BME1450 Intro to MRI February 2002 The Basics The Details – Physics The Details – Imaging

February, 2002BME 1450 Introduction to MRI 2 Example of MRI Images of the Head Bone and air are invisible. Fat and marrow are bright. CSF and muscle are dark. Blood vessels are bright. Grey matter is darker than white matter.

February, 2002BME 1450 Introduction to MRI 3 MRI Imagers GE 1.5 T Signa Imager GE 0.2T Profile/i imager

February, 2002BME 1450 Introduction to MRI 4 MR Imaging: Parts of an Imager Main Magnet – High, constant,Uniform Field, B 0. Gradient Coils – Produce pulsed, linear gradients in this field. – G x, G y, & G z RF coils – Transmit: B1 Excites NMR signal ( FID). – Receive: Senses FID. Basics B0B0 B0B0 B0B0 B1B1

February, 2002BME 1450 Introduction to MRI 5 MR Imaging: Pulse Sequence __________________ A B C D E Excitation Slice Selection Phase Encode Readout RF Detected Signal K Space  DFT  Excitation RF pulse Gz GXGX Gy Basics Coherent detector Complex numbers Image Space ‘Real numbers’

BME595 Intro to MRI October 2000 The Basics The Details – Physics The Details – Imaging

February, 2002BME 1450 Introduction to MRI 7 Magnetic Resonance (MR) An object in a magnetic field B 0 will become magnetized and develop a net Magnetization, M. Most of M arises from the orbital electrons but a small part is the Nuclear Magnetization . The nucleus has a magnetic dipole moment, , and angular momentum, J. |  |/|J| = , the gyromagnetic ratio. For Hydrogen  = 43 MHz/T. J and  The Details - Physics  Magnetization is “magnetic dipole moment per unit volume”.

February, 2002BME 1450 Introduction to MRI 8 MR: Precession The 1.5T magnetic, B 0 field of the MR Imager makes the Hydrogen Nuclei precess around it. The precession rate,, is the Larmor frequency. f L =  B 0 = 43*1.5 = 64MHz for Hydrogen. Y Z J or  X B0B0 |B 0| t The Details - Physics

February, 2002BME 1450 Introduction to MRI 9 MR : Summary The magnetization,M, is the density of nuclear magnetic dipole moments. If you tip M away from B 0 it will precess, at frequency  B 0, producing a measurable RF magnetic field. Y Z J or  or M X B0B0 |B 0| t The Details - Physics

February, 2002BME 1450 Introduction to MRI 10 MR Excitation You can tip M by applying a circularly polarized RF magnetic field pulse, B 1, to the sample. If B 1 is at the Larmor frequency,  B 0 you get this. M is now precessing about two magnetic fields. B 1 is effective because it tracks M. Y Z J or  or M X B0B0 |B 0 | t B1B1 |B 1 | t The Details - Physics B0B0 B1B1

February, 2002BME 1450 Introduction to MRI 11 Magnitisation Relaxation The transverse (M  ) and longitudinal (M || ) components of the magnitization change with time. Two relaxation times T 1 (longitudinal) and T 2 (transverse). T 1  T 2 M(t) || M 0 t T 2 The Details - Physics

BME595 Intro to MRI October 2000 The Basics The Details – Physics The Details – Imaging

February, 2002BME 1450 Introduction to MRI 13 Magnitisation Relaxation MRI Contrast is created since different tissues have different T1 and T2. Gray Matter: (ms) T1= 810, T2= 101 White Matter: (ms) T1= 680, T2= 92 The Details - Imaging

February, 2002BME 1450 Introduction to MRI 14 MR: The FID As the magnetization precesses it creates its own RF magnetic field. This field is much smaller than the Exciting RF field. It can be detected with a standard radio receiver. The resulting signal from precession is called the FID. Y Z J or  or M X B0B0 |B 0 | t How do you maximize the FID? The Details - Imaging Lab Frame M + V(t) -

February, 2002BME 1450 Introduction to MRI 15 MR: The MR Signal The FID can be detected by a ‘read out coil’ and amplified in a standard RF amplifier. It is then input to a coherent detector with two outputs, I and Q. The detector is phase locked to the excitation pulse. Thus – M y’  “In Phase” output, I – M x’  “Quadrature output, Q = 0 Y’ Z M X’ M y’ MZMZ The Details - Imaging Rotating Frame M + V(t) -

February, 2002BME 1450 Introduction to MRI 16 Gradient Pulses __________________ A B C D E Excitation Slice Selection Phase Encode Readout RF Detected Signal K Space  DFT  Excitation RF pulse Gz GXGX Gy Details - Imaging Coherent detector Complex numbers Image Space ‘Real numbers’ {

February, 2002BME 1450 Introduction to MRI 17 MRI: The imaging pulses The phase gradient pulse will cause more precession. Precession occurs during the readout gradient pulse as well. During readout I and Q are digitized into a complex value I+jQ and stored in K space. Y’ Z M X’ MZMZ xG x I  M y’ Q  M x’ xG x  t Details - Imaging

February, 2002BME 1450 Introduction to MRI 18 MRI: Kspace If k x (t) and k y (t) are defined as shown, then they represent the row and column that the value, digitized at time t, should be assigned to in Kspace Details - Imaging

February, 2002BME 1450 Introduction to MRI 19 MRI: Driving through Kspace  times the integral of the G x (t) and G y (t) gives the position in Kspace A B C D E RF pulse Gz Gx Gy Details - Imaging

BME595 Intro to MRI October 2000 The Basics The Details – Physics The Details – Imaging Details not discussed

February, 2002BME 1450 Introduction to MRI 21 MR: The Rotating Frame It is much easier to visualize all this if you observe it from a frame of reference which is rotating at the Larmor frequency, f L =  B 0. B 1 appears motionless in this rotating frame and B 0 effectively disappears and… During the excitation pulse, M precesses only about B 1 at frequency  B 1 !! Y’ Z M X’ B1B1 |B 1 | t M y’ MZMZ The Details - Physics Rotating Frame

February, 2002BME 1450 Introduction to MRI 22 MR: The Rotating Frame When the excitation pulse is over, M is stationary in the rotating frame. In the Lab frame, however, it is still precessing. Y’ Z M X’ M y’ MZMZ The Details - Physics Rotating Frame

February, 2002BME 1450 Introduction to MRI 23 MRI – Meaning of “Z Gradient” X Y Z A “Z gradient” introduces a gradient in the magnetic field in the Z direction. The gradient is produced with resistive coils. Traditionally the Z gradient is associated with the RF excitation pulse and slice selection. z  Gz Details - Imaging B0B0

February, 2002BME 1450 Introduction to MRI 24 MRI – Meaning of “X&Y Gradients” x  Gx X Y Z Details - Imaging B0B0 An “X or Y gradient” introduces a gradient in the B 0 magnetic field in the X or Y direction. These gradients are traditionally associated with readout and phase encode, respectively.