bioe 485 (imaging) notes

7/29/2019 BIOE 485 (Imaging) Notes

1/5

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


Quick, low dose - stationary

Mammography

Energy ~ 20 kEV

Current: ma


Breast must be compressed to reduce

scatter

CT

Energy ~ 100 kEV

Current: variable

high contrast, depth

360 view, image reconstruction


2/5

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.


3/5

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


4/5

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).


5/5

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.

bioe 485 (imaging) notes

Documents