The physics US and MRI Prof. Peter Bogner
Sound waves mechanical disturbance, a pressure wave moves along longitudinal wave compression rarefaction zones c = nl, (c: velocity, n: frequency, l: wavelength diagnostic range: 1-100 MHz
Ultrasound Production Transducer produces ultrasound pulses (transmit 1% of the time) These elements convert electrical energy into a mechanical ultrasound wave Reflected echoes return to the scanhead which converts the ultrasound wave into an electrical signal
Characteristics of ultrasound velocity 1540 m/s in soft tissues frequency depends on trancducer wavelength 2.25 MHz = 0,6 micron 5.0 MHz = 0,31 micron 10 MHz = 0,15 micron amplitude intensity of ultrasound
Frequency vs. Resolution The frequency also affects the QUALITY of the ultrasound image The HIGHER the frequency, the BETTER the resolution The LOWER the frequency, the LESS the resolution A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz Low Frequency 3 MHz High Frequency 12 MHz
Image Formation Electrical signal produces dots on the screen Brightness of the dots is proportional to the strength of the returning echoes Location of the dots is determined by travel time. The velocity in tissue is assumed constant at 1540m/sec Distance = Velocity Time
Ultrasound velocity Relaitvely low in gases higher in solids Velocity changes with the wavelength, frequency remains approx. constant c = E/ (E ~ elasticity, ~ density) Variation in velocity artifacts
Interactions of Ultrasound with Tissue Reflection Refraction Transmission Attenuation
Interactions of Ultrasound with Tissue Reflection The ultrasound reflects off tissue and returns to the transducer, the amount of reflection depends on differences in acoustic impedance acoustic impedance ~ density + elasticity (similarly to velocity) The ultrasound image is formed from reflected echoes transducer
Interactions of Ultrasound with Tissue Transmission Some of the ultrasound waves continue deeper into the body These waves will reflect from deeper tissue structures transducer
Interactions of Ultrasound with Tissue Attenuation Defined - the deeper the wave travels in the body, the weaker it becomes -3 processes: reflection, absorption, refraction Air (lung)> bone > muscle > soft tissue >blood > water
Reflected Echo s Strong Reflections = White dots Diaphragm, tendons, bone Hyperechoic
Reflected Echo s Weaker Reflections = Grey dots Most solid organs, thick fluid isoechoic
Reflected Echo s No Reflections = Black dots Fluid within a cyst, urine, blood Hypoechoic or echofree
Transducer
What determines how far ultrasound waves can travel? The FREQUENCY of the transducer The HIGHER the frequency, the LESS it can penetrate The LOWER the frequency, the DEEPER it can penetrate Attenuation is directly related to frequency
Ultrasound Beam Profile Beam comes out as a slice Beam Profile Approx. 1 mm thick Depth displayed user controlled Image produced is 2D tomographic slice assumes no thickness You control the aim 1mm
Accomplishing this goal depends upon... Resolving capability of the system axial/lateral resolution spatial resolution contrast resolution temporal resolution Processing Power ability to capture, preserve and display the information
Types of Resolution Temporal Resolution the ability to accurately locate the position of moving structures at particular instants in time also known as frame rate
Types of Resolution Contrast Resolution the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects - dependent on the dynamic range
Presentation modes A(mplitude) B(rightness) M(otion) doppler A B
Presentation modes M doppler
MRI - magnetic resonance imaging Sir Peter Mansfield Paul C. Lauterbur The Nobel Prize in Medicine 2003
Damadian R Tumor detection by nuclear magnetic resonance. Science 1971, 171:1151-3.
Hydrogen without magnetic field Normally, spins are randomly oriented No net magnetization (M=0)
Hydrogen in magnetic field Two possible orientations Paralell: lower-energy state Anti-paralell: higher-energy state Natural systems tend to have minimal energy Thermal agitation : only small imbalance
Hydrogen in magnetic field At 1.5 T only 1 spin in a 1 000 000 contribute to the net magnetization (M) M
Larmor equation w=gxb 0
Excitation
Signal detection
Signal detection
Relaxation in z direction: T1
Relaxation in the x-y plane: T2
spin echo (SE) pulse sequence
sequence parameters TR 90º TE TR: time to repeat TE: time to echo
Basis of imaging : spatial localization
Gradient magnetic field Magnetic field gradients are used to change the strength of the main magnetic field (B0) Different spatial locations become associated with different precession frequencies
Spatial encoding It is done using magnetic gradients Slice selection - z Frequency encoding - x Phase encoding - y
Slice selection Based on Larmour equation: ω 0 =γb 0 The central frequency of RF pulse determines the particular location excited Different slice positions are achieved by changing the central frequency Hydrogen nuclei located outside the slice plane are not excited, they will not emit a signal
Frequency encoding It causes range of Larmor frequencies to exist in the direction in which it is applied
Frequency encoding Fourier transform to separate the different frequencies out after the signal detection
Phase encoding Protons located at different positions in the phase encoding direction experience different amounts of phase shift. Repeating the signal detection multiple times with different amplitude of phase-encoding gradient.
Review: Image Formation Fourier transform k y k x k-space image space Data gathered in k-space (Fourier domain of image) Gradients change position in k-space during data acquisition (location in k-space is integral of gradients) Image is Fourier transform of acquired data
The timing of spin echo sequence
Magnet types in MRI In Permanent magnets the magnetic field is always there and always at full strength (<0,5T). Resistive magnets are made from many coils of wire wrapped around a cylinder through which an electric current is passed. This generates a magnetic field. When the electricity is shut off, the magnetic field dies. Superconducting magnets are somewhat similar to resistive magnets - coils of wire with a passing electrical current create the magnetic field. The important difference is that in a superconducting magnet the wire is continually bathed in liquid helium. Always on at full strength.
MR safety!
MR safety!
Coils
MR contrast, methods T1W pre- and post-contrast T2W T1/T2W mixed Fat or water sturation (pl. FLAIR, STIR, Dixon ) Diffusion weighted (DWI, DTI) Susceptibility weighted T2* (SWI, fmri) Flow-sensitive MR angiography (TOF, PC, 3D CE) in vivo spectroscopy
molecular basis of MRI tissue contrast 1. water content Relaxation rate is directly proportional to water content 2. restricted water movement Originates from the interaction of water and macromolecules. This phenomenon is common in pathologic tissues. 3. macromolecular motion It also influences water motion. Other parameters might also be important, like ph, ion concentration, polimerisation of macromolecules, etc. 4. lipid content Hidrophobic lipids membranes 5. paramagnetic ions Primarily paramagnetic iron; contrast agents..
T1 weighted
T2 weighted
FLAIR & T2 weighted
Myelinisation T1 weighted
T2 weighted submilimeter slice thickness
Pre- and postcontrast T1 weighted Note the metallic artifact in the mouth!
T2 weighted and DWI
Inversion Recovery TR = 2000 ms TI (ms) 50 100 250 500 750
1.5T vs. 3T
fat suppression
Late enhancement method myocardial infarction
MR-guided neurosurgery
Fetal imaging