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
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 uses the transmission and reflection of acoustic energy.
prenatal ultrasound image
clinical ultrasound system
both by the transducer.
- 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.
Tradeoff between resolution and attenuation -
↑higher frequency ↓shorter wavelength ↑ higher attenuation
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?
Philips Intera CV
General Electric CV/i
Spins in a magnetic field: analogous to a spinning top in a gravitational field.
Axis of top
Top precesses about the force caused by gravity
Dipoles (or spins) will precess about the static magnetic field
Magnetic field (B0) aligned with z (longitudinal axis and
long axis of body)
There are 3 magnetic fields of interest in MRI.
The first is the static field Bo.
1) polarizes the sample:
2) creates the resonant frequency:
γ is constant for each nucleus:
density of 1H
ω = γB
The magnetic dipole moments can be summed to determine
the net or “bulk” magnetization, termed the vector M.
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) = γBo (x,y,z)
B1 Radiofrequency Field
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.
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.
Diagram: Nishimura, Principles of MRI
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
1Tip Bulk Magnetization
Rotating Reference FrameImagine you are rotating at Larmor frequency in transverse plane
Fig. Nishimura, MRI Principles
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
Water Coronal Knee Image
Why do we need multiple modalities?
Each modality measures the interaction between energy and
- Provides a measurement of physical properties of tissue.
- Tissues similar in two physical properties may differ in a third.
- 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
Measures attenuation coefficient
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.
Measures acoustic reflectivity
Safety: Appears completely safe
Use: Used where there is a complete soft tissue and/or fluid path
Severe distortions at air or bone interface
Reflection: Variations in c (speed) affect depth estimate
Diffraction: λ ≈ desired resolution (~.5 mm)
M(x,y,z) proportional to ρ(x,y,z) and T1, T2.
(the relaxation time constants)
Safety: Appears safe
Static field - No problems
-Some induced phosphenes
Higher levels - Nerve stimulation
RF heating: body temperature rise < 1˚C - guideline
Distortion: Some RF penetration effects
- intensity distortion
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