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Result : Searchterm 'Pulse Sequence' found in 5 terms [] and 166 definitions []
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Magnetic Resonance MyelographyMRI Resource Directory:
 - MR Myelography -
 
MR myelography is studying the spinal canal and subarachnoid space by high-resolution MRI with a technique in which a sequence with strong T2 weighting is used to provide high contrast between the "dark" spinal cord and its nerves and the surrounding "bright" cerebrospinal fluid. MR myelography as part of an entire MR examination has virtually replaced X-ray myelography. Used sequences are T2 weighted fast spin echo pulse sequences or a refocused gradient echo pulse sequence with strong T2 weighting.

See also the related poll result: 'MRI will have replaced 50% of x-ray exams by'
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Ultrasound  (1) Open this link in a new window
Partial Fourier Technique
 
The partial Fourier technique is a modification of the Fourier transformation imaging method used in MRI in which the symmetry of the raw data in k-space is used to reduce the data acquisition time by acquiring only a part of k-space data.
The symmetry in k-space is a basic property of Fourier transformation and is called Hermitian symmetry. Thus, for the case of a real valued function g, the data on one half of k-space can be used to generate the data on the other half.
Utilization of this symmetry to reduce the acquisition time depends on whether the MRI problem obeys the assumption made above, i.e. that the function being characterized is real.
The function imaged in MRI is the distribution of transverse magnetization Mxy, which is a vector quantity having a magnitude, and a direction in the transverse plane. A convenient mathematical notation is to use a complex number to denote a vector quantity such as the transverse magnetization, by assigning the x'-component of the magnetization to the real part of the number and the y'-component to the imaginary part. (Sometimes, this mathematical convenience is stretched somewhat, and the magnetization is described as having a real component and an imaginary component. Physically, the x' and y' components of Mxy are equally 'real' in the tangible sense.)
Thus, from the known symmetry properties for the Fourier transformation of a real valued function, if the transverse magnetization is entirely in the x'-component (i.e. the y'-component is zero), then an image can be formed from the data for only half of k-space (ignoring the effects of the imaging gradients, e.g. the readout- and phase encoding gradients).
The conditions under which Hermitian symmetry holds and the corrections that must be applied when the assumption is not strictly obeyed must be considered.
There are a variety of factors that can change the phase of the transverse magnetization:
Off resonance (e.g. chemical shift and magnetic field inhomogeneity cause local phase shifts in gradient echo pulse sequences. This is less of a problem in spin echo pulse sequences.
Flow and motion in the presence of gradients also cause phase shifts.
Effects of the radio frequency RF pulses can also cause phase shifts in the image, especially when different coils are used to transmit and receive.
Only, if one can assume that the phase shifts are slowly varying across the object (i.e. not completely independent in each pixel) significant benefits can still be obtained. To avoid problems due to slowly varying phase shifts in the object, more than one half of k-space must be covered. Thus, both sides of k-space are measured in a low spatial frequency range while at higher frequencies they are measured only on one side. The fully sampled low frequency portion is used to characterize (and correct for) the slowly varying phase shifts.
Several reconstruction algorithms are available to achieve this. The size of the fully sampled region is dependent on the spatial frequency content of the phase shifts. The partial Fourier method can be employed to reduce the number of phase encoding values used and therefore to reduce the scan time. This method is sometimes called half-NEX, 3/4-NEX imaging, etc. (NEX/NSA). The scan time reduction comes at the expense of signal to noise ratio (SNR).
Partial k-space coverage is also useable in the readout direction. To accomplish this, the dephasing gradient in the readout direction is reduced, and the duration of the readout gradient and the data acquisition window are shortened.
This is often used in gradient echo imaging to reduce the echo time (TE). The benefit is at the expense in SNR, although this may be partly offset by the reduced echo time. Partial Fourier imaging should not be used when phase information is eligible, as in phase contrast angiography.
See also acronyms for 'partial Fourier techniques' from different manufacturers.
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MRI Resources 
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Refocused Gradient Echo SequenceInfoSheet: - Sequences - 
Intro, 
Overview, 
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etc.MRI Resource Directory:
 - Sequences -
 
Refocused GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize (refocus) remaining xy- (transverse) magnetization at the time when the next excitation is due, while the other two gradients are, in any case, balanced.
When the next excitation pulse is sent into the system with an opposed phase, it tilts the magnetization in the a direction. As a result the z-magnetization is again partly tilted into the xy-plane, while the remaining xy-magnetization is tilted partly into the z-direction.
Companies use different acronyms to describe certain techniques.
Different terms for these gradient echo pulse sequences:
R-GRE Refocused Gradient Echo,
FAST Fourier Acquired Steady State,
FFE Fast Field echo,
FISP Fast Imaging with Steady State Precession,
F-SHORT SHORT Repetition Technique Based on Free Induction Decay,
GFEC Gradient Field Echo with Contrast,
GRASS Gradient Recalled Acquisition in Steady State,
ROAST Resonant Offset Averaging in the Steady State,
SSFP Steady State Free Precession.
STERF Steady State Technique with Refocused FID
In this context, 'contrast' refers to the pulse sequence, it does not mean enhancement with a contrast agent.
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S-SCANInfoSheet: - Devices -
Intro, 
Types of Magnets, 
Overview, 
etc.
 
www.esaote.com/products/MRI/sScan/products1.htm From Esaote S.p.A.; Esaote introduced the S-SCAN at RSNA in November 2007. The S-SCAN is a dedicated joint and spine MR scanner derived from the company's earlier G-SCAN system. Unlike the G-SCAN, neither the patient table nor the magnet can rotate from horizontal to vertical position. The patient table can only moved manually. Improved electronics, new coils for lumbar and cervical spine, new pulse sequences, a modified version of the magnet poles and gradient coils are used with a new software release in the S-SCAN.
Esaote North America is the exclusive U.S. distributor of this MRI device.
Device Information and Specification (Under Development)
CLINICAL APPLICATION Musculoskeletal, extremity
CONFIGURATION Open MRI
SURFACE COILS 4-channel phased array spine coil, extremity, shoulder, flex coil, knee dual phased array, ankle//foot dual phased array, hand//wrist dual phased array
PULSE SEQUENCES SE, GE, IR, STIR, TSE, 3D CE, GE-STIR, 3D GE, ME, TME, HSE
IMAGING MODES Single, multislice, volume study, fast scan, multi slab, cine
FOV 25 cm
DISPLAY MATRIX 512 x 512
MEASURING MATRIX 256 x 256 maximum
MAGNET TYPE Permanent
BORE DIAMETER
or W x H
33 cm H, open
POWER REQUIREMENTS 3 kW; 110/220 V single phase
FIELD STRENGTH 0.25 T
STRENGTH 25 mT/m
5-GAUSS FRINGE FIELD, radial/axial 180 cm
SHIMMING Passive
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Further Reading:
  News & More:
Wichita medical facility gets first Hologic S-scan MRI in the United States
Friday, 19 October 2007   by wichita.bizjournals.com    
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Signa Infinity 1.0T™InfoSheet: - Devices -
Intro, 
Types of Magnets, 
Overview, 
etc.MRI Resource Directory:
 - Devices -
 
From GE Healthcare;
www.gehealthcare.com/usen/mr/index.html the Signa Infinity Magnetic Resonance system is a short bore, high performance, whole-body imaging system operating at 1.0 Tesla. The system can image in any orthogonal or oblique plane (including single and double axis oblique), using a wide variety of pulse sequences.

Device Information and Specification
CLINICAL APPLICATION Whole body
CONFIGURATION Short bore
SURFACE COILS Head and body coil standard; all other coils optional; open architecture makes system compatible with a wide selection of coils
SPECTROSCOPY No
SYNCHRONIZATION ECG/peripheral, respiratory gating
PULSE SEQUENCES Standard: SE, IR, 2D/3D GRE and SPGR, Angiography;; 2D/3D TOF, 2D/3D Phase Contrast;; 2D/3D FSE, 2D/3D FGRE and FSPGR, SSFP, FLAIR, optional: EPI, 2D/3D Fiesta, FGRET, Spiral
IMAGING MODES Localizer, single slice, multislice, volume, fast, POMP, multi slab, cine
TR 4.4 msec to 12000 msec in increments of 1 msec
TE 1.0 to 2000 msec; increments of 1 msec
SINGLE/MULTI SLICE Simultaneous scan and reconstruction;; up to 100 images/second with Reflex 100
FOV 1 cm to 48 cm continuous
SLICE THICKNESS 2D 0.7 mm to 20 mm; 3D 0.1 mm to 5 mm
DISPLAY MATRIX 1280 x 1024
MEASURING MATRIX 128x512 steps 32 phase encode
PIXEL INTENSITY 256 gray levels
SPATIAL RESOLUTION 0.08 mm; 0.02 mm optional
MAGNET TYPE Superconducting
BORE DIAMETER 60 cm
MAGNET WEIGHT 3613 kg
H*W*D 172 x 208 x 216 cm
POWER REQUIREMENTS 480 or 380/415 V
COOLING SYSTEM TYPE Closed-loop water-cooled gradient
CRYOGEN USE Less than 0.03 L/hr liquid helium
FIELD STRENGTH 1.0 T
STRENGTH SmartSpeed 23 mT/m, HiSpeed Plus 33 mT/m
5-GAUSS FRINGE FIELD 4.0 m x 2.8 m axial x radial
SHIMMING Active
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MRI Resources 
IR - Cochlear Implant - PACS - Raman Spectroscopy - Anatomy - Hospitals
 
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