BMME 560 Medical Imaging: X-ray, CT, and
Nuclear Methods
Summary and Finale
Today
• Some perspective on other imaging modalities
• Course summary
• Course evaluations
Perspective
• We should have a basic idea of how the other imaging modalities work– MRI– Ultrasound– Optical
Magnetic Resonance
• Some atomic nuclei have a magnetic dipole: 1H, 13C, 17O, 19F, 23Na, 31P
Principles
H
+
-
N
S
Nuclear spin
Magnetic Resonance
• When placed in a magnetic field, some of the dipoles will spin at a characteristic rate– This rate is a property of the material.
• By disturbing the field with a radio-frequency pulse, the spins are perturbed.– As they return to resting state, they give off an RF signal
which can be detected with an antenna.– Dynamic effects are characteristic also – the decay time is
related to the molecular structure
• We can encode spatial position by varying the magnetic field with position.
Magnetic Resonance
• MR produces several pieces of information– Spin density (Local concentration of molecule)– Spin relaxation (The dynamics of how fast the spins return
to equilibrium state)• T1: Spin-lattice interaction: How the molecules lose energy to their
surroundings• T2: Spin-spin interaction: How the molecules transfer energy to
each other
• Thus, MR can produce multiple pieces of information and yield multiple images with different combinations and influences of all of these properties.
MRI
• Distinctive views of neuroanatomy and other soft tissue
• Contrast is related to chemical and molecular properties of tissue
• Not so good with mineralized tissue (bone)
• Contrast agents exist– Gadolinium– Iron oxide particles
Ultrasound
• An acoustic wave (2-10MHz) is created by a vibrating transducer and propagates into the subject.
Principles
Ultrasound
• The wave is partially reflected from a tissue interface.
Principles
The return wave is detected by the transducer
Ultrasound
• Totally safe imaging
Applications - Obstetrics
Ultrasound
• Contrast is developed at tissue boundaries– Changes in acoustic impedance
• Usually anatomical boundaries
• Neat application: Doppler – Imaging dynamics of fluid
• Real-time and interactive
• Small field-of-view
• Interventional ultrasound
Optical Imaging
• Like ultrasound, but using visible and near-visible light.
Principles
Fiber optic cable
Some reflected,Some transmitted
Optical Imaging
• Layers of the retina
Applications - Optical Coherence Tomography
Optical Imaging
• Can be done various ways– Reflective mode: (optical coherence tomography)– Transmission mode: (diffuse optical tomography)– Emission mode: (fluorescence molecular
tomography)
• Depth penetration is chief problem for in vivo applications– Near-IR is reasonably well transmitted through
modest depths of tissue
Ionizing Radiation for Imaging
• Substantial depth penetration
• High sensitivity (PET and SPECT)– What is the smallest physical contrast that can be
detected?
• Radiation dose
• Mostly inexpensive, though it can be expensive
New Research: PET/MRI
• Simultaneous PET/MRI with Siemens Biograph mMR
• MRI-friendly PET detectors
New Research:PET/MRI
New Research: PET/MRI
New Research: PET/MRI
• The big problem:– How to correct attenuation like PET/CT
• Opportunities:– Motion correction of PET
– MRI-guided PET reconstruction via anatomical and functional MRI information
– Improved PET quantitative imaging
• Clinical Applications: – Cancers (Head/neck, abdominal, soft tissues)
– Neurological
– Cardiac
Summary
• Medical Imaging requires– Physics– Signal Processing– Biology– Chemistry– More physics– Medicine
Concept MapImaging Concepts
Physics of Radiation
X-ray Imaging
PET Imaging SPECT Imaging CT Imaging
Radiation Biology
Tomographic Reconstruction
Medical Imaging Systems
Hardware
SoftwareApplications
X-ray sourcesScreen-film detectorsDigital DetectorsGamma Cameras
FilteringCT reconstructionImage quantification
MammographyFluoroscopyFunctional imagingCancer staging
Linear, Shift-Invariant Systems
• LSI systems are characterized by their impulse response– In imaging, we call it a point spread function
(PSF).
System
Compare Two MTFs
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Frequency, cycles per pixel
MT
F
System A
System B
Zero frequencyIs here
Contrast
Input intensity
Out
put
inte
nsity
Input intensity
Out
put
inte
nsity
Linear map Piecewise linear map
Spatial Resolution
• An abbreviated way of characterizing spatial resolution:– The full-width at half-maximum (FWHM) of the
PSF
0 50 100 150 200 2500
0.2
0.4
0.6
0.8
1
Find the peak of PSFTake ½ of the peakMeasure the width
What are the units of FWHM?
Key Point
• There is an essential and inescapable tradeoff between noise and resolution in every imaging system.
Noise variance
Res
olut
ion
(FW
HM
)
Basic radiation concepts & definitions
• Radiation - Energy (with or without mass or charge)
emitted from a source that travels through space.
• Ionization - Event that an atom is separated into free electrons and an ion (original atom minus the released electrons) thus capable of causing structural damage.
radiation
“White” bremsstrahlung x-rays
Filtered x-rays
Compton scattering
1. It is also referred as inelastic or
non-classical scattering 2. an interaction of incident x-ray
with outer shell electrons;
3. almost atomic (proton) number (Z) independent;
4. proportional to e;
5. weakly energy dependent until energy is high (1MeV);
6. a disturbance to x-ray image quality.
X-ray interactions with matter
Possible outcomes for an x-ray when traveling through matter are:
– Nothing happened (a)
(it travels along the original
path with original energy)
– Disappeared (b)
(it is absorbed in the matter)
– Scattered (c)
(It changed its direction of
travel and/or energy)
(a) (b)(c)
Imaging detector
X-Ray Tube• Filament (Tungsten Coil)
emits electron thermionically
• Electrons, accelerated by electric field, hit rotating beveled anode at focal track
• Arrangement enclosed into vacuumed glass housing of Leaded Pyrex, with thin spot (window) for X-ray exit to collimatorCollimator
Radiographic Cassette
• Ensures firm and uniform contact between intensifying screens and film sandwiched in between
• Optical mirrors located outside screens to direct light towards film, maximize light conversion efficiency
• Contains ID card and loaded only one way into X-ray machine
Images source: The essentials Physics of Medical Imaging
Example Problem
• A nonuniform object as shown is imaged. What is its contrast in the detected image?
X-rays
Det
ecto
r
10 cm
3 cm
= .01 cm-1
= .1 cm-1
X-ray Applications
• Conventional Radiography
• Angiography: imaging of blood vessels
• Fluoroscopy: real-time imaging, interventional radiology
• Mammography: breast-cancer screening and diagnosis
NCRP limits• Occupational Exposures
– Effective Dose limits• Annual 50mSv (5rem)• Cumulative 10mSv (1rem) * age
– Equivalent Dose Limts• Lens of eye 150mSv (15rem)• Skin, Hands, feet 500mSv (50rem)
• Public exposures (annual)– Effective dose limit
• Continuous or frequent exposure 1mSv (100mrem)• Infrequent exposure 5mSv (500mrem)
– Equivalent dose limits• Lens of eye 15mSv (1.5rem)• Skin, Hands, Feet 50mSv (5rem)
• Embryo/fetus– Monthly equiv. dose 0.5mSv (50mrem)
• Background in US 2.5mSv (250mrem)/yr• Chest X-ray 0.1mSv (10mrem)
Large scale radiation effects are divided into 2 categories: Deterministic and Stochastic
• Deterministic – implies there is a threshold dose below which no effect will be observed– Includes acute radiation sickness– Typically only important for high doses
• Stochastic – Implies there is some probability of effect for any exposure– Measured in terms of likelihood of effect if entire
population were given same dose
Common cancers induced by radiation
• Leukemia – by far the most common cancer attributed to radiation exposure– Only acute and chronic myeloid leukemia in adults
– Only acute chronic lymphocytic leukemia in children
– Data comes from survivors of Hiroshima and Nagasaki; 20 – 50 rad received.
• Thyroid Cancer– Data from radiotherapy treatment studies on children
• Breast Cancer– Increased incidence in atomic bomb survivors, also in women treated for
postpartum mastitis with radiotherapy
Tomography
To reconstruct a distributed object in 3D, we need a lot of views.
Tomographic Reconstruction
f(x,y)
p(t,)t
ty
x
s
Therefore
( , ) ( cos , sin )t t tP F u v
This is called the Fourier Slice Theorem or the Projection Slice Theorem
Simple Backprojection
• An LSI system model for projection followed by simple backprojection:
FourierTransform
1
t
filter
InverseFourier
Transform
ˆ ( , )sbpf x y( , )f x y
ProjectionSimple
backprojectionˆ ( , )sbpf x y( , )f x y ( , )p t
Filtered Backprojection
• Example
True image Simple backprojection Filtered backprojection
Limitations
• Example: Detector is too small
Two-sided truncation One-sided truncation
Cone-beam Imaging
Issues:•Data sufficiency•Inexact reconstruction methods•Field of view
Tc 99m-labeled
Tc-99m sodium pertechnetate
Tc-99m arcitumomab
Tc-99m apcitide
Tc-99m bicisate
Tc-99m depreotide
Tc-99m disofenin (DISIDA)
Tc-99m exametazine (HMPAO)
Tc-99m gluceptate (Gluco)
Tc-99m macroaggregated albumin (MAA)
Tc-99m mebrofenin
Tc-99m medronate (MDP)
Tc-99m mertiatide
Tc-99m oxidronate
Tc-99m pentetate (DTPA)
Tc-99m pyrophosphate (PYP)
Tc-99m labeled red blood cells
Tc-99m sestamibi
Tc-99m succimer
Tc-99m sulfur colloid
Tc-99m tetrofosmin
Other
Carbon-14 urea
Cobalt-57 cynocobalamin
Chromium-51 sodium chromate
Flourine-18 fluorodeoxyglucose (FDG)
Flourine-17 sodium fluoride (NaF)
Gallium-67 citrate
Indium-111 capromab pendetide
Indium-111 chloride
Indium-111 ibritumomab tiuxetan
Indium-111 pentetate (In-111 DTPA)
Indium-111 oxyquinoline (In-111 oxine)
Indium-111 pentetreotide
Iodine I-125 human serum albumin (HSA)
Iodine I-125 sodium iothalamate
Iodine I-131 human serum albumin (HSA)
Iodine I-131 iobenguane sulfate (mIBG)
Phosphorus P-32 sodium phosphate
Rubidium-82 chloride
Samarium-153 lexidronam
Thallium-201 chloride
Xenon-133 gas
Yttrium-90 ibritumomab tiuxetan
List of Radiopharmaceutical :
Scintillation Camera (Gamma camera)Scintillation Camera (Gamma camera)
Collimator
NaI Crystal
PMT
Lead Shield
Source
Electronic boards
Acquisition &
processing computer
Projections
ReconstructedTransaxial Slices
SPECTSingle Photon Emission Computed Tomography
180º ± 0.25º
FinitePositron range
Non-collinearity180º
LOR
Detected LOR
True LOR
Ideal PET Real PET
The finite positron range and the non-colinearity of the annihilation photons give rise to an inherent positional inaccuracy not present in SPECT. However, other characteristics of PET more than offset this disadvantage.
CTI – HR+
No Attenuation correction Attenuation correction
Example 3 Example 2
Extremely Large Patient – poor scan quality
Imaging time was increased to 5min/bed – probably needs 10-15 min
Key Point
• Note that the different imaging modalities represent different physical and physiologic properties– X-ray: absorption, electron density
• Contrast is produced by the difference in tissue density
• We see anatomy
– PET/SPECT: radiotracer concentration• Contrast is produced by the affinity of the radiotracer
for a particular tissue
• We see tissue function (as defined by the targeting molecule).
The Works• Data corrupted by attenuation, detector
response, scatter, and Poisson noise.
Ideal datareconstructed
with no corrections
Realistic datareconstructed
with no corrections
Realistic datareconstructed
with all corrections
ML-EM Algorithm
Iteration numbers
Iterative vs. FBPTrue Feldkamp FORE-FBP FORE-OSEM 3D OSEM Attenuation
Iterative OS-EM reduces noise compared to FBPAlso, it permits correction for attenuation
Bone Scans
Calcified uterine fibroids above the bladderSource: http://www.uhrad.com/spectarc/nucs020.htm
Stress fracture of the footSource: http://www.uhrad.com/spectarc/nucs012.htm
SPECT Cardiac Imaging
The left ventricle takes most of the blood flow to the heart.
It is shaped like a rounded cone.
Dark regions indicate reduced blood flow to a portion of the myocardium
PET Cancer Imaging
Source: http://www.bocaradiology.com/Procedures/PET.html
Liver, but no metsDiffuse spread of prostate cancer to bone
PET Neuroimaging
Abnormally low activity in right temporal lobe in epileptic patient
Source: http://www.bocaradiology.com/Procedures/PET.html
CT Generations
1. Scanning pencil beam
2. Scanning fan beam
3. Full fan-beam with rotating detector*
4. Full fan-beam with stationary detector ring*
5. Electron-beam CT (EBCT)
6. Spiral CT*
7. Multislice CT*
* Most common today
CT Numbers of Common Materials
material Min Max
Bone 400 1000
Soft tissue 40 80
Water 0 0
Fat -100 -60
Lung -600 -400
Air -1000 -1000
Most CT scanners range up to 2000, but some can go up to 4000 to accommodate metal implants. What determines the peak CT number resolvable in a scanner? Reference 1
CT Artifacts
• Beam hardening – streaks occur near highly-absorbing regions (bone, iodine)
Reference 2
Bottom Line
• Imaging systems have– A source of contrast– A means of spatial localization
• There are many tradeoffs in imaging systems– Noise versus resolution– Dose versus image quality– Cost versus everything– Dreams versus physics
• Each system has strengths and weaknesses
What you have learned
• How radiation is produced and used• How radiation interacts with tissues and
materials• How imaging systems are characterized and
measured• How different modalities work• How basic processing tasks are done• Why we have several different modalities for
imaging
Where can you go next?
• BMME 550? – Ultrasound, MRI, and optical
• BMME 890 – Bioimaging Practicum
• Advanced courses– Tomographic reconstruction – by request
• Image processing and analysis– Computer Science – Image analysis– BME 712 – Image Processing