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Questions and Answers
What type of image predominantly shows the contrast due to the differences in T1 recovery times of tissues?
What type of image predominantly shows the contrast due to the differences in T1 recovery times of tissues?
Which parameter needs to be short in T1 weighting to diminish T2 effects?
Which parameter needs to be short in T1 weighting to diminish T2 effects?
What type of tissues appear bright in T1 weighted images?
What type of tissues appear bright in T1 weighted images?
Why are T2 weighted images best for demonstrating pathology?
Why are T2 weighted images best for demonstrating pathology?
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In a T2 weighted image, what type of tissues appear dark?
In a T2 weighted image, what type of tissues appear dark?
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What is predominantly responsible for the contrast in an AT2 weighted image?
What is predominantly responsible for the contrast in an AT2 weighted image?
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What type of image best demonstrates anatomy but also shows pathology after contrast enhancement?
What type of image best demonstrates anatomy but also shows pathology after contrast enhancement?
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Why is a long echo time (TE) selected for T2 weighting?
Why is a long echo time (TE) selected for T2 weighting?
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Which type of tissue is bright on a sagittal T1 weighted image of the spine?
Which type of tissue is bright on a sagittal T1 weighted image of the spine?
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Study Notes
Contrast in MRI
- Areas of high signal appear white, indicating a large transverse component of magnetization.
- Areas of low signal appear dark, indicating a small transverse component.
- Intermediate signals show shades of grey, representing medium transverse components.
- Contrast in MR images is influenced by extrinsic contrast parameters and intrinsic contrast mechanisms.
Extrinsic Contrast Parameters
- Repetition Time (TR): Time from one RF pulse application to the next, measured in milliseconds. Affects relaxation period duration.
- Echo Time (TE): Time between RF excitation pulse and signal collection, also measured in milliseconds.
- Flip Angle: The angle the net magnetization vector (NMV) is tilted by an RF pulse.
Intrinsic Contrast Mechanisms
- Governed by tissues' T1 recovery (longitudinal magnetization recovery) and T2 decay (transverse magnetization decay).
- Includes proton density and tissue flow effects.
Molecular Motion and Tissue Composition
- Molecular motion consists of rotational and translational movements. Faster motion complicates energy release.
- Fat has large molecules with low motion rates and efficient energy absorption due to low inherent energy.
- Water consists of smaller molecules with high motion rates and inherent energy, making it less effective at energy absorption.
Relaxation Processes
- Following RF pulse removal, the NMV decreases due to relaxation processes and field inhomogeneities, resulting in signal decay.
- The NMV's transverse plane decay is measured as Free Induction Decay (FID).
T1 Recovery and T2 Decay
- T1 Recovery: Time for 63% of longitudinal magnetization to recover through spin-lattice energy transfer, which occurs at varying rates across tissues.
- T2 Decay: Time for 63% loss of transverse magnetization, caused by spin-spin energy transfer, leading to phase coherence loss.
T1 Recovery in Tissues
- Fat has a short T1 time and recovers quickly, while water has a long T1 time due to inefficient energy absorption.
- TR influences T1 recovery; short TRs may not allow full NMV recovery, creating contrast differences.
Saturation Effects
- Short TRs lead to saturation, pushing NMV beyond the transverse plane, creating contrast differences.
- Long TRs allow full recovery of longitudinal components, minimizing contrast between fat and water.
T2 Decay Characteristics
- T2 decay is exponential and varies by tissue; efficiency correlates with molecular motion and proximity of spins.
- Short TE does not allow complete dephasing, leading to reduced contrast between fat and water.
- Long TE permits full dephasing, leading to distinct contrast differences between fat and water due to varied T2 decay times.
T2* Loss of Coherence
- Loss occurs via intrinsic magnetic field interactions between adjacent nuclei and external field inhomogeneities.
- Variations in B0 lead to different precessional frequencies, contributing to dephasing of NMV and transverse magnetization decay.
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Description
Test your knowledge on MRI image contrast mechanisms including T1 recovery, T2 decay, T1 weighting, T2 weighting, and proton density weighting. Learn about areas of high, low, and intermediate signal in MR images.
