neural imaging

8/7/2019 Neural Imaging

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Group 3


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1. Introduction

2. Nuclear Medicine: SPECT, PET

3. Computed Tomography

4. Magnetic Resonance Imaging


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` Whats so great aboutmedical imaging?

We canobserve internal anatomy non-

invasively

` Whats neuroimaging?Neuroimaging includes the use ofvarious

techniques to either directly or

indirectly image the structure,

function/pharmacology ofthe brain.


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` 1918: Ventriculography,

Pneumoencephalography(Walter Dandy)

` 1927: Cerebral angiography (Egas Moniz)

` 1970s: CT (Allan McLeod Cormack andGodfrey Newbold Hounsfield)

` 1980s: SPECT, PET, MRI

` 1990s: fMRI

` 2000s: Brain computer interphase


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Medical

Imaging

Neuro

imaging

SPECT

PET

MRI

CT

etc


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A branch orspecialty ofmedicine and medicalimaging that uses radionuclides and relies onthe process ofradioactive decay inthediagnosis and treatment of disease.

Medical significancex Provides biochemical and physiologic

information for modern diagnosisx Molecularimaging transcends the simple,

gross morphologic data of traditionalimaging


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x Patientis given an appropriate radiopharmaceutical byintravenous injection

x The pharmaceutical concentrates inthe organ or tissue

ofinterest

x

The radionuclide tagged to the pharmaceutical,

Tcm,emits gamma rays which are detected by a gamma

camera and animage of the radioactive distribution is

produced on a monitor

x Gamma rays cannot be focused, instead of a lens, a

multihole collimatoris used to delineate the imagex NM images provide functional information, butnot high

spatial resolution


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Desired qualities:

x Pure gamma emitter

x Easily made

x Localization only inthe area ofinterestx Suitable energy range

x Short half life

Eg.Tcm (Technetium)


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Multi-hole Collimator` Lead disc with closely packed holes

` Only accepts gamma rays from a narrow channel

` Only locates radioactive sources along its line ofsight

` Thus improves image formation by controlling the

number of photons reaching the crystal

` Types: parallel, converging, diverging, pinhole


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` Single crystal consisting ofsodium iodide

activated with thallium

` High atomic number and density absorbs 90% of

gamma rays of

Tcm` Each gamma photon produces a flash of light

when absorbed by the crystal

` The distribution of light leaving the crystal

depends onthe collimator hole the gamma raypasses through


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` The light patternis measured by

photomultipliers packed in hexagonal array

` Evacuated glass tube containing a photocathode

coated with a material that absorbs light and emitsphotoelectrons

` Converts lightphoton energy into an electron

beam


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` Records the frequency distribution ofthe pulse

amplitude


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` Digitizes the pulses with a digital converter

` The computer records the pulses inmemory

locations

` The counts build up in each location on a digitalimage matrix (128x128 pixels)

` Once complete the image is displayed on a screen

` The brightness of each pixel depends onthe

number of counts stored in memory forthat pixel


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` Tomographic imaging used inbone scans,

myocardial perfusion and cerebral blood flow

imaging

` Improved anatomical localization and contrastresolution

` Uptake of radiophamaceutical reflects cerebral

blood flow, well suited forepilepsy imaging

` Differentiate different kinds of disease processeswhich produce dementia


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` Tcm (Technetium) used has a longer half life

(6hr) and easierto make thantracers used in PET

(at most110min), hence do not suffer problems oftransport and availability


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` Positive electrons are antimatter with brief

existance

` It combines with a negative electron and

annihilate each other` E=mc2 :The energy fortotal annihilation of an

electronis 511keV

` The annihilationproduces 2 photons of

annihilation radiation each of 511 keV travelling inopposite directions


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` The most commonpositron emitterused in PET is 18F

` PET imaging is based on detecting these 2annihilation photons in coincidence and identifying

theirorigin inthe patientto locate the radioactive source

` A PET camera comprises a ring surrounding the patient

and a very large number of solid scintillation detectors

` The emission data received are computer-processed

to produce 2- or 3-dimensional images ofthe distribution

ofthe chemicals throughoutthe brain


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` A wide array ofligands used to map different

aspects ofneurotransmitter activity, with by far

the most commonly used PET tracer being alabeled form of glucose (see Fludeoxyglucose

(18F) (FDG)).


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` Uses low atomic number radionuclides ofvery

short half lives

` Accurate localization, high spatial resolution

` To show blood flow and oxygenand glucose metabolism inthe tissues ofthe

working brain.

` To study the areas of the brain activated by a

particulartask (limited to shorttasks)


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` Diagnose brain tumors, strokes, and neuron-

damaging diseases which cause dementia

before gross damage can be observed by CT or

MRI


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x CT scanning uses x-ray tubes and detectorarrays rotating around the patient

xA thinly collimated, high energy x-ray

beam(from an X-ray tube) is used to obtainmultiple projections ofthe internal structureof an object

x X-ray absorption at a large number ofposition and angles are measured

x The data is thenprocessed to produce animage


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` The measurements are treated by a Fourier

transformation which calculates cross sectional

images

` Image is reconstructed by filtered backprojection to give animage thatis made up of a

matrix ofnumbers in Hounsfield units or CT

number

` Filtered back projection: A complicatedmathematical algorithm to sharpen the image


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` Pixel: Individual component of a matrix, 2Drepresentation of an area withinthe patient

` Voxel: a specific volume within 3D space, which isessentially cubic, depth=thickness of section

` Current CT image data are acquired as a volumetricdata setinvoxels

` Image construction techniques can map CT data

from each voxel to corresponding pixels in many ways` This produces CT slices that depict anatomy in

1. Axial, coronal and sagital planes

2. Oblique and curved planes

3. Projectional views

4. 3D displays


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` CT numberrepresents the average linear

attenuation coefficient ofthe tissues ie. a measure

ofthe ability of a tissue to weakenthe X-raypassing through it, withinthe voxel

Substance Range of CT numbers

Bone 500-1500

Muscle 40-60Brain(grey matter) 35-45

Brain(white matter) 20-30

Water : used for callibration 0

Fat -60 to -150

Lung -300 to -800

Air: used for callibration -1000


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` CT number = K(p-w)/w

K = magnification constant

p = pixel linear attention coefficientw = linear attenuation coefficient of water

` Tissues have variable CT numbers due to their

heterogeinity

` These are related to the attenuation coefficients ofthe differenttissues and are represented by a

gray scale

` Pixel brightness: determined by the attenuation

ofthe voxel, represented by a grey scale of

approx.256 shades


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` The range oftissue densities captured exceeds

ourvisual discriminatory abilities among shades of

gray` Contrast in displayed images is enhanced by

windowing, ie.Image display is achieved by

using appropriate window level(central CT

number)` Window: selection ofthe width ofthe CT density

spectrum thatis presented

` Level: mean CT density presented as a median

shade of gray


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` Quantum noise is the main limitto low contrast

resolution, which is a variationinthe number of photons

detected after passing through the patient` Doses are higher forCT than other X-ray imaging

techniques, because increasing the number of photons

reduces quantum noise

SUMMARY:

Higher density substances: whiter shade

Lower density substances: darker shade


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` CT scanning provides cross sectional imagesofthe body

` It provides high tissue contrast resolution

` From old to new generations, there is a vastimprovement in temporal resolution (Past:slow; Present: fast) ie.Fasterimage productionspeeds This enables CT angiographies becauseinjected contrast material does not remain

intravascular forvery long. Even a beating heartcan be visualized.


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` Multi-slice scanners have several rows of

detectors that are able to collect data

simultaneously

` Helical scanning allows the acquisition of datafrom a large volume ofthe patient compared

with non-helical scanning

` Typically used forquickly viewing brain

injuries


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a) First generation: Pencil beams,

translate/rotate

b) Second generation: Multiple

detectors

c) Third generation: rotate/rotate,gas/solid state detectors

d) Fourth generation: rotate/fixed, solid

state detectors

Helical scaning


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x The patient is placed within static and gradient

magnetic fieldsx A complex series ofradiofrequency(RF) pulses

(radio waves) is then applied to the patient

x The spin of water protons thatis affected by the

magnetic fields and RF pulsesx The RF pulse tilts the proton out of alignment with

the main magnetic field

x It emits an RF pulse as it returns to its state before

the application ofthe RF pulse

x Echoes from the RF pulses are detected by a

receiver coil (radio antenna)

x The detected signals are processed to produce the

image


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` Physiochemical environment ofthat proton

` Strength ofthe magnetic field

` Timing ofthe intervals between applied RF

pulses` Time interval between an applied pulse and the

measurement ofthe returning RF echo

` Presence of strongly paramagnetic, intravenous

contrast agents eg. gadolinium containingcompounds


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` The timing and character ofMR pulse sequences affect

the appearance oftissue contrast

` High MR signal in a returning RF echo bright onthe

image reconstruction

` A large variety of MR pulse sequences are available

` Some results in high signal from fluid

` Some suppress the MR signal from fat

` MRI protocols include imaging in several planes with

variable MR pulse sequences to reveal desired tissuecharacteristics, depending on suspected pathology or

body part studied


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Magnetic resonance angiography

1. Vessels containing slow flowing blood (vein) may

appearbright (strong echo)

2. Vessels containing fast-flowing blood (aorta) appear

dark orvoid (no echo)

3. Turbulent flow produces a rapid loss of coherence,

appearing dark orvoid (weak echo)

4. Flowing blood and CSF usually appearbright

compared to stationary tissue in a GRE scan, a MRIequivalent of a CT angiography (No contrast agent

required)


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PerfusionImaging` Uses a paramagnetic contrast agentto measure

the rate at which blood is delivered to thecapillary bed and thus metabolic activity

DiffusionImaging` L

ow signal : Tissue with normal diffusion (normalrandom thermal motion)

` High signal: Tissue with restricted diffusion (Eg.in edema)

` Ability to assess quantitatively, tissue integrity and

connectivity


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Functional MRI` Acquire images ofthe brain during anactivity or

stimulus and compares them with at restimages` Detectionis based onblood oxygenation level` Oxyhemoglobin is diamagnetic (magnetic properties

are weakly opposed to the main field) ie. produces aweak signal

` Deoxyhemoglobin is paramagnetic and produces

magnetic field inhomogeneities inneighbouring tissuesie. produces a strong signal` Anincrease inmetabolic activity increases

deoxyhemoglobin content of a tissue ie. active areas ofthe brain produces a strong MR signal

` Due to the short lived effects, very rapid sequences suchas EPI or fast GRE is needed

` Areas ofthe subtracted images (stimulus minus rest)that show increased signal intensity correspond to thebrain area activated by the stimulus


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` MRI can produce anatomic images axial, sagittal and

coronal planes well before the CT scan

` Some MRI enables volumetric data acquisition to

reformatimages comparable to CT

` MRIis unequalled inits exquisite soft tissue contrastresolution, which enables the detection ofimperceptible

pathologies

` MRIis capable of producing astonishing spatial

resolution, sometimes showing fine anatomy seeninvivo only with magnification

` Many variations of MRI can achieve different functions


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Reported Adverse Effects

` Mild cutaneous sensation

` Involuntary muscle contractions

` Cardiac arrhythmias` Sensation of light flashes

` Softtissue heating


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Contraindications

Absolute:` Intraoccular foreign body

` Pacemaker

Relative:

` Metallic implants eg.intracranial aneurismal clips

` Cochlearimplants

` Pregnancy (1st trimester)

` Claustrophobia


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1. Otherneural imaging techniques:Diffuse optical imaging, Event-related optical

signal, Electroencephalography,

Magnetoencephalography etc.

2. Physics for medical imaging

3. Radiologic anatomy and pathology

4. Radiologic diagnosis

5. Pharmacology of radiology

neural imaging

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