8/4/2016. MRI for Radiotherapy: MRI Basics. Nuclear Magnetic Resonance. Nuclear Magnetic Resonance. Wilson Miller

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1 MRI for Radiotherap: MRI asics Wilson Miller Universit of Virginia Department of Radiolog & Medical Imaging AAPM 2016 August 4, 2016 Nuclear Magnetic Resonance Magnetic resonance images are created using the magnetic resonance (MR) properties of hdrogen nuclei in fat and water. Magnetiation vector Spinning proton aligned with magnetic field Magnetiation vector points along ais at thermal equilibrium Nuclear Magnetic Resonance When the spins are tipped awa from the longitudinal () direction b a radiofrequenc (RF) electromagnetic pulse, the begin to precess about the ais at the Larmor frequenc (f = 128 MH at 3 Tesla). Flip angle θ < 90 θ Spinning proton precessing about magnetic field Transverse component rotates about the ais 1

2 Nuclear Magnetic Resonance When the spins are tipped awa from the longitudinal () direction b a radiofrequenc (RF) electromagnetic pulse, the begin to precess about the ais at the Larmor frequenc (f = 128 MH at 3.0 Tesla). Flip angle θ = 90 Spinning proton precessing about magnetic field Transverse component rotates about the ais Nuclear Magnetic Resonance The rotating transverse component of the magnetiation induces an oscillating signal in a nearb RF receiver coil. Demodulated signal Flip angle θ = 90 The signal amplitude is proportional to the transverse magnetiation. Transverse component rotates about the ais T1 and T2 relaation An ecitation RF pulse converts longitudinal magnetiation into transverse magnetiation, which then decas with constant T2. The longitudinal magnetiation regrows toward thermal equilibrium with constant T1. After 90 RF pulse T2 deca ~ e t/t2 T1 recover ~ 1 e t/t1 2

3 T1 and T2 relaation An ecitation RF pulse converts longitudinal magnetiation into transverse magnetiation, which then decas with constant T2. The longitudinal magnetiation regrows toward thermal equilibrium with constant T1. Relaation T2 deca ~ e t/t2 T1 recover ~ 1 e t/t1 T1 and T2 relaation T1 and T2 var widel across different tissue tpes, which contributes to the ecellent soft tissue contrast offered b MRI. Relaation T2 deca ~ e t/t2 T1 recover ~ 1 e t/t1 CT vs. MRI Standard planning CT of pelvis Corresponding T2-weighted MRI T Koch et al, in MReadings: MR in RT. siemens.com/magnetom-world-rt 3

4 Contrast Manipulation: and The repetition is the between ecitation RF pulses. Longitudinal magnetiation vs. [s] 90 RF pulse 90 RF pulse controls T1 weighting The echo is the between the ecitation RF pulse and signal acquisition. Transverse magnetiation vs. [ms] 90 RF pulse Readout controls T2 weighting T2-weighted MRI Use >> T1, ~ T2 Signal intensit stratifies according to T2 Longer T2 brighter on image T2 is generall elevated in cancerous tissue T1-weighted MRI Use < T1, short. Inherentl faster than T2-w MRI. Signal intensit stratifies according to T1 Shorter T2 brighter on image T1 is also generall elevated in cancerous tissue 4

5 T1 and T2 weighted MRI of Liver CT T1-weighted MRI T2-weighted MRI Liver metastases appear dark on T1 weighted MRI and bright on T2 weighted MRI Unrecogniable on CT (in this particular case) RN Low, Oncolog (Williston Park), 14(6 Suppl 3):5-14; Contrast-Enhanced MRI Gadolinium-based contrast agents shorten T1 Results in bright signal on T1-weighted MRI Glioblastoma: Post-contrast T1 Delineation of enhancing volume Especiall useful in the brain Leak vasculature in high-grade gliomas T2-weighted MRI can be used for non-enhancing (usu. low grade) gliomas N Joe et al, Radiolog 212: (1999) Magnetic Resonance Imaging Spatial localiation is accomplished b using linearl varing magnetic fields ( gradients ) to map position to resonance frequenc. Z gradient X gradient f f 5

6 Magnetic Resonance Imaging We sample the NMR signal in the presence of magnetic field gradients, in order to measure the spatial frequenc components of the magnetiation distribution in k space. k 2D k-space amplitudes ift 2D magnitude image Then reconstruct the image b appling the inverse discrete Fourier transform to the k-space data matri. k MRI Pulse Sequences An MRI pulse sequence is the set of instructions given to the scanner, to tell it when to appl the RF pulses, when to turn magnetic field gradients on and off, and when to read out the MR signal, in order to accumulate the k-space data needed to construct an image. Main pulse sequences used for radiotherap applications: Fast spin echo: T2 weighted MRI Spoiled gradient echo: T1 weighted MRI Set of D k-space matrices Fast Spin Echo Pulse Sequence A.k.a. Turbo spin echo. ut, relativel slow Refocus magnetiation for ever k-space line Achieve T2 (or T1) weighting b adjusting and RF EX. REFOC. REFOC. REFOC. EX. REFOC. G SLICE G PHASE G READ ADC 6

7 Spoiled Gradient Echo Pulse Sequence A.k.a. FLASH. Ver fast, use shortest possible. Perfrom new RF ecitation before ever k-space line. Inherentl T1 weighted (because << T1) θ << 90 RF G Z (slice select) G Y (phase encode) G X (freq. encode) ADC Fast Imaging Spoiled gradient-echo pulse sequence (a.k.a. FLASH) T1 weighted Strong, fast gradients (high readout bandwidth) θ << 90 RF G Z (slice select) G Y (phase encode) G X (freq. encode) ADC θ << 90 RF Fast Imaging Spoiled gradient-echo pulse sequence (a.k.a. FLASH) T1 weighted Strong, fast gradients (high readout bandwidth) G Z (slice select) G Y (phase encode) G X (freq. encode) ADC 7

8 θ << 90 RF Fast Imaging Spoiled gradient-echo pulse sequence (a.k.a. FLASH) T1 weighted Strong, fast gradients (high readout bandwidth) Replace Spoilers with Rewinders G Z (slice select) G Y (phase encode) G X (freq. encode) ADC Fast Imaging Stead-state free precession (SSFP, a.k.a. TrueFISP, FIESTA) T2/T1 weighted; highest possible SNR for short Tradeoff: banding artifacts in regions of field nonuniformit θ << 90 RF Replace Spoilers with Rewinders G Z (slice select) G Y (phase encode) G X (freq. encode) ADC Diffusion Weighted MRI Use magnetic field gradients to encode displacement (changes in position over some interval) Measures random rownian motion of individual water molecules. Apparent diffusion coefficient (ADC) Sensitive to tissue organiation on microscopic scale. Higher cellularit lower ADC Necrosis higher ADC 8

9 G G G No displacement 9

10 G No displacement G G 10

11 G Displacement G Displacement G S b ADC 1 S 0 e Use this equation to etract ADC (Apparent Diffusion Coefficient) So-called b value can be calculated from gradient waveform S 0 S 1 11

12 Diffusion Weighted MRI Mucoepidermoid carcinoma of the right parotid gland Enhancing outer region T2 Post-contrast T1 ADC map Nonenhancing inner region Nonenhancing part of lesion is bright on T2 and ADC Histopatholog confirms low cellularit and necrosis Abdel Raek et al, Acta Radiologica 2008 Geometric Distortion ecause the spatial position is encoded into the resonance frequenc, MRI suffers geometric distortion in the presence of magnetic field nonuniformities. Scanner-related distortion Warps image at edges of large FOV Correctable using built-in tools on scanner Subject-related distortion Primaril due to magnetic field disturbances at air-tissue interfaces Minimiation strategies: use high readout bandwidth, refocusing RF pulses (e.g. fast spin echo) Worst in echo planar imaging (EPI) Scanner-Related Distortion Arises from gradient nonlinearit near the edges of the maimum field-of-view cm 10 cm 12

13 Scanner-Related Distortion efore correction After correction Gradient nonlinearit is constant and well characteried b the manufacturer Can be corrected for using integrated scanner software Thank You for staing until the bitter end! 13