MRI Gradients and Spatial Encoding

Gradients and Resolution in MRI

Encoding creates resolution.
Encoding and resolution are interconnected.
The end goal of encoding is to create resolution.
Resolution is analogous to the sharpness of an image on a TV.
- Higher resolution: Sharper image.
- Lower resolution: Blurry image.

Resolution is the end result; gradients are physical components within the MRI machine.
Gradients create resolution.
RF (Radio Frequency) creates contrast.

Gradients are the second magnetic force that alters the strength of the main magnetic field (B0).
Electricity runs through a coil of wire, creating a magnetic force.
Gradients are coils of wires placed at specific points in the machine.
There are three gradients.
Gradients change over time, unlike B0, which remains constant.
Electricity runs through the coils of wire, causing them to expand, which generates loud sounds during MRI scans.
Gradients are located in the warm bore of the MRI machine.

Example: 1.5 Tesla magnet.
- When gradients are turned off, the magnetic field is uniform at 1.5 Tesla throughout the magnet.
- When gradients are turned on, one part of the magnet becomes stronger, and another part becomes weaker.
- The middle (isocenter) remains at 1.5 Tesla.
Analogy: A seesaw, where the magnetic field strength can be interchanged but isocenter remains constant.
The isocenter is the best place to scan because the magnetic field remains constant there.

Amplitude: The main magnetic strength; the strength of electricity.
Maximum amplitude of the gradient determines the maximum resolution.
Amplitude can have multiple meanings in MRI, but generally refers to maximum strength.
Polarity: Gradients have both north and south poles.
The direction of electricity determines which part of the magnet is weaker or stronger.

Larmor equation relates precessional frequency to the strength of the magnetic field.
Changing precessional frequency is a goal of the gradient.
- Stronger magnet: Protons precess faster.
- Weaker magnet: Protons precess slower.
Gradients change precessional frequency to locate signals within the body.
- Altering precessional frequency changes the shape of the echo.

When a patient is placed in the MRI machine, the Larmor equation applies.
As the magnetic field strength changes, protons precess at different rates.
The gradient changes the amount of magnetism across the body.
Magnetism directly changes precessional frequency.
The computer knows how much the gradient changed the precessional frequency.
Example:
- Protons in the middle (isocenter): 100 mph (arbitrary units).
- Protons at the head: 50 mph.
- Protons at the feet: 50 mph.
Based on the speed (precessional frequency), the computer can determine the location of the protons.
The gradient creates different frequencies across the body, allowing the machine to differentiate where signals originate.
Amplitude and polarity are determined by the computer based on the scan requirements.

Isocenter is similar to the landmark set on the MRI machine.
The landmark indicates the location of the isocenter.
The body part being scanned is aligned with the lasers at the isocenter.
The isocenter is the optimal place for scanning because the shallowest gradient slopes can be used there.

Protons are initially excited and in the transverse plane.
Without a gradient, all protons spin at the same rate due to the main magnetic field (B0) and Larmor frequency.
Turning on the gradient causes protons to spin at different speeds.
The gradient allows differentiation of protons from left to right based on their varying speeds.
Protons at isocenter spin slower; the further from isocenter, the faster the protons spin.
The gradient changes the speed at which protons spin, which is the precessional frequency.

Different proton speeds alter the appearance of the echo.
The waves of the echo vary based on proton speed (slower, medium, faster).
The computer interprets the echo based on the number of peaks and their proximity.
Faster protons create narrower peaks in the echo wave.
The computer decodes these peaks to determine the location of the signal.

Frequency encoding occurs as the signal is being read.
Protons at the strongest point of the gradient (red) precess fastest; those at the weakest point (blue) precess slowest.
Changing precessional frequency forces protons to dephase.
Fastest protons eventually overtake the slowest protons.
The gradient determines the height and distance between peaks in the echo.
The computer uses these differences to map the location of the signal along the body.
A steeper gradient results in more peaks and higher resolution.
A shallower gradient results in fewer peaks and lower resolution.

Gradients are used to spatially encode information.
Encoding involves embedding a code into the echo.
Gradients alter precessional frequency to reveal the spatial origin of the echo.
Processional frequency represents the speed of the protons within the body.
Isocenter is the point around which there are the different speeds within the body.
Precessional frequency is measured in megahertz (MHz) because protons rotate millions of times per second.
Precession refers to the motion of protons rotating around B0.
Altering processional frequency allows us to change the phase of the protons.
Changes in processional frequency allow the height of those peaks and the distance between the peaks to change, which allows the computer to determine where the signal is coming from and create the image.

Three ways to encode an echo in MRI:
- Slice selection.
- Phase encoding.
- Frequency encoding.
These three types of encoding exist in three dimensions (x, y, z).
The computer selects these parameters based on the chosen settings.

A pulse sequence diagram is a visual representation of the timing and strength of RF pulses and gradients during a scan.
The diagram displays when:
- RF pulses are active.
- Slice select gradients are active.
- Phase encoding gradients are active.
- Frequency encoding gradients are active.

Shows the excitation pulse with a specific symbol.
Indicates when a gradient is turned "on" with a box and "off" with a flat line.
Illustrates the sequence of pulses (gradient and RF) during the scan.
Depicts the recipe and order of events during a scan.
Phase encoding changes strength every TR (repetition time).
Frequency encoding occurs when the echo is being sampled.
Different pulse sequences have different diagrams (e.g., spin echo pulse sequence).
Helps visualize which gradient turns on during the RF pulse or when the echo is collected.
The diagrams are visual representations of events during the scan and are assessed with quizzes.