MR TRACKING METHODS Dr. Dan Gamliel, Dept. of Medical Physics, Ariel University Center in Samaria

Overview Magnetic Resonance Imaging (MRI) Nuclear Magnetic Resonance (NMR) Magnetic Resonance Imaging (MRI) Magnetic Resonance Motion Effects Magnetic Resonance Tracking Methods

The NMR method Nuclear Magnetism Macroscopic magnet: collection of magnetic moments single magnetic moment: electric current loop nuclear or electronic magnetic moment: from (orbital + spin) angular momentum non-zero nuclear moment: with Pauli principle - usually odd number of nucleons 1H , 13C, 17O , 23Na , 31P , … (non-zero spin)

The NMR method magnetic moment in external field Classically: Energy term of magnetic moment in external magnetic field: Larmor Precession of magnetic moment around direction of external field

The NMR method magnetic moment in external field Quantum mechanically: Splitting of energy levels For S = ½ (e.g., 1H nucleus): Each type of nucleus has its g value (n0 = w0 / 2p = Zeeman frequency) Longitudinal projection of spin angular momentum “spin” = nuclear magnetic moment

The NMR method Effect of time dependent transversal field Static (constant) magnetic field: B0 = B0 z Generates net magnetization along Z - parallel to static field (longitudinal direction) Time dependent magnetic field: B1 = B1 cos(w t) x (transversal = perpendicular to static field) Effect of time dependent field: Excitation of transitions between energy levels of static field – rotates some spins from Z direction to XY plane (or to the –Z direction)

The NMR method The resonance phenomenon For static field, transition (or precession) frequency is w0 = - g B0 (typical: 107 – 108 Hz) For time dependent field, strength (amplitude) is equivalent to w1 = - g B1 (typical: 103 Hz) The excitation is effective only if | w - w0 | < w1 (close to resonance) Classical Bloch equations: precession and decay

The NMR method The energies Transition frequencies for a single atom: Nuclear or electronic processes (g rays, X rays) ~ 1018 - 1020 Hz Chemical processes – electronic transitions (visible – UV): ~ 1014 – 1017 Hz Nuclear magnetic transitions (NMR): ~ 107 – 109 Hz (RF - radio frequencies - range)

The NMR method The importance of resonance Net magnetization depends on population difference between “parallel” and “anti-parallel” The population difference depends on the Boltzmann factor for the energy difference At room temperature: exp(- hn / kT) ~ 10-5 - very small net magnetization (paramagnetic) in the strong static magnetic field - For a sufficient signal: need - resonance effect - a large (macroscopic) sample

The NMR method Modes of operation CW (continuous wave) (frequency domain): Constant static field (constant resonance value) “sweep” over oscillation frequency of the time-dependent field Measure signal for each oscillation frequency : resonance peak at w = w0 Pulsed operation (time domain experiment): Operating the time-dependent field for a short time, exactly needed to rotate magnetization from the Z axis to the XY plane - Measure the signal for many time values  do a Fourier transform from t to w  get the same resonance graph !

NMR method Relaxation of magnetization After time dependent field stops operating: spins return gradually to original state Thermal equilibrium (final) relaxation – for longitudinal magnetization - T1 time constant Earlier change – signal decay: Loss of coherence – for transversal magnetization - T2 time constant (partial relaxation) T2 with field inhomogeneity – T2* time constant

The NMR method the change in magnetization due to relaxation Transversal magnetization decays as MXY ~ exp(- t / T2 ) Longitudinal magnetization recovers as MZ ~ M0 (1 – exp(- t / T1 )

The NMR method The resonance graph (CW or pulsed method) Time domain signal (pulsed method): Mx + iMy ~ exp{-i(w0 - w) t – t / T2 } The peak of the frequency domain graph: at w = w0 The width at half the peak height:

NMR pulse sequences Typical experiments (pulsed method) Overall structure of pulse sequence: Preparation – e.g. inversion Excitation – cause change of state Evolution – e.g. refocusing or other pulses Detection – measurement of signal (as a function of time) Data processing (Fourier transform)

NMR pulse sequences Typical experiments (pulsed method) Essential steps: Excitation (by an RF “90º pulse”– rotating magnetization) Measurement of signal as a function of time Fourier transform of signal from time to frequency Some additional options (with many possible combinations): Refocus Mxy (by an RF “180º pulse”) – to undo T2* decay - “spin echo” experiment Invert Mz (by an RF “180º pulse”) Add a changeable time interval before another pulse

NMR pulse sequences Typical results (spectrum: signal vs. frequency) This is the spectrum of a sample containing two types of chemical groups – in each group the hydrogen nucleus has a different resonance frequency. In addition, interactions between spins cause splitting of each resonance to several spectral lines

NMR Experimental system Superconducting magnet (cooling – liquid nitrogen, liquid helium) Transmitter/ receiver Spectrometer

NMR Main applications Main nucleus: 1H (water, lipids, …) Study chemical structure by: - chemical environment of atom - interactions between atomic nuclei Study dynamic processes involving spins: - diffusion processes - exchange processes Study details of structure and processes by special pulse sequences

The MR Imaging (MRI) method Transmission of NMR frequencies in body X-ray images of human body are possible because X-rays are (partly) transmitted through the human body Also RF waves are partly transmitted through the body ! - The following graph shows absorption of electromagnetic radiation in the human body – as a function of wavelength

The MRI method Background – other medical imaging modalities Optical images (visible light): reflection and diffraction good resolution in diffraction (short wavelength) high contrast (absorption differences) X-ray images: transmission and diffraction good resolution in CT (beam collimation) contrast: absorption differences and contrast materials Nuclear medicine: g emission from radionuclide low resolution, low contrast very good functional information

The MRI method Problems for NMR imaging Needed: spatial resolution contrast Problem for resolution: In optical images: resolution ~ wavelength (very short) – but NMR wavelength ~ 1 meter ! In X-ray images: resolution ~ focusing of beam – difficult for NMR wavelength Problem for contrast: Water density in body – similar in different tissues

The MRI method Solution for spatial resolution Spatial dependence of resonance frequency by modification of “static” magnetic field: w0 = g B0  g Bz = g { B0 + Gz (t) z + Gy (t) y + Gx (t) x } During excitation pulse: slice selection Gz  z-dependent excitation resonance During signal readout (sampling): frequency encoding Gx  x-dependent readout resonance (many time points for resolution) Between excitation and readout: phase encoding Gy  y-dependent added phase (many repetitions of sequence with different phase)

The MRI method Basic pulse sequence Excitation, field gradients, signal readout with two time parameters: TE, TR initial dephase in view axis – for (k-space) symmetry around echo TE TR

The MRI method Solution for image contrast TE (Time to Echo) = time from excitation to (refocusing moment of) readout = time for decay of signal - determines contrast by T2 differences between tissues TR (Time to Repeat) = time from excitation to next excitation = time for return of magnetization to equilibrium - determines contrast by T1 differences between tissues Relative density of 1H (“proton density”) – minor contrast factor, useful in some applications

The MRI method Solution for image contrast Some typical values (times in ms): Tissue T1 (0.5 T) T1 (1.5 T) T2 proton density grey matter 680 1130 100 10.6 % white matter 450 720 90 10.6 % skeletal muscle 560 1180 34 9.3 % liver 360 720 60 9.7 % Blood 200 1200 30(v)-250(a) Fat 200 260 60 9.6 % tumors (longer) (longer)

The MRI method Solution for image contrast Signal amplitude vs. time for two tissues with different T2 values Recovery of MZ after excitation for two tissues with different T1 values

The MRI method Useful timing combinations for image contrast short TE, long TR (TE << T2 and TR >> T1 ): little decay, "full relaxation" - "proton density" contrast - signal increases with spin density long TE, long TR (TE ~ T2 and TR >> T1 ): much decay, "full relaxation" - T2 contrast signal increases with T2 3. short TE, short TR (TE << T2 and TR ~ T1 ): little decay, little relaxation - T1 contrast signal decreases when T1 increases (Note: T2* replaces T2 where appropriate)

The MRI method Clinical utility Advantages: - Non-ionizing radiation (unlike CT and NM) - Many different contrasts available (various pulse sequences - T1, T2, spin density, static tissue, blood vessels, …) - No limitation on imaging plane (same as in CT) - Both anatomic and (more limited) functional information

The MRI method Clinical utility MRI system Superconducting magnet Gradients Transmit/receive system + coils

The MRI method Clinical utility Some images: Top left: T1 contrast (useful to distinguish tumors) Top right: T2 contrast (anatomic detail)

The MRI method Clinical utility Some images (Joseph Hornak – online course):

The MRI method Measured signal and image reconstruction Measured signal (in “k-space”) – without relaxation: Reconstructed image (spin density) – without relaxation:

The MRI method Time scales in imaging and in internal motion MRI techniques (examples): Standard (grad. echo, spin echo): ~ 100 – 1000 s Fast (fast spin echo, FLASH, etc.): ~ 50 s Very fast (EPI, single shot FSE etc.) ~ 0.1 s – 0.5 s Internal motion in body (examples): Respiratory cycle ~ 2 – 4 s Cardiac cycle ~ 1 s Gastro peristaltic motion cycle ~ 10 - 20 s blood velocity ~ 0.1 - 1 m/s

MR Motion Effects Phase change due to motion Some motion effects: Some spins feel only early part of “imaging sequence” Some spins feel only late part of “imaging sequence” Some spins acquire a time dependent phase, reconstructed as a “change in position“. Example: x(t) = x0 + v t  (time dependent phase)

MR Motion Effects Avoiding motion artifacts Some ways of avoiding motion artifacts: Change gradient pattern in pulse sequence to compensate for common motion effects (blood motion) Cardiac/peripheral (ECG) gating Respiratory gating (bellows) Breathholding Fast pulse sequence Tagging (e.g. cardiac) Dynamic correction using “navigator” Spatial “suppression” of moving region in image

MR Tracking Methods The need for tracking a position Compare stages in time change in anatomic structure Interventional procedure: - imaging while operation is being carried out - follow position of instrument (e.g. needle) - follow changes in anatomic region

MR Tracking Methods Using External Markers markers seen in MR image, placed in known positions reference points for position of special object (e.g. needle) reference points for position of relevant anatomic region employs simple and accurate calculations enables directing treatment to desired location requires: “static” region

MR Tracking Methods Using External Markers Scan for locating external markers: Fast, short TE (gradient echo type) geometrical information used for operation example: locating ultrasound transducer during Focused UltraSound ablation of tissue

MR Tracking Methods Using External Markers A possible way to monitor (with MRI) temperature of ablated region: Chemical shift (change in resonance frequency) depends on temperature temperature difference  off-resonance difference  phase difference: Df = g DB t  temperature mapping

MR Tracking Methods Using Navigator Pulse Sequence For a “dynamic” region (large motion – mainly breathing): must follow region dynamically Navigator Pulse Sequence: Sequence generates partial image data (e.g. a straight line) – to mark a specific anatomic structure (e.g. diaphragm) reference for position of relevant region (e.g. liver)

MR Tracking Methods Using Navigator Pulse Sequence Using reference image: Take a reference image(s) Check correlation of specific image with a reference image Check cross- correlation between images Using a navigator sequence: Run a reference navigator Run navigators between some of the repetitions of the main pulse sequence Check correlation between reference navigator and a current navigator, correct current image

MR Tracking Methods Using Navigator Pulse Sequence (Commercial sequence) The Cardiac Navigator feature combines a cardiac gated, 3D Fast GRE or 3D FIESTA sequence with a navigator pulse that tracks the motion of the diaphragm. By placing the navigator tracker pulse over the right hemi-diaphragm, the acquisition is synchronized to the end-expiration respiratory phase of the patient thus minimizing respiratory ghosting artifacts.