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| | | | • View the DATABASE results for 'Cartesian Sampling' (4).
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Coherent gradient echo sequences can measure the free induction decay ( FID), generated just after each excitation pulse or the echo formed prior to the next pulse. Coherent gradient echo sequences are very sensitive to magnetic field inhomogeneity.
An alternative to spoiling is to incorporate residual transverse magnetization directly into the longitudinal steady state.
These GRE sequences use a refocusing gradient in the phase encoding direction during the end module to maximize rem aining transverse (xy) 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 ag ain partly tilted into the xy-plane, while the rem aining xy-magnetization is tilted partly into the z-direction.
A fully refocused sequence with a properly selected and uniform f would yield higher signal, especially for tissues with long T2 relaxation times (high water content) so it is used in angiographic, myelographic or arthrographic examinations and is used for T2* weighting.
The repetition time for this sequence has to be short. With short TR, coherent GE is also useable for breath hold and 3D technique. If the repetition time is about 200 msec there's no difference between spoiled or unspoiled GE. T1 weighting is better with spoiled techniques.
The common types include GRASS, FISP, FAST, and FFE.
The T2* component decreases with long TR and short TE. The T1 time is controlled by flip angle. The common TR is less than 50 ms and the common TE less than 15 ms
Other types have stronger T2 dependence but lower SNR. They include SSFP, CE-FAST, PSIF, and CE-FFE-T2.
Examples of fully refocused FID sequences are TrueFISP, bFFE and bTFE. | | | | • View the DATABASE results for 'Coherent Gradient Echo' (6).
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Contrast enhanced GRE sequences provide T2 contrast but have a relatively poor SNR. Repetitive RF pulses with small flip angles together with appropriate gradient profiles lead to the superposition of two resonance signals.
The first signal is due to the free induction decay FID observed after the first and all ensuing RF excitations.
The second is a resonance signal obt ained as a result of a spin echo generated by the second and all addicted RF-pulses.
Hence it is absent after the first excitation, it is a result of the free induction decay of the second to last RF-excitation and has a TE, which is almost 2TR.
For this echo to occur the gradients have to be completely symmetrical relative to the half time between two RF-pulses, a condition that makes it difficult to integrate this pulse sequence into a multiple slice imaging technique.
The second signal not only cont ains echo contributions from free induction decay, but obviously weakened by T2-decay.
Since the echo is generated by a RF-pulse, it is truly T2 rather than T2* weighted. Correspondingly it is also less sensitive to susceptibility changes and field inhomogeneities.
Companies use different acronyms to describe cert ain techniques.
Different terms (see also acronyms) for these gradient echo pulse sequences:
CE-FAST Contrast Enhanced Fourier Acquired Steady State,
CE-FFE Contrast Enhanced Fast Field Echo,
CE-GRE Contrast Enhanced Gradient-Echo,
DE-FGR Driven Equilibrium FGR,
FADE FASE Acquisition Double Echo,
PSIF Reverse Fast Imaging with Steady State Precession,
SSFP Steady State Free Precession,
T2 FFE Contrast Enhanced Fast Field Echo (T2 weighted).
In this context, 'contrast enhanced' refers to the pulse sequence, it does not mean enhancement with a contrast agent. | | | | • View the DATABASE results for 'Contrast Enhanced Gradient Echo Sequence' (4).
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Contrast enhanced MRI is a commonly used procedure in magnetic resonance imaging. The need to more accurately characterize different types of lesions and to detect all malignant lesions is the m ain reason for the use of intravenous contrast agents.
Some methods are available to improve the contrast of different tissues. The focus of dynamic contrast enhanced MRI (DCE-MRI) is on contrast kinetics with demands for spatial resolution dependent on the application. DCE- MR imaging is used for diagnosis of cancer (see also liver imaging, abdominal imaging, breast MRI, dynamic scanning) as well as for diagnosis of cardiac infarction (see perfusion imaging, cardiac MRI). Quantitative DCE-MRI requires special data acquisition techniques and analysis software.
Contrast enhanced magnetic resonance angiography (CE-MRA) allows the visualization of vessels and the temporal resolution provides a separation of arteries and veins. These methods share the need for acquisition methods with high temporal and spatial resolution.
Double contrast administration (combined contrast enhanced (CCE) MRI) uses two contrast agents with complementary mechanisms e.g., superparamagnetic iron oxide to darken the background liver and gadolinium to brighten the vessels. A variety of different categories of contrast agents are currently available for clinical use.
Reasons for the use of contrast agents in MRI scans are:
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Relaxation characteristics of normal and pathologic tissues are not always different enough to produce obvious differences in signal intensity.
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Pathology that is sometimes occult on unenhanced images becomes obvious in the presence of contrast.
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Enhancement significantly increases MRI sensitivity.
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In addition to improving delineation between normal and abnormal tissues, the pattern of contrast enhancement can improve diagnostic specificity by facilitating characterization of the lesion(s) in question.
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Contrast can yield physiologic and functional information in addition to lesion delineation.
Common Indications:
Brain MRI : Preoperative/pretreatment evaluation and postoperative evaluation of brain tumor therapy, CNS infections, noninfectious inflammatory disease and meningeal disease.
Spine MRI : Infection/inflammatory disease, primary tumors, drop metastases, initial evaluation of syrinx, postoperative evaluation of the lumbar spine: disk vs. scar.
Breast MRI : Detection of breast cancer in case of dense breasts, implants, malignant lymph nodes, or scarring after treatment for breast cancer, diagnosis of a suspicious breast lesion in order to avoid biopsy.
For Ultrasound Imaging (USI) see Contrast Enhanced Ultrasound at Medical-Ultrasound-Imaging.com.
See also Blood Pool Agents, Myocardial Late Enhancement, Cardiovascular Imaging, Contrast Enhanced MR Venography, Contrast Resolution, Dynamic Scanning, Lung Imaging, Hepatobiliary Contrast Agents, Contrast Medium and MRI Guided Biopsy. | | | | | | | | | | | • View the DATABASE results for 'Contrast Enhanced MRI' (14).
| | | • View the NEWS results for 'Contrast Enhanced MRI' (8).
| | | | Further Reading: | | Basics:
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News & More:
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FDA Approves Gadopiclenol for Contrast-Enhanced Magnetic Resonance Imaging Tuesday, 27 September 2022 by www.pharmacytimes.com | | |
Effect of gadolinium-based contrast agent on breast diffusion-tensor imaging Thursday, 6 August 2020 by www.eurekalert.org | | |
Artificial Intelligence Processes Provide Solutions to Gadolinium Retention Concerns Thursday, 30 January 2020 by www.itnonline.com | | |
Accuracy of Unenhanced MRI in the Detection of New Brain Lesions in Multiple Sclerosis Tuesday, 12 March 2019 by pubs.rsna.org | | |
The Effects of Breathing Motion on DCE-MRI Images: Phantom Studies Simulating Respiratory Motion to Compare CAIPIRINHA-VIBE, Radial-VIBE, and Conventional VIBE Tuesday, 7 February 2017 by www.kjronline.org | | |
Novel Imaging Technique Improves Prostate Cancer Detection Tuesday, 6 January 2015 by health.ucsd.edu | | |
New oxygen-enhanced MRI scan 'helps identify most dangerous tumours' Thursday, 10 December 2015 by www.dailymail.co.uk | | |
All-organic MRI Contrast Agent Tested In Mice Monday, 24 September 2012 by cen.acs.org | | |
A groundbreaking new graphene-based MRI contrast agent Friday, 8 June 2012 by www.nanowerk.com |
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