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Inversion TimeInfoSheet: - Sequences - 
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(TI) The time period between the 180° inversion pulse and the 90° excitation pulse in an inversion recovery pulse sequence. The inversion time controls the signal of different tissues and with the change of this parameter also fat and water suppression is attainable.
 
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    • Ultrafast Gradient Echo Sequence
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Study identifies new way to predict prognosis for heart failure patients
Tuesday, 10 December 2013   by medicalxpress.com    
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Double Inversion Recovery T1 MeasurementInfoSheet: - Sequences - 
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(DIR or DIRT1) Double inversion recovery T1 measurement is a T1 weighted black blood MRA sequence in which the signal from blood is suppressed. The inversion time to suppress blood is described as the duration between the initial inversion pulse and time point that the longitudinal magnetization of blood reaches the zero point. The readout starts at the blood suppression inversion time (BSP TI) and blood in the imaging slice gives no signal. This inversion time is around 650 ms with a 60 beat per minute heart rate at 1.5 T.
The TI can be decreased by using a wider receive bandwidth, shorter echo train length and/or narrow trigger window. Wide bandwidth also decreases the blurring caused by long echo trains at the expense of signal to noise ratio. In case of in plane or slow flow the suppression of the signal from blood may be incomplete. With increased TE or change of the image plane the blood suppression can be improved.
Double inversion recovery is a breath hold technique with one image per acquisition used in cardiovascular imaging. The patient is instructed to hold the breath in expiration (if not possible also inspiration can be taken), so that the end diastolic volume in the cardiac chambers would be the same during entire scanning. DIR provides fine details of the boundary between the lumen and the wall of the cardiac chambers and main vascular and heart structures, pericardium, and mediastinal tissues.
 
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 Normal Dual Inversion Fast Spin-echo  Open this link in a new window
      

Courtesy of  Robert R. Edelman

 
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• View the DATABASE results for 'Double Inversion Recovery T1 Measurement' (2).Open this link in a new window

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Field Inhomogeneity ArtifactInfoSheet: - Artifacts - 
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Quick Overview

Artifact Information
NAME Field inhomogeneity
DESCRIPTION Image distortion signal loss
REASON B0 inhomogeneity, paramagnetic or ferromagnetic implants
HELP Larger FOV, oversampling

A disturbance of the field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the coil, the homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase. The same problem appears by scanning at a distance from the isocenter in left-right direction or too large field of view.
There are different types of bad image quality, the images are noisy, distorted or the fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the coil the same inversion time is sufficient to suppress water. Since the inversion time is not changed, the T1 recovery rates will increase.


Image Guidance
Take a smaller imaging volume (and for fat suppression a volume shimming), take care that the imaged region is at the center of the coil and that no magnetic material is inside the imaging volume.
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• View the DATABASE results for 'Field Inhomogeneity Artifact' (3).Open this link in a new window

 
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MRI Artifact Gallery
   by chickscope.beckman.uiuc.edu    
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Short T1 Inversion RecoveryInfoSheet: - Sequences - 
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(STIR) Also called Short Tau (t) (inversion time) Inversion Recovery. STIR is a fat suppression technique with an inversion time TI = T1 ln2 where the signal of fat is zero (T1 is the spin lattice relaxation time of the component that should be suppressed). To distinguish two tissue components with this technique, the T1 values must be different. Fluid Attenuation Inversion Recovery (FLAIR) is a similar technique to suppress water.
Inversion recovery doubles the distance spins will recover, allowing more time for T1 differences. A 180° preparation pulse inverts the net magnetization to the negative longitudinal magnetization prior to the 90° excitation pulse. This specialized application of the inversion recovery sequence set the inversion time (TI) of the sequence at 0.69 times the T1 of fat. The T1 of fat at 1.5 Tesla is approximately 250 with a null point of 170 ms while at 0.5 Tesla its 215 with a 148 ms null point. At the moment of excitation, about 120 to 170 ms after the 180° inversion pulse (depending of the magnetic field) the magnetization of the fat signal has just risen to zero from its original, negative, value and no fat signal is available to be flipped into the transverse plane.
When deciding on the optimal T1 time, factors to be considered include not only the main field strength, but also the tissue to be suppressed and the anatomy. In comparison to a conventional spin echo where tissues with a short T1 are bright due to faster recovery, fat signal is reversed or darkened. Because body fluids have both a long T1 and a long T2, it is evident that STIR offers the possibility of extremely sensitive detection of body fluid. This is of course, only true for stationary fluid such as edema, as the MRI signal of flowing fluids is governed by other factors.
See also Fat Suppression and Inversion Recovery Sequence.
 
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 Sagittal Knee MRI Images STIR  Open this link in a new window
      

 
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• View the DATABASE results for 'Short T1 Inversion Recovery' (3).Open this link in a new window

 
Further Reading:
  Basics:
Techniques of Fat Suppression(.pdf)
   by cds.ismrm.org    
MRI GLOSSARY
   by fonar.com    
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Contrast mechanisms in magnetic resonance imaging
2004   by www.iop.org    
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Black Blood MRAForum -
related threadsInfoSheet: - Sequences - 
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With this magnetic resonance angiography technique flowing blood appears dark.
MR black blood techniques have been developed for cardiovascular imaging to improve segmentation of myocardium from the blood pool. Black blood MRA techniques decrease the signal from blood with reference to the myocardium and make it easier to perform cardiac chamber segmentation.
ECG gated spin echo sequences with presaturation pulses for magnetization preparation will show strong intravascular signal loss due to flow effects when appropriate imaging conditions including spatial presaturation are used. The sequence use the flow void effect as blood passes rapidly through the selected slice.
For dark blood preparation, a pair of nonselective and selective 180° inversion pulses are used, followed by a long inversion time to null signal from inflowing blood. A second selective inversion pulse can also be applied with short inversion time to null the fat signal. These in cardiac imaging used black blood techniques are referred to as double inversion recovery T1 measurement turbo spin echo or fast spin echo, and double-inversion recovery STIR.
 
Images, Movies, Sliders:
 Normal Dual Inversion Fast Spin-echo  Open this link in a new window
      

Courtesy of  Robert R. Edelman

 
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• View the DATABASE results for 'Black Blood MRA' (6).Open this link in a new window

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