MRI 24th Oct 23 Part 2.pptx

MAGNETIC RESONANCE IMAGING MAKING THE SIGNAL VISIBLE PHYSICAL PRINCIPLES – PART 2 24TH OCTOBER 2023 DR MARCUS THOMAS JACKSON Reviewed and revised 17th October 2023 Generating the MR signal rf B0...

MAGNETIC RESONANCE IMAGING MAKING THE SIGNAL VISIBLE PHYSICAL PRINCIPLES – PART 2 24TH OCTOBER 2023 DR MARCUS THOMAS JACKSON Reviewed and revised 17th October 2023 Generating the MR signal rf B0 0 =   B0 NMV CREATING A VISIBLE IMAGE  Need a scanner to generate B0  Need to localise the signals using gradients  Gradients change magnetic field strength in a defined way, and protons change their resonant frequencies to match  Need to excite a signal many times to complete the image and optimise signal-to-noise ratio (SNR)  Need to get signals into the computer for reconstruction Generating B0 Main magnet Cylindrical bore ~ 60cm Field strength 1.5-3T Superconducting Gradients & body rf coil (inside magnet covers) Gradients localise signal Body coil for torso scans Knee rf coil Fits closely to anatomy to improve signal Patient lies on couch Feet first or head first Couch moves into bore Couch controls Coil connection Buzzer for patient alert LOCALISING MR SIGNAL Larmor freq Low freq High freq ISOCENTRE 0.999 T 1.000 T 1.001 T 42.5275 MHz 42.5700 MHz 42.6125 MHz SLICE SELECTION Larmor freq Low freq High freq z X Y 0.999 T 1.000 T 1.001 T 42.5275 MHz 42.5700 MHz 42.6125 MHz SIGNALS LOTS OF SIGNALS Signal now contains many frequencies instead of just one, all added together and looking like a complete mess signal time  TOTAL SIGNAL & FOURIER TRANSFORM Signal Fourier Transform time Signal frequency DISPLAYING AN IMAGE MR Signal ‘K’ Space Signal Image Matrix frequency Fourier Transform IMAGE MATRIX – VISIBLE IMAGE REVIEW MRI Gradients modify the frequency of the signals in a controlled way By measuring frequency & amplitude we can localise signals within the magnet Creating an Image  SUMMARY  Protons in water and fat precess in a magnetic field  RF pulses excite the protons and produce signals in a coil  Gradients are used to localise the signals  Pulse sequences can be controlled to produce images with different contrast …..coming up in the next presentation. WHAT IS A PULSE SEQUENCE  A pulse sequence is set of changing magnetic gradients. Each sequence will have a number of parameters, and multiple sequences grouped together into an MRI protocol.  In this presentation we will review some commonly used pulse sequences  Understanding a pulse sequence diagram will enable you to use equipment from different manufactures as they use different terminology for the same pulse sequence PULSE SEQUENCES  Combination of rf pulses to excite the signal and gradients to localise it  Different pulse sequences produce different kinds of image  Spin echo – basic, for high SNR and resolution  Fast spin echo – spin echo with go-faster stripes  Gradient echo – common, for fast scans  Time-of-flight and phase-contrast – for MRA  Echo-planar imaging – for ultra fast sub-second image times, low resolution SPIN ECHO PULSE SEQUENCE Data collection 90° 180° RF Gradients Time to Echo (TE) SPIN ECHO PULSE SEQUENCE 90° 180° 90° 180° Data Data Collection Collection Time to Repeat (TR) 900 & 1800 RF PULSES RF Wave 1800 900 Rephasing Excitation Pulse Pulse REPHASING  Note: 900 is followed by two 1800 DUAL ECHO SEQUENCE pulses – Each giving a signal GRADIENT ECHO SEQUENCE Note: 900 pulse is not followed by 1800 but rephased by a gradient reversal INVERSION RECOVERY SEQUENCE Note: 1800 inversion pulse precedes excitation pulse. Introduces new parameter TI IMAGE CONTRAST  TR and TE can be altered by the user  Contrast in image depends strongly on TR and TE  TR controls how much T1 relaxation occurs  Short TR gives T -weighted images 1  TE controls how much T2 relaxation occurs  Long TE gives T -weighted images 2  What are T1 and T2? RELAXATION – REVISITED ! Spin-spin relaxation refers to rate at which Mxy decays to zero Depends on tissue composition and how easily protons can move within lattice More “solid” tissues have short T2  SPIN-SPIN RELAXATION Diffusion & vibration of molecules Uneven proton density causes local inhomogeneities SPIN-SPIN RELAXATION M0 T2 decay magnetization Transverse 37% Time (ms) T2 T2 RELAXATION TIMES  Definition: time for Mxy to decay to 37% of initial Mxy  Typical values:  muscle 40 ms  fat 110 ms  grey matter110 ms  white matter 105 ms  CSF 800 ms SPIN-LATTICE RELAXATION RF pulse deposits energy into protons Immediately after end of rf pulse, tissues will start to relax Absorbed energy is released in the form of heat SPIN-LATTICE RELAXATION Longitudinal magnetization M0 T1 recovery 63% T1 Time (ms) T1 RELAXATION TIMES  Definition: time for Mz to grow back to 63% of M0  Typical values:  muscle 630 ms  fat 190 ms  grey matter825 ms  white matter 685 ms  CSF 1200 ms DIFFERING BIOLOGICAL TISSUES -T1 Mz TR T1 & T2 long T1 short, T2 medium eg T1 medium, T2 short eg eg oedema fat muscle DIFFERING BIOLOGICAL TISSUES – T2 Mx y TE T1 & T2 long T1 short, T2 medium eg T1 medium, T2 short eg eg oedema fat muscle SPIN ECHO PD WEIGHTING  Must use maximum TR in order to get PD contrast  Use long TR  TR > 1250ms for 0.5T  TR > 1500ms for 1.5T  Create spin echo as soon as possible after rf pulse to avoid T2 effects  Use short TE  TE 10-35ms SPIN ECHO T1 WEIGHTING  Need to decrease TR in order to get T1 contrast  Use short TR  TR 150-600ms for 0.5T  TR 300-800ms for 1.5T  Collect spin echo as soon as possible after 90° rf pulse to avoid T2 effects  Use short TE  TE 10-35ms  “Anatomy” scans SPIN ECHO T2 WEIGHTING  Must increase TR in order to avoid T1 contrast  Use long TR  TR > 1250ms for 0.5T  TR > 1500ms for 1.5T  Allow time for some T2 decay before creating echo  Use long TE  TE 70-100ms  “Pathology” scans SPIN ECHO TR 240 400 800 1000 1500 SPIN ECHO TE 25 50 75 120 REVISION QUESTION 90° 180° A B C WEB SITES TO VISIT  An excellent overview of MRI – highly recommended https://www.imaios.com/en/e-Courses/e-MRI  For those who would like more physics …… http://www.cis.rit.edu/htbooks/mri/inside.htm http://www.simplyphysics.com/page2_1.html THE END !