Frequency Encoding Gradient | MRI Signal Localisation | MRI Physics Course #8

Introduction and Slice Selection

In this section, the speaker introduces the topic of selecting a specific slice along the z-axis in the Cartesian plane for generating an MRI image. The process involves using a slice selection gradient and matching frequencies with an RF pulse.

Selecting a Specific Slice (0:00:00 - 0:01:35)

• A slice selection gradient is used to select a specific slice along the z-axis.

• Matching the radio frequency pulse frequency with the processional frequency causes flipping of spins in the transverse plane only along that specific slice.

• Net magnetization vectors from different regions in the selected slice process in phase with each other.

• The amplitude of the signal generated depends on tissue type, while the procession frequency remains constant.

Sampling Over Time (0:01:35 - 0:04:18)

• The entire tissue resonates in phase, generating a sinusoidal wave at the same frequency as spin procession.

• Multiple data points are needed to generate an image, so sampling is done at multiple points surrounding te (time period after applying 90-degree RF pulse).

• The analog signal is converted into discrete digital values over time for storage and analysis.

Signal Changes and Digital Conversion

This section discusses how signals change over time and how they are converted from analog to digital values.

Signal Changes Over Time (0:04:18 - 0:05:55)

• Signals change over time due to processes like free induction decay and re-phasing of spins.

• Transverse magnetization vector reaches its maximum when spins re-phase, indicating in-phase spins in the transverse plane.

Analog to Digital Conversion (0:05:55 - 0:06:17)

• An analog signal represents continuous movement, while a digital signal consists of discrete values over time.

• Sampling the analog signal at multiple points creates discrete digital values for analysis and storage.

Conclusion

The transcript provides an overview of slice selection in MRI imaging, the importance of sampling signals over time, and the conversion from analog to digital values. Understanding these concepts is crucial for analyzing MRI images and generating accurate diagnoses.

T2 Decay and Frequency Encoding Gradient

This section explains the concept of T2 decay and introduces the use of a frequency encoding gradient to differentiate signals based on their x-axis location within a selected slice.

T2 Decay and Signal Measurement

• T2 decay refers to the loss of signal intensity over time due to local inhomogeneities in the magnetic field.

• By using a 180-degree RF pulse, we can rephase the spins and measure a signal similar to what the T2 signal would have been.

• The rephasing helps negate some of the local inhomogeneities, resulting in a stronger signal measurement.

Sampling Multiple Points Surrounding Te

• During sampling, we don't just sample at one point in Te (time echo), but multiple points surrounding it.

• The time period at the middle of these sampling points is denoted as t e.

• The signal increases slightly and decreases slightly during this time period.

Frequency Encoding Gradient for Spatial Differentiation

• To differentiate signals based on their x-axis location within a selected slice, we need to overcome the problem of all spins being in phase with each other.

• A frequency encoding gradient is applied along the x-axis using different currents on two halves of a coil.

• This creates a magnetic field strength difference along the x-axis of the selected slice, allowing us to encode spatial information.

Correlating Frequency with X-Axis Location

• When applying the frequency encoding gradient, spins at different x-axis locations experience different frequencies.

• The frequency difference allows us to determine where signals are coming from within the slice based on their specific frequencies.

• The magnetic field strength varies from left to right, resulting in faster processing rates or frequencies on one side compared to the other.

Reconstructing Net Magnetization Vector

• Initially, when spins lose phase due to spinning out of phase, the net magnetization vector becomes very low.

• To address this, an equal and opposite frequency encoding gradient is applied prior to readout.

• This helps re-phase the spins with each other during the data acquisition time, resulting in a more in-phase net magnetization vector.

• The different frequencies of spins contribute to a non-sinusoidal wave when measuring the net transverse magnetization vector.

Data Acquisition and Gray Scale Representation

• Multiple periods of time are sampled during data acquisition to obtain discrete values representing the amplitude of the net magnetization vector.

• These discrete values can be represented as grayscale values, where higher amplitudes correspond to higher numerical data values.

• Each data acquisition point represents the entire net magnetization vector for the whole slice at a given period in time.

Summary

This section summarizes the key points discussed regarding T2 decay and frequency encoding gradient.

• T2 decay refers to signal loss over time due to magnetic field inhomogeneities.

• A 180-degree RF pulse helps rephase spins and measure a signal similar to T2 signal.

• Sampling occurs at multiple points surrounding Te, which is the middle of these sampling points.

• Frequency encoding gradient is used to differentiate signals based on x-axis location within a selected slice.

• Different frequencies correlate with specific x-axis locations within the slice.

• An equal and opposite frequency encoding gradient is applied prior to readout to re-phase spins for better measurement accuracy.

• Data acquisition involves sampling multiple periods of time, obtaining discrete values representing net magnetization vector amplitude.

• Grayscale representation reflects numerical data values of the measured wave's amplitude.

New Section

The speaker asks if it is possible to see which frequencies are present.

Can we see which frequencies are present?

• The speaker raises the question of whether it is possible to visualize the frequencies that are present.

• This implies a desire to have a visual representation of the different frequencies in a given context or scenario.

No specific timestamp was provided for this section.