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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?
Visible
Electromagnetic Spectrum
Visible
Endoscopy
Laparoscopy - ovary
X-RAYs
Roentgen :
1895 Discovered X-rays
1901 Nobel Prize
Advantages of X-Ray
-cheap
- fast
- good diagnostic value for many things
Disadvantages
- ionizing radiation
- contrast is just density differences
X-RAYs
Electromagnetic Spectrum
How X-rays work
Simple Fit
Number of protons
Roughly Proportional to mass
X-ray Radiography - 2D (ie Chest)
Advantage
very fast
high resolution
Disadvantage
ionizing radiation
xray contrast
X-ray Radiography - 2D
100KVp
X-ray Radiography - 2D
Compton
Scattering
(Photoelectric
effect)
[everything going digital now]
X-ray Radiography - 2D
Bone healing study
on rats
Mammography
Low energy X-ray b/c
all tissue
“Pretty pictures, but they will never
replace radiographs” –
Neuroradiologist 1972
(X-ray) CT – computed tomography – 3D
Godfrey Hounsfield 1972
(nobel prize 1979)
Advantage
high resolution 1mm x0.4mm x 0.4mm
3D
Disadvantage
ionizing radiation
xray contrast
Axial - abdomen
Axial - head
What is tomography
Red dots are areas of high density
Peaks are number of Xrays absorbed
(note : normally would do axially and not sagittally)
(X-ray) CT – computed tomography – 3D
Axial - head
Substance
Axial - abdomen
HU
Air
-1000
Fat
-120
Water
0
Muscle
+40
Bone
+1000
Houndsfield
Units
Why is water in brain dark compared
to 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?
Image Feature
Property
Sample tissues
Hypodensity
(dark)
Not much x-ray Air, fat, water,
absorbed
CSF
Hyperdensity
(bright)
Lots of x-ray
absorbed
Isodensity
(gray)
Some x-ray
absorbed
Bone, newly
congealed
blood
Gray matter,
white matter
(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
CT - Hemmorage
(Xray) Flouroscopy – 2D
Xray
advantage :
Realtime Imaging
Heart
Blood Flow
Surgery
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
Advantages
better resolution
(smaller detectors
source closer to detector)
Disadvantage
small :}
mouse microCT/PET
13um resolution mouse
placenta vasculature
PET – positron emission tomography
Electromagnetic Spectrum
PET – positron emission tomography
Inject Patient with Radioactive Drug
Late 1960’s
Drug travels to metabolically active sites (many
tumors have high metabolic activity)
Drug emits (+) positrons (basically a positively
charged electron)
FDG - Fluorodeoxyglucose (most common drug)
(F18 – + emitter – two hour half-life)
Advantage
functional imaging
Disadvantage
some ionizing radiation
low resolution (4mm x 4mm x 4mm)
need to make/buy FDG (cyclotron)
PET/CT - together
PET
CT
(Xray) CT
PET – positron emission tomography
• 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
Treated Tumor
growing again
on periphery
PET – positron emission tomography
functional brain activity (mostly done with MRI now)
PET/CT - together
PET/CT - together
CT
PET
PET
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 noise
good
bad
Electromagnetic
Spectrum
Ultrasound
Discovered (Norris) 1952, clinical 1962
Sound waves 1-15MHz (ear 20 – 20KHz)
Echos (reflections) from different density interfaces
are recorded
Image soft tissue and blood flow (Doppler)
Advantages:
high resolution (mm)
cheap
real time imaging
safe
Disadvantages:
skilled technician & interpretation
small field of view (~20cm)
bone and air problematic
Ultrasound
Typical ultra sound – sound reflections off surface
Fetocopsy
Image
Example
probe
Arterial Blood Flow
MRI – Magnetic Resonance Imaging
Electromagnetic Spectrum
MRI – Magnetic Resonance Imaging
Mansfield and Lauterbur nobel prize
1978 first images
1st published MRI
images of abdomen
First brain MR
Modern T2 image
“Interesting images, but will never be as
useful as CT”
neuroradiologist, 1982
3 Tesla MRI Scanner
MRI
Advantages
safe
great soft tissue contrast
many contrast options
Disadvantages
expensive
long time
bad for bones
mediocre resolution
3 Tesla Magnetic
Field (60,000 times
Earths field)
B0
B0
MRI
3 Tesla magnet field
MRI
Not all the protons
line up – thermal
energy
Protons (hydrogen
nuclei act like little
magnets)
B0
Collective Magnetic
Moment of Protons
MRI
Stage I
Excite
Radio Waves
B0
end
start
Collective Magnetic
Moment of Protons
Make image based on
Protons loosing energy
Make image based on
Protons dephasing
MRI
Stage II
listen
start
Fat and
water loose
energy and
dephase at
different
rates
end
T1 (energy lose time constant)
Imaging
Slow Precession
Fast
Precession
T2 (dephasing time constant)
Imaging
Axial MRI Head
bright
dark
Water
bright
Fat
dark
CT versus MRI
CT
MRI
+Excellent bone imaging
+Excellent grey/white matter
contrast & spatial resolution
+Excellent new acute
hemorrhage detection
+Skull fracture, calcified
lesion
+Better for old hemorrhage
(and new with Diffusion?)
-Long scan time
+Short scan time, metal
devices allowed
-Pts cannot have metal
devices
-Poor contrast and
resolution
-Claustrophobia, obesity
problems
-Radiation
+No radiation
- expensive
MRI: “Normal” Anatomy
corpus callosum
fornix
thalamus
midbrain
pons
medulla
MRI: “Normal” Anatomy
superior frontal g.
precuneus
cingulate g.
cuneus
lingual g.
g. rectus
MRI: Imaging deep structures
(thalamus and basal ganglia)
thalamus
Caudate nucleus
Putamen &
globus pallidus
MRI
Disease
Multiple Sclerosis – Active Lessions
(basically edema – water)
Tumor (can be combination of
Edema and tumor tissue characteristics)
Why MRI : Detection of Acute Stroke
MRI
“Diffusion Weighted Imaging (DWI) has proven to be the most effective
means of detecting early strokes” Lehigh Magnetic Imaging Center
Conventional T2 WI
DW-EPI (advanced technique)
Sodium ion pumps fail, water goes in cells and can not diffuse.
MRI
Brain Injury
MRA – magnetic resonance angiogram
MRI
Single slice from MRA
Excite
Protons
Wait then
Listen to
Protons
MRI
Stack the slices to produce 3D image
MRA (arteries)
MRV (Veins) – reverse
excite and listen slices
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
3s
2s
4s
Xray: Imaging Vasculature
1. Obtain
scout
2. Reverse
image of
scout =
“mask”
4. Take
second
image
5. Subtract
second
image from
mask
3. Inject
contrast
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
r. Internal carotid injection
Lateral view
X-ray angiography
MCA and
branches
ACA and
branches
ophthalmic artery
carotid siphon
internal carotid
l. vertebral injection
Lateral view
X-ray angiography
l. vertebral injection
Lateral view
X-ray angiography
PCA and
branches
basilar artery
vertebral artery
Posterior inf.
cerebellar artery
Venous sinuses
Lateral view
X-ray angiography
Venous sinuses
Lateral view
X-ray angiography
Superior
sagittal sinus
Superior
sagittal sinus
Great cerebral
vein of Galen
Straight
sinus
Confluence
of sinuses
Jugular vein & bulb
Anterior view
MRA
Anterior view
MRA
MCA
ACA
MCA
Internal carotid
Carotid siphon
Basilar artery
r. Internal carotid injection
AP view
X-ray angiography
r. Internal carotid injection
AP view
ACA
X-ray angiography
Carotid siphon
MCA
Internal carotid
l. Vertebral artery injection
AP view
X-ray angiography
l. Vertebral artery injection
AP view
X-ray angiography
PCA
vertebral artery
r. Internal carotid injection
AP view
X-ray angiography
r. Internal carotid injection
Superior
sagittal sinus
AP view
X-ray angiography
Confluence
of sinuses
Transverse
sinus
Sigmoid
sinus
Jugular vein
& bulb
Unused slides
Sound – density
determines reflection
Sound reflections
like light (E&M) – index of refraction
determines reflection for light (E&M)
Sonar = 10 – 200KHz
Incident
Reflected
Refracted (penetrated)
Ultrasound
Doppler (frequency shift due to movement)
Arterial blood flow
Heart Valve functionality