Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Take Aways: Five Things You should be able to Explain after the MRI Lectures ¬ Magnetic Resonance Imaging – Chapter 15 ¬ ¬ ¬ Brent K Stewart, PhD, DABMP Professor, Radiology and Medical Education Director, Diagnostic Physics ¬ a copy of this lecture may be found at: http://courses.washington.edu/radxphys/PhysicsCourse04 http://courses.washington.edu/radxphys/PhysicsCourse04 05.html 05.html © UW and Brent K Stewart, PhD, DABMP © UW and Brent K Stewart, PhD, DABMP Localization of the MR Signal ¬ ¬ ¬ ¬ 2, and 16 June 2005 Magnetic Field Gradients Spatial localization requires the imposition of magnetic nonuniformities Linear gradients are superimposed on the homogeneous and much stronger main magnetic field (B0) The change in Larmor frequency of the precessing nuclei are used to distinguish position of the NMR signal within the object Conventional MRI involves RF excitations (NMR) combined with magnetic field gradients to localize the signal from volume elements (voxels) within the patient © UW and Brent K Stewart, PhD, DABMP How the MR signal is localized within the patient (2D) How the collected FID echoes are collected (‘k(‘k-space’ data acquisition) and how these are reconstructed into the grayscale image data visualized on PACS (2D) How 3D volume data is acquired and reconstructed What factors of the MRI data collection process play into the resulting quality of reconstructed image slices and volumes How consideration of artifacts, safety/bioeffects and instrumentation play into the decisions you will be making in the future with regards to image interpretation, magnet operation and system purchase ¬ ¬ ¬ ¬ Linear magnetic field gradients with prescribed directionality and strength are produced in paired wire coil configurations energized with a DC current of specific polarity and amplitude Gradient null point; reverse grad polarity w/ opp opp current Linear over a predefined field of view (FOV) Three sets: x, y and z; can also generate oblique w/ superpos c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 416416-7 © UW and Brent K Stewart, PhD, DABMP Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Magnetic Field Gradients ¬ ¬ ¬ ¬ Magnetic Field Gradients Larmor freq changes along gradient: e.g., Gz = ∂B/∂z, B/∂z, Gx = ∂B/∂x B/∂x Location of nuclei along gradient is determined by their frequency frequency (∆f = (γ ∂z·∆ ∆z) and phase (∆φ (γ/2π /2π)·∂B/ ·∂B/∂z· (∆φ = 2π 2π·∆f·∆t) Peak amplitude of gradient (G) field (‘steepness’): [1,80] mT/m Slew rate (‘quickness’ of gradient ramping): [5,200] mT/m/msec ¬ ¬ ¬ Gradient amplitude and number of samples over the FOV determines the frequency bandwidth across each pixel 10 mT/m · 42.58 MHz/T · 1T/1,000 mT · m/100 cm = 4258 Hz/cm Localization of nuclei in 2D requires the application of three distinct distinct and orthogonal gradients during the pulse sequence: slice select, select, frequency encode and phase encode gradients From the above calculation, it’s easy to see that with gradients our old friend: π= friend: γ/2 γ/2π 426 HzHz-cm-1/mT/mT-m-1, so then it’s just a matter of multiplying the number of mT/m by this factor to get the bandwidth (Hz)/cm c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 417 © UW and Brent K Stewart, PhD, DABMP c.f Hashemi, Hashemi, et al MRI the Basics, p 105 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 418 Slice Select Gradient (SSG) ¬ ¬ ¬ ¬ ¬ ¬ ¬ Applied RF pulse bandwidth (BW) Gradient strength across the FOV ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 418 © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 Slice Select Gradient (SSG) RF pulse antennas can’t spatially direct the RF energy within FOV FOV In conjunction with a selective frequency narrowband RF pulse applied to the entire volume, the SSG determines the imaging slice slice Slice thickness (ST) determined by: ¬ © UW and Brent K Stewart, PhD, DABMP For a given gradient strength, ST determined by RF BW For fixed RF BW, the gradient strength determines ST Excite a rectangular slab (slice) of nuclei ‘sinc’ waveform: sinc(t) = sin(t)/t Need an infinitely long sinc pulse to get a perfectly rectangular slice Truncation in time of sinc pulse leads to rounded slice profiles c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 419419-20 © UW and Brent K Stewart, PhD, DABMP Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Slice Select Gradient (SSG) Frequency Encode Gradient (FEG) ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Width of sinc pulse determines the output frequency BW Both narrow BW w/ weak gradient and wide BW w/ strong gradient same ST SNR ∝ [SQRT(BW)][SQRT(BW)]-1 Narrow BW SNR Narrow BW chemical shift Gradients cause spin dephasing: phase important! ReRe-establish original phase with opp polarity gradient with ½ integrated area (∆f ∝ G·∆ G·∆t) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 421 © UW and Brent K Stewart, PhD, DABMP ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 422 Frequency Encode Gradient (FEG) ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 © UW and Brent K Stewart, PhD, DABMP 10 Phase Encode Gradient (PEG) Composite signal is amplified, digitized and decoded by Fourier Transform (FT) ∆f = (γ ∆f ∝ ∆x (γ/2π /2π)·Gx·∆x Rotation of FEG direction provides projections through object as a function of angle Like CT: filtered backprojection However, due to sensitivity to motion artifacts phase encoding gradients used c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 423 FEG aka readout gradient Applied to SSG ∆f = (γ ∆f ∝ ∆x (γ/2π /2π)·Gx·∆x Applied throughout formation and decay of the FID echo from slab excited by the SSG Demodulation of the composite signal produces a net frequency variation that is symmetrically distributed from +fmax to –fmax at FOV edges Spatial projection: column sum 11 Short duration gradient applied before FEG and after SSG to provide 3rd spatial dimension After SSG all spins in φ coherence During PEG application linear variation in precessional frequency introducing a persistent phase shift across the slice slab (∆φ ∝ By·∆y·t) After all FID data collected, a FT is applied to decode the spatial position along the PE direction Motion during data collection produces ghosting in along PE c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 424 © UW and Brent K Stewart, PhD, DABMP 12 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Gradient Sequencing ¬ ¬ ¬ Raphex 2001 Diagnostic Questions For the SE pulse sequence Timing of the gradients in conjunction with RF excitation pulses and data acquisition during echo evolution and decay Sequence repeated periodically (TR) with only slight changes in the PEG amplitude to provide the 3D identity of protons of the object object in the resulting image ¬ D43 In MRI, the RF frequency is dependent on the: ¬ A Diameter of the body part being imaged B Magnetic field strength C Pulse sequence D Relaxation time E RF coil ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 425 © UW and Brent K Stewart, PhD, DABMP 13 © UW and Brent K Stewart, PhD, DABMP Raphex 2001 Diagnostic Questions ¬ D46 Gradient fields in MRI are principally used to: ¬ A Eliminate perturbations in the magnetic field due to site location B Maintain a uniform magnetic field in the field of view C Measure the spin coupling D Provide spatial localization E Shorten T1 to reduce scan time ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 14 Raphex 2000 Diagnostic Questions ¬ D48 D48 In MRI images, motion during the scans results in ghost images which appear in the direction ¬ A Amplitude B Frequency encoding C Phase encoding D Relaxation E Slice thickness ¬ ¬ ¬ ¬ 15 © UW and Brent K Stewart, PhD, DABMP 16 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course ‘K-space’ Data Acq and Image Reconstruction TwoTwo-dimensional Data Acquisition Max signal in center of kk-space ¬ ¬ ¬ ¬ ¬ MRI data initially stored in a ‘k‘kspace’ matrix (spatial frequency domain corr time domain; x : k, f : t – FT pairs; Larmor relation through gradients: ∆f = (γ (γ/2π /2π)·Gx·∆x) k-space divided into quadrants w/ origin at center FID data encoded in kx by FEG and in ky by PEG Spat Freq enc.: [[-kmax,kmax] Complex conjugate symmetry: only ½ matrix + one line req adapted from Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 426 +k ·· · ¬ ·· · ¬ ¬ +k -k © UW and Brent K Stewart, PhD, DABMP +k adapted from Hashemi, Hashemi, et al MRI the Basics, p 140 17 ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 427 © UW and Brent K Stewart, PhD, DABMP 18 Amendment to Bushberg Figure 1515-15 Pulse Sequences ¬ Example: cycles/sec in kx MR data acquired as a complex, composite frequency waveform With methodical variations of the PEG during each excitation, the the kkspace matrix is filled (or partially filled) to produce the desired desired variations across the FE and PE directions Tailoring pulse sequences emphasizes the image contrast dependent on ρ, T1 and T2 Timing, order, polarity, pulse shaping, and repetition frequency of RF pulses and x, y and z gradient application Major pulse sequences ¬ ¬ ¬ ¬ ¬ Spin Echo (SE) Inversion recovery (IR) Fast Spin Echo (FSE) Gradient Recalled Echo (GRE) Echo Planar Image (EPI) c.f http://www.indianembassy.org/dydemo/page3.htm © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 19 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 428 © UW and Brent K Stewart, PhD, DABMP 20 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Summary of 2D SE Acquisition Steps ¬ ¬ ¬ ¬ Summary of 2D SE Acquisition Steps (1) Narrowband RF pulse applied simultaneously with SSG (center t=0); SSG: ∂(∆ ∂(∆f)/∂z )/∂z (1) Mz converted to Mxy, the extent determined by the flip θ (2) PEG applied to SSG for short time (encoding precessional ∆φ along PE grad.) and with differing amplitudes for each repetition to create ∂(∆φ)/ ∂y along PE direction: ∂(∆φ)/∂y multiple views along ky (3) Refocusing 180° 180° RF pulse delivered at t = TE/2: inverting spins c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 428 © UW and Brent K Stewart, PhD, DABMP ¬ ¬ ¬ 21 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 428 Summary of 2D SE Acquisition Steps ¬ ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 © UW and Brent K Stewart, PhD, DABMP 22 Summary of 2D SE Acquisition Steps (5) ADC sampling rate determined by the excitation BW (6) Data stored in kk-matrix row (k (kx) the position (k (ky) determined by the PEG magnitude (6) Inc changes in PEG mag fills matrix one row at a time (may be nonnon-sequential) (6) When filled partially then copy complex conjugate data into remaining blank rows (7) 2D FT decodes time (spatial frequency - k) domain data piecewise along the rows (k (kx) and then columns (k (ky) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 428 (4) ReRe-establishment of phase coherence at t = TE (FID echo) (4) During echo formation and subsequent delay, FEG (∂(∆ ∂(∆f)/∂x) )/∂x) applied to both SSG and PEG, encoding precessional frequency along the readout gradient (5) Simultaneous to application of FEG and echo formation, the computer acquires the timetime-domain signal (FID echo) using ADC ¬ ¬ ¬ ¬ 23 (8) Object spatial and contrast characteristics manifested in the resulting image (8) Final image a spatial representation of the ρ, T1, T2 and flow characteristics of the tissues in each voxel using a graygray-scale range Voxel thickness determined by SSG and RF freq bandwidth Pixel dimension determined by varying PEG magnitudes and readout digitization rate c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 428 © UW and Brent K Stewart, PhD, DABMP 24 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Summary of 2D SE Acquisition Steps ¬ ¬ TwoTwo-dimensional MultiMulti-planar Acquisition Bulk of information representing lower spatial frequencies near center of kk-space – provides large area contrast in the image Higher spatial frequency nearer the periphery – provides resolution and detail in the image ¬ ¬ ¬ Max signal in center of kk-space ¬ +k ¬ Axial (SSG: z, PEG: y, FEG: x) Coronal (SSG: y, PEG: x, FEG: z) Sagittal (SSG: x, PEG: y, FEG: z) Oblique (SSG: a1x + a2y + a3z, etc.) Data acquisition into the kk-space matrix same for all ·· · y z z ·· · +k x -k c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 429 © UW and Brent K Stewart, PhD, DABMP adapted from Hashemi, Hashemi, et al MRI the Basics, p 140 25 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 430 Acq Time, 2DFT SE and Multislice Acq ¬ ¬ ¬ ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 y © UW and Brent K Stewart, PhD, DABMP 26 Data Synthesis Acq time = TR · no PE steps · NEX (number of excitations) Example (256x192 matrix, TR=600, NEX=2) 230 sec PE along lesser matrix dimension to speed acquisition Multiple slice acquisition also speeds image collection Max number slices = TR/(TE+C) C dependent on MRI system capabilities Longer TR more slices c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 431 x +k ¬ ¬ ¬ ¬ 27 Take advantage of symmetry and redundant characteristics of kk-space domain signals In PE direction ‘½ Fourier’, ‘½ NEX’ or ‘phase conjugate symmetry’ techniques reduce data collection to ½ ky matrix dimension + line In FE direction ‘fractional echo’ and ‘read conjugate symmetry’ shorten FID echo sampling time Both SNR and artifacts c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 432 With quadrature detection, have real and “imaginary” (90° out of phase) components of induced voltage from FID (t): V(t) = V1·cos(2πft) + i·V2·sin(2πft) Two data values per digitized FID sample Complex conjugate = V1·cos(2πft) – i·V2·sin(2πft) © UW and Brent K Stewart, PhD, DABMP 28 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Inversion Recovery (IR) Acquisition ¬ ¬ ¬ ¬ 180180-(TI)(TI)-9090-(TE/2)(TE/2)-180180-(TR) SSG, PEG and FEG as SE TR long many slices per TR STIR ¬ ¬ ¬ ¬ Fast Spin Echo (FSE) Acquisition ¬ ¬ ¬ ¬ Short Tau IR Eliminate Fat TI = 180 msec ¬ ¬ FLAIR ¬ ¬ ¬ ¬ FLuid FLuid Attenuated IR Eliminate CSF TI = 2,400 msec ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 434 © UW and Brent K Stewart, PhD, DABMP 29 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 433 Gradient Recalled Echo (GRE) Acquisition ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 © UW and Brent K Stewart, PhD, DABMP 30 Echo Planar Image (EPI) Acquisition Similar to SE but with readout gradient reversal for 180° 180° pulse Repetition of acq for each PE With small flip angles and gradient reversals large reduction in TR and TE fast acq PEG rewinder pulse (opp polarity) to maintain φ relationship between pulses (due to short TR) Acq time=TR time=TR·· no PE steps · NEX Example (256x192 matrix, TR=30): 15.5 sec SNR and artifacts; one slice GRASS, FISP, FLASH, etc c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 434 FSE uses multiple PE steps w/ multiple 180° 180° pulses per TR First echos placed near ky=0 Best SNR least T2 decay Immunity from B0 inhomogen with up to 16x faster collection Lower SNR for highhigh-freq ky Fewer slices collected per TR SE: 8.5 (TR=2000, 256 PE) FSE: 2.1 (TR=2000, 256 PE steps and echos per TR) aka: ‘turbo SE’ SE’ & RARE (R (Rapid Acq w/ Refocused Echoes) ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ 31 Extremely fast imaging Single (1 TR) and multimulti-shot 90° 90° flip, PEG/FEG, 180° 180° flip Oscillating PEG/FEG ‘blips’ stimulate echo formation Rapid ‘zig‘zig-zag’ kk-space filling Acq occurs in a period < T2*: 2525-50 msec High demands on sampling rate, gradient coils and RF deposition limitations Poor SNR, low res (642) and many artifacts ‘Real‘Real-time’ snapshot c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 435 © UW and Brent K Stewart, PhD, DABMP 32 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Spiral KK-space Acquisition ¬ ¬ ¬ ¬ Gradient Moment Nulling Simultaneous oscillation of PEG/FEG to sample data during echo formation in a spiral starting at kk-space origin Regridding to 2D kk-space array for 2D FT Efficient method placing maximum samples in the lowlowfrequency are of kk-space Like EPI sensitive to T2*: field inhomogeneities and susceptibility agents ¬ ¬ ¬ ¬ ¬ In SE and GRE SSG/FEG balanced so that the uniform dephasing caused by the initial gradient application is rephased by an opposite polarity gradient of equal area Moving spins phase dispersal not compensated Constant flow: spins can be rephased with a gradient triplet HigherHigher-order corrections Applied to both SSG/FEG to correct motion ghosting and pulsatile flow A = -1, B = and C = -3 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 436 © UW and Brent K Stewart, PhD, DABMP 33 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 437 Raphex 2001 Diagnostic Questions ¬ D50 D50 Which of the following does NOT generally affect the total exam time of an MRI study? ¬ A # of acquisitions B # of frequency encoding steps C # of phase encoding steps D # of pulse sequences in the study E TR ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 34 3D Fourier Transform Image Acquisition ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP ¬ ¬ ¬ ¬ ¬ 35 Uses a broadband, nonnonselective RF pulse to excite a large spin volume Acq time = TR · no PE steps (z) · no PE steps (y) · NEX SE: TR=600, 1283 164 GRE: TR=50, 1283 14 Isotropic or anisotropic ( 20 T: membrane permeability, enzyme kinetic changes and altered altered biopotentials < 10 T: these effects have not been demonstrated 63 Machine Dependent Artifacts Susceptibility Artifacts Gradient Field Artifacts Radiofrequency Coil Artifacts Radiofrequency Artifacts K-space Errors Chemical Shift Artifacts Ringing Artifacts Wraparound Artifacts Partial Volume Artifacts © UW and Brent K Stewart, PhD, DABMP 64 16 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Machine Dependent Artifacts ¬ ¬ ¬ Susceptibility Artifacts ¬ Magnetic field inhomogeneities distortion or misplacement of anatomy Proper site planning, selfself-shielded magnets, automatic shimming and PM procedures > homogeneity Focal field inhomogeneities ferromagnetic objects: field distortions, signal void ¬ ¬ ¬ ¬ Magnetic susceptibility: ratio of induced internal magnetization in a tissue to external magnetic field (B0) Drastic changes in mag suscept distort B0 TissueTissue-air interfaces: lungs and sinuses rapid T2* Metal: ferrous or not Paramagnetic agents (Gd) ¬ ¬ ¬ Paramagnetic effects shorten T2 Hydration layer interactions shorten T1 Mag suscept of blood degradation products ¬ Diagnose the age of a hemorrhage EPI diffusion study suffers from severe susceptibility artifact due to retained metal after surgery Courtesy, GE Medical Systems © UW and Brent K Stewart, PhD, DABMP 65 © UW and Brent K Stewart, PhD, DABMP Gradient Field Artifacts ¬ ¬ ¬ ¬ Radiofrequency Coil Artifacts Reconstruction algorithm assume linear gradients Tendency for gradient field strength at periphery of FOV to deviate from linear assumption Reduce FOV or lower gradient strength Need balanced gradient strength for PEG and FEG ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 Stray RF signals ¬ ¬ ¬ ¬ ¬ ¬ 67 TV, radio, electric motors, fluorescent lights & computers Narrowband: zipper artifact perpendicular to FEG direction Broadband: herringbone artifact across larger area RF shielding : Faraday cage RF quadrature coils: imbalanced amplifiers DC offset ¬ Otherwise nonnon-square pixels c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 449 66 Causes ghosting of objects diagonally in image Surface coils variations in uniformity across the image caused by RF attenuation, RF mismatching and sensitivity falloff with distance The scanner room door was left open during the acquisition causing the zipper artifacts shown c.f., www.spectroscopynow.com/Spy/pdfs/mritutor.pdf © UW and Brent K Stewart, PhD, DABMP 68 17 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Radiofrequency Artifacts ¬ K-space Errors NonNon-rectangular RF pulses: sliceslice-toto-slice interference ¬ ¬ ¬ ¬ ¬ ¬ T2SNR T2-weighted T1image T1-weighted contrast Interslice gaps and pseudopseudorectangular RF pulses Slice interleaving c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 450 © UW and Brent K Stewart, PhD, DABMP 69 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 451 Motion Artifacts ¬ Mostly occur along the PEG direction ghost images Compensation methods: ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Cardiac/respiratory gating Respiratory ordering Signal averaging Short TE SE sequences Gradient moment nulling Presaturation pulses applied outside the imaging region ¬ ¬ ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 451451-2 © UW and Brent K Stewart, PhD, DABMP 2, and 16 June 2005 © UW and Brent K Stewart, PhD, DABMP 70 Chemical Shift Artifacts ¬ ¬ Artifactual superimposition of wave patterns across the FOV Even one bad pixel can produce a significant artifact, especially especially when at or near kk-space DC data point (center) 71 f0 variations resulting from intrinsic magnetic shielding f = (1 - σ) · (γ/2π /2π) · B0 Distinct peaks in MR spectrum Fat: 3.5 ppm lower than H20 B chemical shift G chemical shift Cannot distinguish freq shift by FEG or chemical shift Misregistration of H20 and fat moieties anatomical shift Cure: G, but SNR Cure: offoff-reson presat pulse Cure: STIR bounce point c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 454 © UW and Brent K Stewart, PhD, DABMP 72 18 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Ringing Artifacts ¬ ¬ ¬ ¬ ¬ ¬ Wraparound Artifacts AKA Gibbs phenomenon Occurs near sharp boundaries and highhigh-contrast transitions Multiple, regularly spaced parallel bands of alternating bright/dark signal fading with distance Lack of highhigh-frequency signals causes ‘ringing’ at sharp transitions Most likely for small matrix dimensions Skull/brain interface ¬ ¬ ¬ Result of mismapping anatomy that lies outside the FOV, but within the slice volume Opposite side of image Caused by: ¬ ¬ ¬ ¬ ¬ NonNon-linear gradients Undersampling of frequencies within the signal envelope (Nyquist sampling limit) FT cannot distinguish freq > Nyquist limit lower freq Cure: lowlow-pass filter ([...]... Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Some Advanced Topics ¬ ¬ ¬ ¬ ¬ Signal from Flow TimeTime-ofof-Flight (TOF) MR Angiography (MRA) Phase Contrast MRA Magnetization Transfer Contrast (MTC) Perfusion and Diffusion Contrast fMRI and BOLD Imaging ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 77 c.f Bushberg, et al The Essential Physics of Medical Imaging, ... head/brain motion Eddy currents c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 410 T1 © UW and Brent K Stewart, PhD, DABMP 2, 9 and 16 June 2005 ρ T2 FLAIR 87 © UW and Brent K Stewart, PhD, DABMP 88 22 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course fMRI and BOLD Imaging ¬ ¬ ¬ Areas of metabolic activity correlated signal (functional... (10 g) require controlled and restricted access w/ signs Stray RF signal protection: Faraday cage (copper sheeting/mesh) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 463 © UW and Brent K Stewart, PhD, DABMP 56 14 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Quality Control ¬ Periodical checking of: ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Raphex 2001... Essential Physics of Medical Imaging, 2nd ed., p 444 © UW and Brent K Stewart, PhD, DABMP 80 20 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Phase Contrast MR Angiography (MRA) ¬ ¬ ¬ ¬ ¬ Phase Contrast MR Angiography (MRA) Relies on phase change (∆φ (∆φ)) for moving protons (blood); ∆φ = ½ · γ/2π /2π · Gx · vx · t2 Application of + and then – polarity gradients... Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Radiofrequency Artifacts ¬ K-space Errors NonNon-rectangular RF pulses: sliceslice-toto-slice interference ¬ ¬ ¬ ¬ ¬ ¬ T2SNR T2-weighted T1image T1-weighted contrast Interslice gaps and pseudopseudorectangular RF pulses Slice interleaving c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed.,... anatomical shift Cure: G, but SNR Cure: offoff-reson presat pulse Cure: STIR bounce point c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 454 © UW and Brent K Stewart, PhD, DABMP 72 18 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Ringing Artifacts ¬ ¬ ¬ ¬ ¬ ¬ Wraparound Artifacts AKA Gibbs phenomenon Occurs near sharp boundaries and... Artifacts Susceptibility Artifacts Gradient Field Artifacts Radiofrequency Coil Artifacts Radiofrequency Artifacts K-space Errors Chemical Shift Artifacts Ringing Artifacts Wraparound Artifacts Partial Volume Artifacts © UW and Brent K Stewart, PhD, DABMP 64 16 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Machine Dependent Artifacts ¬ ¬ ¬ Susceptibility... costs, cryogen costs, difficulty turning B off in emergency and extensive fringe fields c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 459 © UW and Brent K Stewart, PhD, DABMP 52 13 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Resistive Magnet ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Ancillary Equipment Either air core or solid core Continuous electrical... Essential Physics of Medical Imaging, 2nd ed., p 451 Motion Artifacts ¬ Mostly occur along the PEG direction ghost images Compensation methods: ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ ¬ Cardiac/respiratory gating Respiratory ordering Signal averaging Short TE SE sequences Gradient moment nulling Presaturation pulses applied outside the imaging region ¬ ¬ ¬ ¬ ¬ ¬ ¬ c.f Bushberg, et al The Essential Physics of Medical Imaging, ... (RF/gradients) © UW and Brent K Stewart, PhD, DABMP 60 15 Magnetic Resonance Imaging – Bushberg Chapter 15 Diagnostic Radiology Imaging Physics Course Safety and Bioeffects Safety and Bioeffects ¬ Static Magnetic Fields ¬ ¬ ¬ Varying Magnetic Field Effects ¬ ¬ ¬ ¬ ¬ ¬ ¬ © UW and Brent K Stewart, PhD, DABMP 61 ¬ ¬ ¬ ¬ D49 D49 Patients who have MRI scans should be screened to eliminate those who have: ¬ ¬ A Internal