bioe 485 (imaging) notes
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7/29/2019 BIOE 485 (Imaging) Notes
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Imaging Notes
Lecture 1Mathematics and Physics of RadiologyIntro lectureexposure to different kinds of imaging techniques. Transmission imaging, ionizing
radiation inherent in X-ray and analogs.
Lecture 2Interaction of radiation with matter
Radiationinteraction with matterElectromagnetic wave
Particle
ExcitationIonization
Bremstrahlung
Types of x-ray interaction (bold most likely)
Rayleigh scatterVery low energies (can occur in mammography)
Compton scatterinelastic scattering (low contrast, most likely due to low z) (low energy ->
higher probability)
Photoelectric effectprobability protional to Z3/E
3.Also, greater probability if incident energy is
just higher than binding energy.
Pair productionincident energy of up to 1.02 MeV
Human body made up of low Z elements
Lecture 3Interaction: attenuation and dose; Image quality: Resolution, Noise, Contrast,
SamplingEnergy ranges1.02 MeV (pair production), 100-110 KeV normal x-ray, fluoro, CT, 20 KeV
Mammography
I = I*exp(-x)
Linear attenuation coefficient sum of all individual linear coefficients for each type of interaction(Rayleigh, photoelectric, Compton, etc.)
Also, linear coefficient of water > linear coefficient of ice > linear coefficient of water vapor
Mass attenuation coefficient
X-ray
Energy ~ 100 kEV
Current: ma
Low contrast, no depth
Quick, low dose - stationary
Fluoroscopy (movie x-ray)
Energy ~ 100-200 kEV
Current: ma
Low contrast, no depth
Quick, low dose - stationary
Mammography
Energy ~ 20 kEV
Current: ma
Low contrast, no depth
Breast must be compressed to reduce
scatter
CT
Energy ~ 100 kEV
Current: variable
high contrast, depth
360 view, image reconstruction
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For a given thickness, the probability of interaction is dependent on number of atoms per
volume. It is the linear attenuation coefficient divided by density, thus the mass coefficient for all
phases is the same.Half Value Layer defined as the thickness of material required to reduce the intensity of an x or
gamma ray beam to one half of its initial value.
HVL = 0.693/linear attenuation coefficient (from equation above)Beam hardeninglower energy photons of a polyenergetic beam cannot pass through matter,thus these values are filtered to reduce patient exposure.
Kerma = kinetic energy released in matter, defined as KE transferred to charged particles by
indirectly ionizing radiation. Units are J/Kg or gray. 100 rads in a gray.Doseenergy deposited by ionizing radiation per mass
Exposureamount of electrical charge produced by ionizing electromagnetic radiation per mass.
C/kg
Equivalent dosein Sieverts, radiation weighting factor applied to absorbed dose to account forrelative effectiveness of various rays
Effective dosein Sieverts, tissue weighting factor to determine relative dose of various tissue.
Lecture 4Image Quality; X-Ray Production/Properties
Contrastdifference in the image gray scale between closely adjacent regions on an image
Digital image contrast (Contrast to Noise ratio (CNR)assessment of digital image quality.
Looks at Density at Region A minus Density at Region B divided by noise term.Spatial domaintwo spatial dimensions of an image (x and y axis)
Point spread functionimage produced from a single point stimulus to a detector. Can be
isotropic (symmetrically spread out) or non-isotropic. If the PSF is measured at many differentlocations and is the same regardless of location, the system is said to be stationary.
Line spread function(LSF)linear set of PSF
Modulation transfer functionplot of the imaging systems modulation versus spatial frequency.
Good representation of the resolution properties of an imaging system.Illustrates the fraction of an objects contrast that is recorded by the imaging system as a function
of size (spatial frequency). Note that spatial frequency increases for smaller objects and
decreases for larger objects.MTF can be found through the Fourier transform of the LSF.
Noisemany sources. Use statistics as an example, essentially fluctuations about the mean
(standard deviation = square root of mean). Example is the Gaussian distribution described bymean and standard deviation while Poisson distribution is only described by mean. We usually
talk about Poisson and use it as a standard for determining covariance = std. dev/mean = 1/square
root(mean) where variance is equal to mean.
Contrast Detail curvedescribes the Contrast and Detail for images and allows for comparison
Lecture 5X-Ray Production; Radiography
Xray tubeelectron source and target. Generatorvoltage source to excite electrons. Collimator
defines x-ray field. Tube housingshielding and coolant for xray tube.Brehmstralung spectrumx-ray photon with equal energy to the kinetic energy lost by the
electron
Highly inefficient, but increasing energy of incident electrons increases efficiency.
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Intensity varies with frequency, essentially higher current increases area under the curve while
higher voltage increases the cut-off frequency
Lecture 6Radiography; Mammography
Lecture 7Mammography, Digital Detectors
Lecture 8Mammography, Fluoroscopy, CT
Lecture 9CT and Exam Review
1Figure 13.15
2One dimensional CT detector array, Figure 13.14
3Figure 13.11
4 - Figure 13.105Single array CT, multiple detector CT, cone beam CT. Differences in slice width
6Factors affecting resolution p 368-369
CT dependent on Compton scatter, can use this principle to work in reverse i.e. determinedensity of sample.
Development is inverse of radiography (in radiography x-rays that pass through will darken film
while those that dont will have lighter areas (bone is white on an x-ray).
CT # Discussion see notes
ProblemBeam hardening can occur, average energy through tissue increases due to
susceptibility of lower energy beams to attenuation. This worsens with prosthetics which can
completely block x-ray
Lecture 10 (Chapter 14)MRIsoft tissue contrast incredibly high, superconducting air-core system.
Magnetic susceptibilityParamagnetic agent: augments local magnetic field
Diamagnetic agent: depletes local magnetic field
Change in magnetic field
Nucleus has magnetic properties due to protons and neutrons
Hydrogen atoms most important/abundant and is the key to MRI
Magnetic Field and sample magnetization
Thermal energy agitates and randomizes spins in the sample
Under external field B0, protons organize in low (parallel) and high(anti-parallel) quantization
energy levels.Element in question dictates spin and gyromagnetic ratio
Precession frequency
1T = 42.58 MHz1.5T = 63.86
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2T = 127.74
All protons spinning at Larmor frequency
Frame of ReferenceApplied magnetic field is parallel to z axis
Think of how a top spins and falls, but has enough energy to restore its original position.
Free Induction Decay
Rotating frame
T2 Versus T1 (T2 always smaller than T1)
Pulse sequences
Excitation, relaxation, echo formation, data acquisition
Image contrast generated from characteristics of the tissue based upon differences of
T1time needed for Mz decayT2
Spin Echo90 degree excitation, 180 degree refocusing pulses
Inversion recovery180 degree inversion pulse
Pulse sequences are a combination of excitation, relaxation, echo formation, and data acquisition
TRtime period between initial RF excitationsTEdelay between excitation and echo formation
TI: inversion time (inversion recovery) to manipulate spin lattice recovery curves and null signal
from specific tissues
Flip angle: amount of excitation by RF pulse
Spin Echo Pulse Sequence
ExcitationTEEchoFID signal gradually decays with rate constant T2 -> Spin Echo peak amplitude depends on T2
T1 > T2 > T2*
TR Time of Repetition Time between similar angle scans
Multiple Spin Echo (T2 versus T2*)
Lecture 11Important aspect of MR is the soft tissue contrast, demonstrated how those parameters could be
changed to increase contrast
TRRadio frequency time period
TEecho timeFIDfree-induction decay See p 391-393.
T1 = time needed for longitudinal (Mz) regeneration of net magnetization (spin-lattice
interaction)T2 = Time needed for transverse (Mxy) (spin-spin interaction).
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T1 and T2 are tissue dependent because it depends on how these interactions manifest.
T1 Weightedmust have short TR and TE (intrinsically linked to TR)
T2 Weightedmust have long TR and long (but not too long) TE (intrinsically linked to TE)Proton DensityLong TR, short TE. Enhances intensity of tissue image, can be acquired while
gaining a T2 image.
Spin Echo Pulse90, 180, and then 180 degree pulses, can see on p 399. 1
st
echo is the firstwaveform after initiation. TR between the two 90 degree pulses (RF in the second wave)Inversion Recovery (IR)Emphasizes T 1 relaxation times of the tissues by extending the
amplitude of the longitudinal recovery by a factor of 2. After a delay (time of inversion) TI a 90
degree RF pulse rotates the recovered fraction of Mz spins into the transverse plane to gain theFID. p. 400. Inversion Time related to T1 and Transverse Decay is T2.
18090180 degree pulses (TR between 180 degree pulses, second 180 pulse in RF)
Short tau inversion recovery (STIR)very short TI and magnitude signal processing to eliminate
tissue such as fat.Fluid attenuated inversion recovery (FLAIR)use of longer TI to reduce signal level of tissue
like CSF
Gradient Recalled ECHO (GRE)magnetic field gradient inducing formation of an echo ratherthan the 180 degree pulse. It relies on purposeful phasing and dephasing of the FID. Spin-spin
interaction and interaction with external inhomogeneities of the magnetic field cause
degradation. This causes faster degradation (T2*)
Image acquisition Look over this
Apply RF frequency to excite certain frequencies in FOV. Apply a gradient of RF values to
select each slice of the patient.Slice select gradient
Frequency encode gradient
Go up to page 437 in Chapter 15 and then look at motion artifacts.
Process is time intensive, serial MRI is a good way to help reduce this time.
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