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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

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Introduction 2 2 comparison of modalities l.jpg
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.


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Ultrasound

Ultrasound uses the transmission and reflection of acoustic energy.

prenatal ultrasound image

clinical ultrasound system


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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.


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Ultrasound: Resolution and Transmission Frequency

Tradeoff between resolution and attenuation -

↑higher frequency ↓shorter wavelength ↑ higher attenuation

Power loss:

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.


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Major MRI Scanner Vendors

Philips Intera CV

Siemens Sonata

General Electric CV/i


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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


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Nuclear Magnetic Dipole Moment

Vector

Representation

Magnetic Dipole

Representation


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Nuclear Magnetic Dipole Moment : Spinning Charge

N

N

P

P

P

P

P

N

Hydrogen

Helium

Helium-3


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No Magnetic Field

=

No Net

Magnetization

Random

Orientation


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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 field


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Static Magnetic Field (B0)

Body RF

(transmit/receive)

Bore

(55 – 60 cm)

Gradients

Shim

(B0 uniformity)

Magnetic field (B0)


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Reference Frame

y

x

z

Magnetic field (B0) aligned with z (longitudinal axis and

long axis of body)




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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:

2) creates the resonant frequency:

γ is constant for each nucleus:

density of 1H

ω = γB


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Dipole Moments from Entire Sample

B0

7 up

6 down

Non-Random

Orientation


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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.


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Static Magnetic Field (B0)

Body RF

(transmit/receive)

Bore

(55 – 60 cm)

Gradients

Shim

(B0 uniformity)

Magnetic field (B0)


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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) = γBo (x,y,z)


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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


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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


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M

B

1

Tip Bulk Magnetization

z'

y'

x'

Rotating Reference FrameImagine you are rotating at Larmor frequency in transverse plane


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B

1

Tip Bulk Magnetization

z'

y'

x'

Rotating Reference Frame


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B

1

Tip Bulk Magnetization

z'

y'

x'

Rotating Reference Frame


Tip bulk magnetization25 l.jpg

B

1

Tip Bulk Magnetization

z'

y'

x'

Rotating Reference Frame


Slide26 l.jpg

Transmit Coils

RF Coil

Demodulate

A/D

Preamp


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Static Magnetic Field (B0)

Body RF

(transmit/receive)

Bore

(55 – 60 cm)

Gradients

Shim

(B0 uniformity)

Magnetic field (B0)


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Gradient Coils

Fig. Nishimura, MRI Principles



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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



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Non-contrast-enhanced MRI

Sagittal Carotid

Coronal





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Fat Coronal Knee Image

Water Coronal Knee Image


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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


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Comparison of modalities:X-Ray

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.


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Comparison of modalities:Ultrasound

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

Distortion:

Reflection: Variations in c (speed) affect depth estimate

Diffraction: λ ≈ desired resolution (~.5 mm)


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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

-Some induced phosphenes

Higher levels - Nerve stimulation

RF heating: body temperature rise < 1˚C - guideline

Use:

Distortion: Some RF penetration effects

- intensity distortion




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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


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