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Introduction (2/2) – Comparison of Modalities

Introduction (2/2) – Comparison of Modalities

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Introduction (2/2) – Comparison of Modalities

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  1. 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.

  2. Ultrasound Ultrasound uses the transmission and reflection of acoustic energy.  prenatal ultrasound image clinical ultrasound system

  3. 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.

  4. 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.

  5. Major MRI Scanner Vendors Philips Intera CV Siemens Sonata General Electric CV/i

  6. 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

  7. Nuclear Magnetic Dipole Moment Vector Representation Magnetic Dipole Representation

  8. Nuclear Magnetic Dipole Moment : Spinning Charge N N P P P P P N Hydrogen Helium Helium-3

  9. No Magnetic Field = No Net Magnetization Random Orientation

  10. 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

  11. Static Magnetic Field (B0) Body RF (transmit/receive) Bore (55 – 60 cm) Gradients Shim (B0 uniformity) Magnetic field (B0)

  12. Reference Frame y x z Magnetic field (B0) aligned with z (longitudinal axis and long axis of body)

  13. Main Magnetic Field B0

  14. Effects of Strong Magnetic Fields

  15. 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

  16. Dipole Moments from Entire Sample B0 7 up 6 down Non-Random Orientation

  17. 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.

  18. Static Magnetic Field (B0) Body RF (transmit/receive) Bore (55 – 60 cm) Gradients Shim (B0 uniformity) Magnetic field (B0)

  19. 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)

  20. 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

  21. 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

  22. M B 1 Tip Bulk Magnetization z' y' x' Rotating Reference FrameImagine you are rotating at Larmor frequency in transverse plane

  23. B 1 Tip Bulk Magnetization z' y' x' Rotating Reference Frame

  24. B 1 Tip Bulk Magnetization z' y' x' Rotating Reference Frame

  25. B 1 Tip Bulk Magnetization z' y' x' Rotating Reference Frame

  26. Transmit Coils RF Coil Demodulate A/D Preamp

  27. Static Magnetic Field (B0) Body RF (transmit/receive) Bore (55 – 60 cm) Gradients Shim (B0 uniformity) Magnetic field (B0)

  28. Gradient Coils Fig. Nishimura, MRI Principles

  29. Spin Encoding

  30. 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

  31. Brain Glioma

  32. Non-contrast-enhanced MRI Sagittal Carotid Coronal

  33. Contrast-enhanced Abdominal Imaging

  34. Time-resolved Abdominal Imaging

  35. Contrast-enhanced MR Cardiac Imaging

  36. Fat Coronal Knee Image Water Coronal Knee Image

  37. 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

  38. 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.

  39. 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)

  40. 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

  41. Clinical Applications - Table

  42. Clinical Applications – Table continued…

  43. 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