The physics of medical imaging US, CT, MRI Prof. Peter Bogner
Clinical radiology curriculum blocks of lectures and clinical practice (7x2) Physics of medical imaging Neuroradiology Head and neck I. Head and neck II. Thoracic radiology Gastrointestinal radiology Urogenital and interventional radiology Final exam: test + oral
Medical imaging Energy matter is required or not Imaging needs energy-matter interaction Part of the energy is deposited
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 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
60-90% US interactions in tissues
Reflection the basis of US imaging it occurs on the interface of two media, that have different acoustic impedances (Z) acoustic impedance ~ density + elasticity (similarly to velocity) impedance (Z) = ( )(v) (rayl = kgm 2 sec -1 )
Transducer
Adjustable focus
A(mplitude) B(rightness) M(otion) doppler Presentation modes
Presentation modes M doppler
Resolution
Resolution
CT - computed tomography Godfrey N. Hounsfield Allan M. Cormack The Nobel Prize in Physiology or Medicine 1979
MRI - magnetic resonance imaging Sir Peter Mansfield Paul C. Lauterbur The Nobel Prize in Medicine 2003
Superimposition
Limitations of radiography superimposition slice thickness in direction! estimation of x-ray attenuation tissue contrast sensitivity: ~ 10%
computer tomography minimal superimposition improves tissue contrast tissue characterization (CT number)
computed tomography
CT principles attenuation coefficient reconstruction algorithm CT number Hounsfield value
element of CT image: voxel FOV field of view image matrix slice thickness
Spatial encoding detectors number of slices : 16,32,64,128,256,384
Detector configuration
CT contrast factor physical (electron) density 1 HU is 0.1% change in attenuation 4 HU can be differentiated 0,008 g/cm 3 between white matter and white matter edema: 2.6 HU 1 % difference in water content (% wet weight) cytotoxic edema can produce a 3% increase in water within 1 hour (hypoxia)
relationship between electron and physical density x-ray attenuation
windowing
windowing
window settings 1 HU is 0.1% change in attenuation 4 HU can be differentiated experienced reader can differentiate 10-15% change of contrast in radiography
CT contrast media Oral contains iodine or barium Intravenous iodinated, non-ionic Injector volume, timing (phases), flow (ml/s) Adverse effects, contraindications, nephrotoxicity (ESUR guidelines)
Pre- and post-contrast (iv.)
CT methods morphologic/structural imaging preand postcontrast (multiple phases) CT angiography, perfusion CT neuro, orbits, sinuses, cervical soft tissue, thorax, abdomen, pelvis, traumaemergency, staging, radiation oncology planning, surgical planning, cardiac, virtual colonoscopy, etc.
isotropic voxel reconstruction
cardiac CT
virtual endoscopy
CT angiography
characteristics of modern CT imaging isotropic voxels powerful reformatting possibilities high contrast good temporal resolution (e.g. cardiac imaging, CT-fluoroscopy) volume scanning: good spatial resolution lower patient dose
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