Download Lecture 1: Introduction (1/1)

University of Wisconsin
Diagnostic Imaging Research
Lecture 1: Introduction (1/2) – History, basic principles, modalities
Class consists of:
1) Deterministic Studies
- distortion
- impulse response
- transfer functions
All modalities are non-linear and space variant to some degree.
Approximations are made to yield a linear, space-invariant system.
2) Stochastic Studies
SNR (signal to noise ratio) of the resultant image
- mean and variance
Wilhelm Röntgen, Wurtzburg
Nov. 1895 – Announces X-ray discovery
Jan. 13, 1896 – Images needle in patient’s hand
– X-ray used presurgically
1901 – Receives first Nobel Prize in Physics
– Given for discovery and use of X-rays.
Radiograph
of the hand of
Röntgen’s
wife, 1895.
Röntgen’s Setup
Röntgen detected:
• No reflection
• No refraction
• Unresponsive to mirrors or lenses
His conclusions:
• X-rays are not an EM wave
• Dominated by corpuscular behavior
Projection X-Ray
attenuation
coefficient
Id  Ioe
μ ( x, y, z )  f (electron density, z)
  μ( x,y,z )dl
Measures line integrals of attenuation
Film shows intensity as a negative ( dark areas, high x-ray detection)
Disadvantage: Depth information lost
Advantage: Cheap, simple
Sagittal
Coronal
Early Developments
• Intensifying agents, contrast agents all developed within
several years.
• Creativity of physicians resulted in significant
improvements to imaging.
- found ways to selectively opacify regions of interest
- agents administered orally, intraveneously, or via catheter
Later Developments
More recently, physicists and engineers have initiated new
developments in technology, rather than physicians.
1940’s, 1950’s
Background laid for ultrasound and nuclear medicine
1960’s
Revolution in imaging – ultrasound and nuclear medicine
1970’s
CT (Computerized Tomography)
- true 3D imaging
(instead of three dimensions crammed into two)
1980’s
MRI (Magnetic Resonance Imaging)
PET ( Positron Emission Tomography)
Computerized Tomography (CT)
Result:
ID ( x, y)  μ( x, y)
1972 Hounsfield announces findings at British Institute of Radiology
1979 Hounsfield, Cormack receive Nobel Prize in Medicine
(CT images computed to actually display attenuation coefficient m(x,y))
Important Precursors:
1917 Radon: Characterized an image by its projections
1961 Oldendorf:
Rotated patient instead of gantry
First Generation CT Scanner
Acquire a projection (X-ray)
Translate x-ray pencil beam and
detector across body and record
output
Rotate to next angle
Repeat translation
Assemble all the projections.
Reconstruction from Back
Projection
1.Filter each projection to account for sampling data on polar grid
2. Smear back along the “line integrals” that were calculated by
the detector.
Modern CT
Scanner
From Webb, Physics of Medical Imaging
Computerized Tomography (CT), continued
Early CT Image
Current technology
Inhalation
Exhalation
Nuclear Medicine
Grew out of the nuclear reactor research of World War II
Discovery of medically useful radioactive isotopes
1948 Ansell and Rotblat: Point by point imaging of thyroid
1952 Anger: First electronic gamma camera
a) Radioactive tracer is selectively taken up by organ of interest
b) Source is thus inside body!
c) This imaging system measures function (physiology)
rather than anatomy.
Example in medical imaging:
Consider a nuclear study of a liver with a tumor point source
at x1= , y1= 
Radiation is detected at the detector plane.
To obtain a general result, we need to know all combinations
h(x2, y2; , )
By “general result”, we mean that we could calculate the image I(x2, y2)
for any source input S(x1, y1)
Nuclear Medicine, continued
Very specific in imaging physiological function - metabolism
- thyroid function
- lung ventilation: inhale agent
Advantage: Direct display of disease process.
Disadvantage: Poor image quality (~ 1 cm resolution)
Why is resolution so poor?
Very small concentrations of agent used for safety.
- source within body
Quantum limited:
CT
109 photons/pixel
Nuclear ~100 photons/pixel
Tomographic systems:
SPECT: single photon emission computerized tomography
PET: positron emission tomography
Combined CT / PET Imaging
Necessary Probe Properties
Probe can be internal or external.
Requirements:
a) Wavelength must be short enough for adequate
resolution.
bone fractures, small vessels < 1 mm
large lesions < 1 cm
b) Body should be semi-transparent to the probe.
transmission > 10-1 - results in contrast problems
transmission < 10 -3 - results in SNR problems
λ > 10 cm
- results in poor resolution
λ < .01Å
- negligible attenuation
Standard X-rays:
.01 Å < λ < .5 Å
corresponding to
~ 25 kev to 1.2 Mev per photon
Necessary Probe Properties: Transmission vs. λ
Graph:
Medical Imaging Systems
Macovski, 1983
Probe properties of different modalities
NMR
• Nuclear magnetic moment ( spin)
• Makes each spatial area produce its own signal
• Process and decode
Ultrasound
• Not EM energy
• Diffraction limits resolution
• resolution proportional to λ
Introduction (2/2) – Comparison of Modalities
Review:
Modalities:
X-ray:
Measures line integrals of attenuation coefficient
CT:
Builds images tomographically; i.e. using a set
of projections
Nuclear: Radioactive isotope attached to metabolic
marker . Strength is functional imaging, as
opposed to anatomical
Ultrasound: Measures reflectivity in the body.
Ultrasound
Ultrasound uses the transmission and reflection of acoustic energy.

prenatal ultrasound image
clinical ultrasound system
Ultrasound
• A pulse is propagated and its reflection is received,
both by the transducer.
• Key assumption:
- Sound waves have a nearly constant velocity
of ~1500 m/s in H2O.
- Sound wave velocity in H2O is similar to that in soft tissue.
• Thus, echo time maps to depth.
Ultrasound: Resolution and Transmission Frequency
Tradeoff between resolution and attenuation -
↑higher frequency ↓shorter wavelength
↑ higher attenuation
dB
Power loss: 1
cm  MHz
Typical Ultrasound Frequencies:
Deep Body
1.5 to 3.0 MHz
Superficial Structures
5.0 to 10.0 MHz
e.g. 15 cm depth, 2 MHz, 60 dB round trip
Why not use a very strong pulse?
• Ultrasound at high energy can be used to ablate (kill) tissue.
• Cavitation (bubble formation)
• Temperature increase is limited to 1º C for safety.
Magnetic Resonance Imaging
Main Magnetic Field
B0
Magnetic Resonance Imaging
There are 3 magnetic fields of interest in MRI.
The first is the static field Bo.
1) polarizes the sample:
M( x,y,z)   ( x,y,z)
2) creates the resonant frequency:
γ is constant for each nucleus: γ
2π
density of 1H
ω = γB
 42.57 MHz/Tesla for 1H
Proton Spin Creates Signal
Source
B0
B0
w = gB
64 MHz for H+ at 1.5T
Second Magnetic Field : RF Field
B1
An RF coil around the patient transmits a pulse of power at the
resonant frequency ω to create a B field orthogonal to Bo.
This second magnetic field is termed the B1 field.
B1 field “excites” nuclei.
Excited nuclei precess at ω(x,y,z) = γB (x,y,z)
Transmit Coils
RF Coil
Demodulate
A/D
Preamp
Spin Encoding
Magnetic Resonance
The spatial location is encoded by using gradient field coils around
the patient. (3rd magnetic field) Running current through these
coils changes the magnitude of the magnetic field in space and
thus the resonant frequency of protons throughout the body.
Spatial positions is thus encoded as a frequency.
The excited photons return to equilibrium ( relax) at different rates.
By altering the timing of our measurements, we can create
contrast. Multiparametric excitation – T1, T2
Brain Glioma
Non-contrast-enhanced MRI
Sagittal Carotid
Coronal
Contrast-enhanced Abdominal Imaging
Time-resolved Abdominal Imaging
Contrast-enhanced MR Cardiac Imaging
Fat Coronal Knee Image
Water Coronal Knee Image
Comparison of modalities
Why do we need multiple modalities?
Each modality measures the interaction between energy and
biological tissue.
- Provides a measurement of physical properties of tissue.
- Tissues similar in two physical properties may differ in a
third.
Note:
- Each modality must relate the physical property it measures
to normal or abnormal tissue function if possible.
- However, anatomical information and knowledge of a large
patient base may be enough.
- i.e. A shadow on lung or chest X-rays is likely not good.
Other considerations for multiple modalities include:
- cost
- safety
- portability/availability
Comparison of modalities:
X-Ray
Measures attenuation coefficient μ ( x, y, z )
Safety: Uses ionizing radiation
- risk is small, however, concern still present.
- 2-3 individual lesions per 106
- population risk > individual risk
i.e. If exam indicated, it is in your interest to get exam
Use: Principal imaging modality
Used throughout body
Distortion: X-Ray transmission is not distorted.
Comparison of modalities:
Ultrasound
Measures acoustic reflectivity
R( x, y, z )
Safety: Appears completely safe
Use: Used where there is a complete soft tissue and/or fluid path
Severe distortions at air or bone interface
Distortion:
Reflection: Variations in c (speed) affect depth estimate
Diffraction: λ ≈ desired resolution (~.5 mm)
Comparison of modalities:
Magnetic Resonance (MR)
Multiparametric
M(x,y,z) proportional to ρ(x,y,z) and T1, T2.
(the relaxation time constants)
Velocity sensitive
Safety: Appears safe
Static field - No problems
dB
 10 T/s - Some induced phosphenes
dt
Higher levels - Nerve stimulation
RF heating: body temperature rise < 1˚C - guideline
Use:
Distortion:
Some RF penetration effects
- intensity distortion
Clinical Applications - Table
Chest
+ widely used
+ CT - excellent
Abdomen
– needs contrast
+ CT - excellent
Ultrasound – no, except for
+ heart
+ excellent
– problems with
gas
Merge w/ CT
X-Ray/
CT
Nuclear
+ extensive use
in heart
MR
+ growing
cardiac
applications
+ minor role
Head
+ X-ray - is good
for bone
– CT - bleeding,
trauma
– poor
+ PET
+ standard
Clinical Applications – Table continued…
Cardiovascular
X-Ray/
+ X-ray – Excellent, with
CT
catheter-injected
contrast
Ultrasound + real-time
+ non-invasive
+ cheap
– but, poorer images
Nuclear
+ functional information
on perfusion
Skeletal / Muscular
+ strong for skeletal
system
MR
+ excellent
+ getting better
High resolution
Myocardium viability
– not used
+ functional - bone marrow
Economics of modalities:
Ultrasound: ~ $100K – $250K
CT: $400K – $1.5 million (helical scanner)
MR: $350K (knee) - 4.0 million (siting)
Service: Annual costs
Hospital must keep uptime
Staff:
Scans performed by technologists
Hospital Income: Competitive issues
Significant investment and return