X-ray Hardware: An Overview for Clinicians .


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MRI Hardware: An Overview for Clinicians. Richard G. Spencer, M.D., Ph.D. Nuclear Magnetic Resonance Unit National Institute on Aging, National Institutes of Health Baltimore, Maryland USA. Why go to a talk?. ii) to give good answers. i) to ask good questions. 1. Magnet 2. Gradients
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Slide 1

X-ray Hardware: An Overview for Clinicians Richard G. Spencer, M.D., Ph.D. Atomic Magnetic Resonance Unit National Institute on Aging, National Institutes of Health Baltimore, Maryland USA

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Why go to a discussion? ii) to give smart responses i) to ask great inquiries

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1. Magnet 2. Angles 3. Transmitter 4. Collector 5. Test • Considerations • Reasonable inquiries • A couple points of interest • A couple of details

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Magnet

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Magnet Considerations • High field for high flag to-commotion proportion • Weight • Diameter and length (understanding background; field homogeneity) • Homogeneity (spatial); Field security (transient) • Configuration (get to; patient experience) • Cryogenic effectiveness (working expenses)

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What kind of magnet is it? Is it protected? Field quality? Bore measure? How steady is the field? What\'s the field homogeneity?

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Resistive Magnet Type Wire Solenoid • Field solidness  require an extremely stable 10\'s of kW power supply • Power prerequisites  B 2  cooling necessities  0.2 T or less

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Permanent Magnet Type • Excellent field security • Configuration: open frameworks are accessible • No power utilization be that as it may... • Weight- - can be tremendous: press 0.2 T entire body weighs around 25 tons • 0.2 T neodymium compound ~ 5 tons • Homogeneity- - can be an issue

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Open Permanent Magnet System Siemens Viva- - 0.2 T

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Superconducting Magnet Type • Required for high field frameworks • Homogeneous field • Stable field notwithstanding... • Expensive • Quench wonder Siemens/Bruker Magnetom Allegra-3T, head just

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Field Strength • Polarization of nuclear cores Larger field  More turn arrangement  More motion from every pixel or voxel

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Trading Rules Signal-to-clamor expanded at high field Resolution Speed

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More flag from every pixel implies: Each pixel can be littler

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More flag per unit time implies: Images can be gained speedier Cardiac MRI Functional MRI RN Berk, UCSD S Smith, Oxford

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• Greater phantom determination Field Strength in Spectroscopy 31 P NMR Spectrum of Skeletal Muscle at 1.9 T 31 P NMR Spectrum of Rat Heart at 9.4 T Baseline uncertain Baseline determined PCr  - ATP  - ATP

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Field Strength Considerations However, at higher field: • expanded synthetic move curios, e.g. fat/water • expanded vulnerability artifacts • expanded siting cost • expanded introductory cost

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Field Strength Ohio State, Bruker 8T Whole-body System Gradient-reverberate Images Typical clinical frameworks: 0.2 T to 1.5 T to 3 T Whole-body: 3 T, 4 T, … , 8 T, 9.4 T accessible

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Field Strength High field particularly helpful for fMRI and spectroscopy- - less clearly so for standard imaging • Typical creature frameworks: 4.7 T, 7 T,… , 9.4 T, 11 T even 9.4 T, 11 T vertical

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Bore Size 600 mm magnet warm bore 570 mm magnet at shoulder incl. shim 360 mm angle loop inward breadth 265 mm RF-headcoil internal measurement Siemens/Bruker Magnetom Allegra-3T, head just • Bigger is better- - e.g. "head just" fits just heads!

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Bore Size • Human i) Whole Body (run of the mill: 100 cm) ii) Head just (run of the mill: 80 cm) •Animal 15 cm, 20 cm, 30 cm, 40 cm, ... In any case: • Larger bore larger periphery field • Larger slope sets can be slower • Cost

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Magnetic Shielding: Containment of the Fringe Field • Fringe field: part of the attractive field that reaches out past the magnet bore • 5 G taken as most extreme safe open presentation • Effect on e.g. pacemakers, steel instruments, and attractive cards • Effect from e.g. moving autos and lifts

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Magnetic Shielding Considerations • Weight • Footprint • Expense including space Note: periphery field is 3D

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Magnetic Shielding Options • Unshielded: • Lightest, least expensive However: biggest field impression, most costly space • Passive protecting: ferromagnetic material put outside magnet • Small field impression: diminish by calculate of 2 all bearings However: heaviest- - 10\'s of huge amounts of iron • Active protecting: electromagnetic counter-windings outside the primary magnet curl • Similar field impression concerning uninvolved protecting • Mild increment in weight versus unshielded However: most noteworthy magnet cost

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Sample Magnet Specifications • Field solidness: superior to 0.1 ppm/hour float Note: fat-water division = 3.5 ppm • Field homogeneity*: superior to 0.3 ppm more than 22 cm and 5 ppm more than 50 cm breadth circular volume (DSV) *without room-temperature shims 1 ppm = 0.00001%

Slide 25

Gradients

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Magnetic Field Gradients: required along every one of the three tomahawks Coils Magnetic Field—changes with position Field Strength I Distance I (flow) heading demonstrated by

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Operation of Gradients No angle: B z,Local = B 0  z With slope: B z,Local = B 0 + z G z Spatial variety of the B field grants spatial mapping of twists: recurrence  spatial position

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Gradient Considerations • Want high in-plane determination • Want contract cut ability in 2D imaging • Want pictures which aren\'t misshaped • Want to have the capacity to picture rapidly

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What\'s the inclination linearity? What\'s the slope quality? What\'s the ascent time of the inclinations? Are the slopes effectively protected?

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Two Different "Bandwidths" in MRI • Excitation Bandwidth of a radiofrequency heartbeat The beat energizes turns in this scope of frequencies • Receiver Bandwidth The recipient can identify motions in this scope of frequencies

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Sampling amid MRI flag obtaining Excitation BW resound flag 90  180  rf G s G PE Detection BW G read ADC Sampling time, t

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Excitation Bandwidth and Gradient Strength

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Excitation beat Frequency band energized Excitation Bandwidth Fourier rf adequacy BW Short heartbeat: Broad band  (kilohertz) t (milliseconds) BW Long heartbeat: Narrow band Fourier rf abundancy t (milliseconds)  (kilohertz)

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Pulse Bandwidth a.k.a. Excitation Bandwidth Longer span beats  • smaller excitation transfer speed be that as it may… • longer resound time—loss of flag from short T 2 species • more prominent specimen warming • unwinding impacts amid heartbeats Would get a kick out of the chance to have the capacity to utilize short heartbeats and still have limit cut

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Slice inclination quality + Pulse data transmission  Slice thickness  G s z =  BW excitation (Hz) 2 G/cm = 20 mT/meter Frequency   G 0.5 G/cm 2000 Hz Pulse Bandwidth Spatial measurement 2.5 mm Slice Thickness 1 cm Slice Thickness

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Effect of Slice Gradient Strength on Slice Thickness  Slice Thickness

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Receiver Bandwidth and Gradient Strength

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MRI maps a recurrence range to a spatial range  G r FOV =  BW recipient (Hz) Receiver BW=4,300 Hz Receiver BW=4,300 Hz G r = 2 Gauss/cm G r = 0.5 Gauss/cm  G  FOV = 2 cm FOV = 5 mm

Slide 39

Effect of Read Gradient Strength on In-plane Resolution

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 B sought  B incited Gradient Eddy Currents Faraday\'s Law Increased streams with more quick exchanging

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B actuated causes B-field twists Gradient intensifier driving waveform Resulting angle waveform Cure #1: Pre-accentuation Gradient enhancer driving waveform Resulting slope waveform

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Cure #2: Shielded Gradients Gradient Coils Magnet bore Gradient Shield Coils

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Gradient Linearity Effect on Image Accuracy B z, Local = B 0 + G (z) Nonlinear angle: geometric bending Linear angle: nondistorted picture B z, Local = B 0 + z G z straight z

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Sample Gradient Specifications • Gradient quality: 2.5 G/cm (clinical)  4 G/cm, 8 G/cm 10-100 G/cm (creature) Increased inclination quality  higher determination, smaller cuts in any case: likewise expanded warming, expanded ascent time (slower) • Gradient exchanging time (rise and fall time) relies on inductance and driving voltage: 0.2 ms to ascend to 2 G/cm Faster exchanging  better execution in quick imaging arrangements • Gradient linearity: 5% more than 22 cm width round volume Better linearity  less picture contortion

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Transmitter Considerations • Need to consistently energize extensive data transmission • Require precise molded heartbeats (time, abundancy) • Desire simple to control yield • Frequency steadiness

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What\'s the transmitter control? What\'s the linearity of the speaker?

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Transmitter Low power rf Transmitter Linearity Input 1 volt Gain set to two Output 2 volts Low power rf Transmitter Gain set to two Input 2 volts Linear: Output 4 volts Nonlinear: Output = 3.5 volts

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Transmitter linearity is critical for precise molded heartbeats Transmitter Low power contribution to transmitter speaker High power yield from transmitter intensifier ...and for aligning heartbeats

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What is a decibel? The dB scale communicates enhancement or constriction as the logarithm of a proportion: dB = 20 log 10 (A 2/A 1 ) A 2/A 1 dB 2 6 10 20 100 40

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Sample Transmitter Specifications • Maximum yield: 15 kW • Linear to inside 1 dB over a scope of 40 dB • Output security of 0.1 dB more than 10 ms beat • Output dependability of 0.1 dB heartbeat to-heartbeat

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Receiver Considerations Goal: Receive the microvolt NMR flag and change over it to a discernible resound/FID • Without debasement by commotion • With devoted sufficiency propagation • With loyal recurrence generation

Slide 54

What\'s the digitizer determination? What\'s the beneficiary data transfer capacity? .:

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