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