Introduction 22 Comparison of Modalities Review Modalities Xray

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 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 H 2 O. - Sound wave velocity in H 2 O 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 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 d. B 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 (B 0) (Chapter 12, Prince) – Creates or polarizes signal – 1000 Gauss to 100, 000 Gauss • Earth’s field is 0. 5 G • Radiofrequency Field (B 1) (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 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 (B 0) Bore (55 – 60 cm) Magnetic field (B 0) Body RF (transmit/receive) Gradients Shim (B 0 uniformity)

Reference Frame y x z Magnetic field (B 0) aligned with z (longitudinal axis and long axis of body)

Main Magnetic Field B 0

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: density of 1 H 2) creates the resonant frequency: γ is constant for each nucleus: ω = γB

Dipole Moments from Entire Sample B 0 7 up 6 down Non-Random Orientation

Sum Dipole Moments -> Bulk Magnetization Net Magnetization B 0 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 (B 0) Bore (55 – 60 cm) Magnetic field (B 0) Body RF (transmit/receive) Gradients Shim (B 0 uniformity)

Second Magnetic Field : RF Field B 1 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 B 1 field “excites” nuclei. Excited nuclei precess at ω(x, y, z) = γB (x, y, z)

B 1 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 B 0 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 B 1 tips magnetization towards the transverse plane. Strength and duration of B 1 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' B 1 Rotating Reference Frame Imagine you are rotating at Larmor frequency in transverse plane

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

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

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

Transmit Coils RF Coil Demodulate A/D Preamp

Static Magnetic Field (B 0) Bore (55 – 60 cm) Magnetic field (B 0) Body RF (transmit/receive) Gradients Shim (B 0 uniformity)

Gradient Coils Fig. Nishimura, MRI Principles

Spin Encoding

Magnetic Resonance The spatial location is encoded by using gradient field coils around the patient. (3 rd 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 – T 1, T 2

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 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 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 T 1, T 2. (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

Clinical Applications - Table Chest + widely used + CT - excellent Abdomen – needs contrast + CT - excellent Head + X-ray - is good for bone – CT - bleeding, trauma Ultrasound – no, except for + heart + excellent – problems with gas Merge w/ CT – poor + minor role + standard X-Ray/ CT Nuclear + extensive use in heart MR + growing cardiac applications + PET

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: ~ $100 K – $250 K CT: $400 K – $1. 5 million (helical scanner) MR: $350 K (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