Overview of medical imaging: Focusing on Neuroimaging

Overview of medical imaging:Focusing on Neuroimaging

Medical imaging • Using the Electromagnetic Spectrum

– Visible light

– X-ray, Fluoroscopy, CT, & Angiography

– gamma rays - PET (positron emission tomography)

– Radio waves from nuclear spin – MRI

• Sound waves - ultrasound

Electromagnetic wave (could use photon picture b/c wave-particle duality)

one wavelength

Wave Particle duality of electromagnetic radiation

Electromagnetic Spectrum

Guiding Questions

1. What is the energy used?

2. How does the energy interact with tissues?

3. How is the image produced?

4. What is represented in the image?

Electromagnetic Spectrum

Visible

Laparoscopy - ovaryEndoscopy

Visible

X-RAYs

Roentgen : 1895 Discovered X-rays1901 Nobel Prize

Advantages of X-Ray-cheap- fast- good diagnostic value for many things

Disadvantages- ionizing radiation- contrast is just density differences

Electromagnetic Spectrum

X-RAYs

How X-rays work

Number of protonsRoughly Proportional to mass

Simple Fit

X-ray Radiography - 2D (ie Chest)

Advantage very fast high resolution

Disadvantage ionizing radiation xray contrast

100KVp

X-ray Radiography - 2D

(Photoelectric effect)

ComptonScattering

X-ray Radiography - 2D

[everything going digital now]

X-ray Radiography - 2D

MammographyLow energy X-ray b/c

all tissue

Bone healing study on rats

(X-ray) CT – computed tomography – 3D

Godfrey Hounsfield 1972(nobel prize 1979)

“Pretty pictures, but they will never replace radiographs” –Neuroradiologist 1972

Axial - head

Axial - abdomen

Advantage high resolution 1mm x0.4mm x 0.4mm 3DDisadvantage ionizing radiation xray contrast

What is tomography

Red dots are areas of high densityPeaks are number of Xrays absorbed(note : normally would do axially and not sagittally)

Axial - head Axial - abdomen

Substance HU

Air -1000

Fat -120

Water 0

Muscle +40

Bone +1000

HoundsfieldUnits

Why is water in brain dark comparedto brain tissue?

(X-ray) CT – computed tomography – 3D

CT: What does the image represent?

hyperdensity

hypodensity

isodensity

(X-ray) CT – computed tomography – 3D

CT: What does the image represent?

Hypodensity (dark)

Not much x-ray absorbed

Air, fat, water, CSF

Hyperdensity (bright)

Lots of x-ray absorbed

Bone, newly congealed blood

Isodensity (gray)

Some x-ray absorbed

Gray matter, white matter

Image Feature Property Sample tissues

(X-ray) CT – computed tomography – 3D

CT: What does the image represent?

(X-ray) CT – computed tomography – 3D

CT: What does the image represent?

(X-ray) CT – computed tomography – 3D

(X-ray) CT – computed tomography – 3D

CT - Hemmorage

(Xray) Flouroscopy – 2D

Xray

Realtime ImagingHeartBlood FlowSurgery

advantage :

disadvantage: high radiation dose

(Xray) CONTRAST – Radiograph, CT, or Flouroscopy

Barium Swallow

Injection (Iodine Compound)

Angiogram

(X-ray) CT – computed tomography – 3D

(X-ray) CT – computed tomography – 3D

13um resolution mouseplacenta vasculature

Advantages better resolution (smaller detectors source closer to detector) Disadvantage small :}

mouse microCT/PET

Electromagnetic Spectrum

PET – positron emission tomography

Inject Patient with Radioactive Drug

Late 1960’s

Drug travels to metabolically active sites (manytumors have high metabolic activity)

Drug emits (+) positrons (basically a positivelycharged electron)

FDG - Fluorodeoxyglucose (most common drug)(F18 – + emitter – two hour half-life)

Advantagefunctional imaging

Disadvantagesome ionizing radiationlow resolution (4mm x 4mm x 4mm)need to make/buy FDG (cyclotron)

PET – positron emission tomography

CT

PET/CT - together

(Xray) CT PET

• β+ decay, positron travels several mm and collides with an electron

• produce a pair of annihilation photons (511kev, 180o)

• simultaneous detection 180o apart

PET – positron emission tomography

Abnormal FDG collection

PET – positron emission tomography

Treated Tumorgrowing againon periphery

PET – positron emission tomography

functional brain activity (mostly done with MRI now)

PET/CT - together

PET/CT - together

PETPETCT PET/CT

microPET/CT– positron emission tomography

Advantages better resolution (smaller detectors source closer to detector)

Disadvantage small :}

physics note: signal on expanding sphere drops as 1/R2 (surface area of sphere), therefore closer is better

signal to noisegood bad

Electromagnetic Spectrum

Ultrasound

Discovered (Norris) 1952, clinical 1962

Sound waves 1-15MHz (ear 20 – 20KHz)

Echos (reflections) from different density interfacesare recorded

Image soft tissue and blood flow (Doppler)

Advantages:high resolution (mm)

cheapreal time imagingsafe

Disadvantages:skilled technician & interpretationsmall field of view (~20cm)bone and air problematic

Ultrasound

Typical ultra sound – sound reflections off surface

Fetocopsy ImageExample

probe

Arterial Blood Flow

Electromagnetic Spectrum

MRI – Magnetic Resonance Imaging

Mansfield and Lauterbur nobel prize1978 first images

MRI – Magnetic Resonance Imaging

1st published MRI images of abdomen

3 Tesla MRI Scanner

“Interesting images, but will never be as useful as CT”neuroradiologist, 1982

First brain MR First brain MR Modern T2 imageModern T2 image

MRI

Advantages Disadvantagessafe expensivegreat soft tissue contrast long timemany contrast options bad for bones

mediocre resolution

3 Tesla MagneticField (60,000 timesEarths field)

MRI

B0

B0

3 Tesla magnet field

B0

Protons (hydrogennuclei act like littlemagnets)

Not all the protonsline up – thermal energy

MRI

Collective MagneticMoment of Protons

MRIStage IExcite

B0

Radio Waves

Collective MagneticMoment of Protons

start

end

MRIStage IIlisten

start

end

Make image based onProtons loosing energy

Make image based onProtons dephasing

Slow Precession

FastPrecession

Fat andwater looseenergy anddephase atdifferent rates

T1 (energy lose time constant)Imaging T2 (dephasing time constant)

Imaging

Axial MRI Head

Water

Fat

bright

bright dark

dark

CT versus MRI

CT

+Excellent bone imaging

+Excellent new acute hemorrhage detection

+Skull fracture, calcified lesion

+Short scan time, metal devices allowed

-Poor contrast and resolution

-Radiation

MRI

+Excellent grey/white matter contrast & spatial resolution

+Better for old hemorrhage (and new with Diffusion?)

-Long scan time

-Pts cannot have metal devices

-Claustrophobia, obesity problems

+No radiation

- expensive

MRI: “Normal” Anatomy

corpus callosum

fornix

thalamus

midbrain

pons

medulla

MRI: “Normal” Anatomy

g. rectus

cingulate g.

cuneus

lingual g.

precuneussuperior frontal g.

MRI: Imaging deep structures(thalamus and basal ganglia)

thalamus Caudate nucleus Putamen & globus pallidus

Multiple Sclerosis – Active Lessions(basically edema – water)

MRI

Tumor (can be combination ofEdema and tumor tissue characteristics)

Disease

Conventional TConventional T22 WI WI DW-EPI (advanced technique)DW-EPI (advanced technique)

Why MRI : Detection of Acute StrokeWhy MRI : Detection of Acute Stroke

“Diffusion Weighted Imaging (DWI) has proven to be the most effective means of detecting early strokes” Lehigh Magnetic Imaging Center

Sodium ion pumps fail, water goes in cells and can not diffuse.

MRI

MRI

Brain Injury

MRA – magnetic resonance angiogram MRI

Excite Protons

Wait then Listen to Protons

Single slice from MRA

MRA (arteries)MRV (Veins) – reverseexcite and listen slices

MRIStack the slices to produce 3D image

Angiography

• Refers to imaging of blood vessels

• Several types:

•conventional x-ray angiography

•Spiral / helical CT angiography

•magnetic resonance angiography

X-Ray Angiography

• inject pt. With contrast agent (e.g. sodium iodide)

• take series of images at intervals following injection (e.g. 1-second intervals)

• early images show arteries; later images show veins

Xray: Imaging Vasculature

1s

2s

3s

4s

Xray: Imaging Vasculature

1. Obtain scout

3. Inject contrast

2. Reverse image of scout = “mask”

4. Take second image

5. Subtract second image from mask

MR Angiography

• often don’t need contrast agent

• pulse sequences accentuate flowing tissues and minimize contrast from stationary ones

• usually both arteries and veins are shown together (but can be separated)

MR Angiography

r. Internal carotid injection

Lateral view

X-ray angiography

carotid siphon internal carotid

ophthalmic artery

ACA and branches

MCA and branches

r. Internal carotid injection

Lateral view

X-ray angiography

l. vertebral injection

Lateral view

X-ray angiography

Posterior inf. cerebellar arteryvertebral artery

basilar artery

PCA and branches

l. vertebral injection

Lateral view

X-ray angiography

Venous sinuses

Lateral view

X-ray angiography

Jugular vein & bulb

Straight sinus

Venous sinuses

Lateral view

X-ray angiography

Great cerebral vein of Galen

Superior sagittal sinus

Superior sagittal sinus

Confluence of sinuses

Anterior view

MRA

Anterior view

MRA

Basilar artery

Internal carotid

MCAACA

MCA

Carotid siphon

r. Internal carotid injection

AP view

X-ray angiography

r. Internal carotid injection

AP view

X-ray angiography

Internal carotidMCA

ACA

Carotid siphon

l. Vertebral artery injection

AP view

X-ray angiography

l. Vertebral artery injection

AP view

X-ray angiography

vertebral artery

PCA

r. Internal carotid injection

AP view

X-ray angiography

r. Internal carotid injection

AP view

X-ray angiography

Jugular vein & bulb

Superior sagittal sinus

Transverse sinus

Confluence of sinuses

Sigmoid sinus

Unused slides

Sound reflectionsSound – densitydetermines reflection

like light (E&M) – index of refractiondetermines reflection for light (E&M)

Sonar = 10 – 200KHz

Incident

Refracted (penetrated)

Reflected

Ultrasound

Doppler (frequency shift due to movement)

Heart Valve functionality

Arterial blood flow