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.
Major MRI Scanner Vendors
Philips Intera CV
Siemens Sonata
General Electric CV/i
MRI Uses Three Magnetic Fields
• Static High Field (B0) (Chapter 12, Prince)
– Creates or polarizes signal
– 1000 Gauss to 100,000 Gauss
• Earth’s field is 0.5 G
• Radiofrequency Field (B1) (Chapter 12, Prince)
– Excites or perturbs signal into a measurable form
– On the order of O.1 G but in resonance with MR
signal
– RF coils also measure MR signal
– Excited or perturbed signal returns to equilibrium
• Important contrast mechanism
• Gradient Fields ( Chapter 13, Prince)
– 1-4 G/cm
– Used to image: determine spatial position of MR
signal
Nuclear Magnetic Dipole Moment
Magnetic Dipole
Representation
Vector
Representation
Nuclear Magnetic Dipole Moment : Spinning
Charge
N
P
P
N
P
N
Hydrogen
Helium
P
P
Helium-3
No Magnetic Field
=
Random
Orientation
No Net
Magnetization
Classical Physics: Top analogy
Spins in a magnetic field: analogous to a spinning top in a
gravitational field.
Axis of top
gravity
Top precesses about the force caused by gravity
Dipoles (or spins) will precess about the static magnetic
Static Magnetic Field (B0)
Bore
(55 – 60 cm)
Magnetic field (B0)
Body RF
(transmit/receive)
Gradients
Shim
(B0 uniformity)
Reference Frame
y
x
z
Magnetic field (B0) aligned with z (longitudinal axis and
long axis of body)
Main Magnetic Field
B0
Effects of Strong Magnetic Fields
Magnetic Resonance Imaging: Static Field
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
Dipole Moments from Entire Sample
B0
7 up
6 down
Non-Random
Orientation
Sum Dipole Moments -> Bulk Magnetization
Net Magnetization
B0
z
z
M
y
y
x
x
The magnetic dipole moments can be summed to determine
the net or “bulk” magnetization, termed the vector M.
Static Magnetic Field (B0)
Bore
(55 – 60 cm)
Magnetic field (B0)
Body RF
(transmit/receive)
Gradients
Shim
(B0 uniformity)
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)
B1 Radiofrequency Field
Polarized signal is all well and good, but what can we do with it? We
will now see how we can create a detectable signal.
To excite nuclei, tip them away from B0 field by applying a
small rotating B field in the x-y plane (transverse plane). We
create the rotating B field by running a RF electrical signal
through a coil. By tuning the RF field to the Larmor frequency,
a small B field (~0.1 G) can create a significant torque on the
magnetization.
Diagram: Nishimura, Principles of MRI
Exciting the Magnetization Vector
z
B1 tips magnetization towards the transverse plane. Strength and
duration of B1 can be set for any degree rotation. Here a 90
degree rotation leaves M precessing entirely in the xy
(transverse) plane.
Laboratory Reference Frame
Tip Bulk Magnetization
z'
M
y'
x'
B1
Rotating Reference Frame
Imagine you are rotating at Larmor
frequency in transverse plane
Tip Bulk Magnetization
z'
y'
x'
B1
Rotating Reference Frame
Tip Bulk Magnetization
z'
y'
x'
B1
Rotating Reference Frame
Tip Bulk Magnetization
z'
y'
x'
B1
Rotating Reference Frame
Transmit Coils
RF Coil
Demodulate
A/D
Preamp
Static Magnetic Field (B0)
Bore
(55 – 60 cm)
Magnetic field (B0)
Body RF
(transmit/receive)
Gradients
Shim
(B0 uniformity)
Gradient Coils
Fig. Nishimura, MRI Principles
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…
X-Ray/
CT
Cardiovascular
Skeletal / Muscular
+ X-ray – Excellent, with + strong for skeletal system
catheter-injected
contrast
Ultrasound + real-time
+ non-invasive
+ cheap
– but, poorer images
Nuclear
+ functional information
on perfusion
– not used
+ Research in elastography
MR
+ excellent
+ getting better
High resolution
Myocardium viability
+ 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