OSRT MRI Seminar February 2019 Barry Southers M
OSRT MRI Seminar February 2019 Barry Southers, M. Ed, RT(R)(MR)
Principles of MRI
Magnetic Resonance Imaging • THEN… 1977 Photo from http: //www. vigyanprasar. gov. in/comcom/feature 69. htm • NOW… 2019 Photo from http: //www. cmiv. liu. se/cmiv/about-cmiv/equipment/Philips 2. jpg
Magnetic Resonance Imaging • THEN… 1977 Photo from http: //fonar. com • NOW… 2019 Photo from Philips
Magnetic Resonance Imaging • THEN… 1977 Photo from http: //fonar. com • NOW… 2019 Photo from Tom Haller
Magnetic Resonance Imaging • Uses powerful magnet and magnetic field to produce images • Can be permanent, resistive, or superconducting
Magnetic Resonance Imaging • Motion within atom – Electrons spinning on own axis – Electrons orbiting the nucleus – Nucleus spinning about its own axis
Magnetic Resonance Imaging Photo courtesy of MRI For Technologists
Magnetic Resonance Imaging • Hydrogen nuclei in patients (protons) behave like tiny magnets – 80% of atoms in body are hydrogen • These atoms are dipoles • Net magnetization - dipoles align with external magnetic field
Magnetic Resonance Imaging • When a H 1 proton spins, a magnetic field is created • The H 1 proton has a magnetic field induced around it and acts like a small magnet
Magnetic Resonance Imaging The Earth Photo courtesy of www. healthcare 4. com
Magnetic Resonance Imaging • 1 Tesla = 10, 000 Gauss • The earth’s magnetic field is app. 0. 5 Gauss • The force that points a compass needle north
Magnetic Resonance Imaging B 0 Photo courtesy of GE
Magnetic Resonance Imaging B 0 Proton Photo courtesy of www. healthcare 4. com
Magnetic Resonance Imaging • Nuclei that have even mass number (protons and neutrons) have NO net spin • Nuclei that have odd mass number (where # of protons is more or less than # of neutrons) have net spin and can be manipulated by MRI • Also called spin angular momentum • Nuclear spin is manipulated by introducing magnetic fields or radio waves (which are comprised of alternating electric and magnetic fields)
Magnetic Resonance Imaging • Hydrogen (H 1), along with carbon (C 13), nitrogen (N 15), oxygen (O 17), fluorine (F 19), sodium (Na 23), and phosphorus (P 31) are all important MR active nuclei. • MRI uses H 1 because of its high abundance in the body
Magnetic Resonance Imaging • H 1 protons spin, causing its nucleus to have a magnetic field around it • They also have a net charge – magnetic moment • Can align with an external magnetic field • What is this called? B 0
Magnetic Resonance Imaging n Spinning Protons Act Like Little Magnets Protons are little magnets Photo from www. simplyphysics. com
Magnetic Resonance Imaging • Like a compass needle, protons align with an external magnetic field Tissue is then MAGNETIZED Photo from www. simplyphysics. com
Magnetic Resonance Imaging Protons align with a magnetic field… Slide courtesy of Dr. Scott Huettel, Duke University
Magnetic Resonance Imaging … but move around the field axis in a motion known as precession. Precession axis Slide courtesy of Dr. Scott Huettel, Duke University
Magnetic Resonance Imaging Spin Image from MRI For Technologists Precession
Magnetic Resonance Imaging In a magnetic field, protons can take low- or high- energy states Slide courtesy of Dr. Scott Huettel, Duke University
Magnetic Resonance Imaging The difference between the numbers of protons in the low- and high- energy states results in a net magnetization vector in the patient. Slide courtesy of Dr. Scott Huettel, Duke University
Magnetic Resonance Imaging • When protons spin around the magnetic field, this is precession • The speed in which they precess around B 0 is called their precessional frequency • The precessional frequency of H 1 is 42. 6 MHz. • This is 42. 6 MILLION cycles per second!! • To define the rate of precession at a certain field strength, use the Larmor Equation 0= B 0 x
Magnetic Resonance Imaging • 0 is the precessional frequency • Omega • Also called Larmor Frequency or resonant frequency • B 0 is the magnetic field strength • is the gyro-magnetic ratio • Gamma • Constant; expressed as the precessional frequency of a MR active nucleus at 1 T • MHz/T
Magnetic Resonance Imaging • Examples: • At 1 T, the Larmor Frequency of H 1 is 42. 6 MHz (42. 6 MHz/T x 1 T) • At 1. 5 T, the Larmor Frequency of H 1 is 64 MHz (42. 6 MHz/T x 1. 5 T) • At 3 T, the Larmor Frequency of H 1 is 128 MHz (42. 6 MHz/T x 3 T)
Magnetic Resonance Imaging • Protons in M precess in unison according to the Larmor Equation Image from MRI For Technologists
Magnetic Resonance Imaging • What is resonance? • When a H 1 nucleus is exposed to a radiofrequency pulse (RF pulse) of exactly the same Larmor Frequency of H 1 • Resonance occurs when H 1 nucleus gains energy, just like a tuning fork • Other MR active nuclei do not resonate at that time, since their precessional frequencies are different from H 1
Magnetic Resonance Imaging • Gyromagnetic ratios of other MR active nuclei: – 17. 25 MHz/T - P 31 – 11. 27 MHz/T - Na 23 – 40. 08 MHz/T - F 19
Magnetic Resonance Imaging • Two tuning forks tuned at the same frequency • If you strike one, the other tuning fork will vibrate also • If they are tuned differently, striking one will not cause the other to vibrate Image from MRI For Technologists
Magnetic Resonance Imaging • After the RF pulse is applied, the net magnetization vector (M) becomes out of alignment with B 0 • The angle to which the NMV(M) moves is the “flip angle” • Flip angle depends on the RF excitation pulse • This is usually 90° • Magnetized tissue is then EXCITED • RF excitation pulses are named after the flip angles they induce: • 90° RF pulses lead to 90° flip angles • 180° RF pulses lead to 180° flip angles
Magnetic Resonance Imaging • 90° pulse rotates M from longitudinal to transverse plane • We now have: • Longitudinal magnetization (LM) • Transverse magnetization (TM) • Flip angle controls amount of TM created • 90° FA more TM than 10° FA, so SNR ↑ when FA ↑ • Maximum signal amplitude created is at 90° • As M returns to equilibrium, LM returns to original full magnitude, and TM returns to 0 • This process is called relaxation
Magnetic Resonance Imaging • • Tissues have different relaxation times Fat: Short relaxation times WM/GM: Medium relaxation times CSF: Long relaxation times • Relaxation times related to LM - T 1 relaxation times • Relaxation times related to TM - T 2 relaxation times
• http: //upload. wikimedia. org/wikipedia/co mmons/9/9 b/Hahn. Echo_GWM. gif • http: //en. wikipedia. org/wiki/File: Hahn. Ec ho_GWM. gif
FID • FID – Free Induction Decay • • After 90° RF pulse is removed protons • immediately begin to return to the longitudinal plane • Protons are in phase but immediately dephase after 90° RF pulse is removed http: //upload. wikimedia. org/wikipedia/commons/ 9/9 b/Hahn. Echo_GWM. gif http: //en. wikipedia. org/wiki/File: Hahn. Echo_GW M. gif
Magnetic Resonance Imaging • Then, a 180° pulse is applied to rephase M • Voltage induced again as all spins are rephased • This is the MR signal (in spin echo sequences it is the SPIN ECHO) 180° 90° 180° echo 90° echo
Magnetic Resonance Imaging • The time in between each 90° pulse is called TR (repetition time) • Measured in milliseconds (ms) • The time in between the 90° pulse and the peak of the induced MR signal is called TE (echo time) • Also measured in ms 180° 90° 180° echo 90° echo
Magnetic Resonance Imaging • TR and TE in a nutshell… • These are controlled by MRI Technologist • Short TR + Short TE = T 1 -weighting • Long TR + Short TE = Proton Density • Long TR + Long TE = T 2 -weighting • TR controls amount of LM allowed to recover before next excitation pulse is applied, so longer TR increases SNR due to more magnetization available to be flipped during next TR period • TE controls amount of TM allowed to decay before an echo is collected, so longer TE decreases SNR due to more decay before echo is collected
Image Contrast and Weighting
MRI Parameters • Parameters are factors that affect image contrast in MR imaging • Two categories: – Intrinsic contrast parameters – parameters that cannot be changed because they are inherent to the body’s tissues – Extrinsic contrast parameters – parameters that can be changed by MR operator
MRI Parameters • Intrinsic contrast parameters – parameters that cannot be changed because they are inherent to the body’s tissues – – – T 1 recovery time T 2 decay time Proton density Flow Apparent diffusion coefficient (ADC)
MRI Parameters • Extrinsic contrast parameters – parameters that can be changed by MR operator – – – TR (repetition time) TE (echo time) Flip angle TI (inversion time) Turbo factor/echo train length b-value • Parameters are typically preset by manufacturer, but MRI Technologist WILL alter use of them accordingly
Relaxation • Relaxation…what is it? • After RF energy is turned off, protons begin to release energy absorbed by RF pulse into surrounding molecular environment • The protons’ net magnetization, which was tilted into the transverse plane, relaxes back to a less energized state in alignment with B 0 • The protons that make up each tissue in the body release absorbed RF energy by a process known as relaxation
Relaxation • Tissues relax at different rates • They release absorbed energy at a different rate unique for that tissue • This causes the ability to distinguish between tissues in an MR image • There actually TWO processes which tissues relax – T 1 relaxation – T 2 relaxation – T 1 and T 2 relaxation occur simultaneously and independently of one another
T 1 relaxation • T 1 Time Values: Approximate T 1 Relaxation Times for Various Tissues TISSUE 1. 5 T 3 T WM 560 832 GM 1100 1331 CSF 2060 3700 Muscle 1075 898 Fat 200 382 Liver 570 809 Spleen 1025 1328 From Picture to Proton, by Donald Mc. Robbie
T 1 relaxation – 63% rule • T 1 times indicates how long it takes for a tissue to regain 63% of its longitudinal magnetization (after having been tipped 90° by a RF pulse) • As each additional T 1 time value elapses, the tissue regains an additional 63% of longitudinal magnetization remaining • It takes about 5 T 1 time values for most if not all tissues to regain 99% of their original level of LM
T 1 CURVE 0. 63 T 1
T 2 relaxation • T 2 Time Values: Approximate T 2 Relaxation Times for Various Tissues (Measured at 1 T) TISSUE T 2 Time (in ms) Muscle 40 ms Liver 50 ms Spleen 80 ms Fat 90 ms White matter 90 -100 ms Gray matter 100 -110 ms Blood 180 ms
T 2 relaxation – 63% rule • T 2 times indicates how long it takes for a tissue to lose 63% of its transverse magnetization (after having been tipped 90° by a RF pulse) • As each additional T 2 time value elapses, the tissue regains an additional 63% of transverse magnetization remaining • It takes about 5 T 2 time values for most if not all tissues to lose 99% of their TM
T 2 CURVE (WM>GM) 63%
• TR – short • TE – short T 1 -weighted
• TR – long • TE – long T 2 -weighted
• TR – long • TE – short PD-weighted
TR/TE times • T 1 -weighting • SHORT TR (<1000 ms) • SHORT TE (<50 ms) • T 2 -weighting • LONG TR (>2000 ms) • LONG TE (>50 ms) • PD-weighted • LONG TR (>2000 ms) • SHORT TE (<50 ms)
- Slides: 55