Medical Imaging Physics Spring Quarter Week 9-1

Transcription

1 Medical Imaging Physics Spring Quarter Week 9-1 NMR and MRI Davor Balzar

2 Intro MRI

3 Outline NMR & MRI Guest lecturer fmri Thursday, May 22 Visit to CUHSC It s not mandatory for this class Your own transportation The address and directions are posted Reading assignment: D 17; CSG D 16; Homework Problems posted Due Tuesday, May 27 Quiz Tuesday, May 27 FINAL EXAM Thursday, May 29, 2-4 PM Olin 205

4 Nuclear Magnetic Resonance (NMR) In 1971, NMR showed differences in magnetic properties between normal and cancerous tissue in rats In 1973, first magnetic resonance image was obtained Birth of Magnetic Resonance Imaging (MRI) technique Principle: Neutrons and protons have spins (small tops that spin in one direction) similar to electrons This gives them magnetic moment Charge movement is equivalent to the current and current produces magnetic field In nucleus, neutrons and protons are packed together with opposing spins Total magnetic moment will be zero for even number and nonzero for odd number of nuclei

5 NMR Thus, some have resulting magnetic moment: 1 H most effective for imaging soft tissues because most contain it Others as well ( 13 C, 19 F, 23 Na, 31 P, ) Even if there is a nonzero magnetic moment of individual nuclei, the sample normally doesn t exhibit any total moment Magnetic moments of individual nuclei are randomly oriented and cancel out If the sample is placed in magnetic field, they orient Parallel orientation is a lower energy state

6 NMR They still spin around its axis Precession around the axis parallel to the external magnetic field Similar to a spinning top precessing around the direction of gravitational force Precessional (Larmor) frequency: ω = gb g gyromagnetic ratio specific of a particular nuclei Proton: T -1 s -1 B magnetic field (typically about 1-4 T) This puts Larmor frequency at MHz Radio frequency (RF) range No radiation-related concerns!

7 NMR Precession around the direction of the magnetic field can be induced by the short RF driving pulse at Larmor frequency Equivalent to pushing a spinning top Precessing spin generates an RF signal at Larmor frequency The signal is picked up by a coil

8 NMR Detected NMR RF signal decreases exponentially: Spin orientations return to equilibrium positions through exchange with environment Defines the spin-lattice relaxation time T 1 Variations in the local magnetic field because of different environment precession of individual nuclei are out of phase with each other Defines the spin-spin relaxation time T 2 The signal magnitude depends on number of 1 H nuclei Fat tissue much stronger signal than bones

9 NMR Time constants provide the most useful information Nature of the nuclei environment Analogy with the spinning top air as opposed to water, For instance (B = 1 T): Fat: T 1 = 240 ms, T 2 = 80 ms Heart tissue: T 1 = 570 ms, T 2 = 57 ms Malignant tissue: higher T 1

10 MRI Magnetic Resonance Imaging (MRI) Challenging as compared to X-ray CT X-rays can be collimated into narrow beams RF frequencies have wavelengths in meters - cannot collimate to give sufficient resolution Problem solved by using a magnetic field gradient in all three directions Magnetic field changes result in Larmor frequency changes Particiular frequency can be associated with x,y,z coordinates which uniquely locates the signal from within the body Resolution of about 0.5 mm can be obtained Sophisticated systems do much better (below 100 nm)

11 MRI Especially useful for imaging of the brain

12 MRI

13 MRI

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16 fmri Functional MRI (fmri) Oxygenated blood (oxyhemoglobin) is diamagnetic (weak magnetic effects manifested by repulsion of external magnetic field) Deoxygenated blood (deoxyhemoglobin) is paramagnetic because of unpaired electrons that increase magnetic moment locally (2-3 times the diameter of the blood vessel nourishing the region) This causes significant shortening of T 2 T 2 -weighted MRI thus reveals the difference When specific part of the brain becomes active, energy requirement rises Neuronal activity stimulates blood flow Therefore, oxyhemoglobin serves as naturally occurring contrast agent for MRI signals corresponding to the spatial and temporal pattern of neuron activity in the brain

17 fmri Uses: Cognitive, motor, and sensory activities Neurological diseases (Alzheimer s, Parkinson s, ) More accurate removal of tumors.

18 Next Time UCHSC visit