other sets by this creator
Principles of Economics Terms
Biomedical Instrumentation L5
Other Quizlet sets
E-6 study guide
Chapter 10: Political Change and Develop…
PSY 150-Final Exam Review
Diabetes RxPrep Quiz