MRI Physics and T2-weighted Imaging

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Questions and Answers

What primarily determines the contrast in a T2-weighted image?

Signal loss due to T1 effects

Density of protons in the tissue

Radiofrequency pulse duration

Signal differences among tissues at long TE (correct)

How does keeping a short TE affect the contrast in a Proton Density (PD) image?

Eliminates T1 effects (correct)

Reduces proton density signal

Enhances T1 effects

Enhances T2 effects

What is the effect of using a long TR in a Proton Density image?

Minimizes T1 effects (correct)

Increases T2 signal differences

Enhances T1 effects

Reduces T2 signal differences

In a T2-weighted image, why does tissue A maintain a good signal while tissue B loses most of its signal?

Tissue A has longer T2 than B Signup and view all the answers

What happens to the signal intensity difference among tissues in a T2-weighted image at short TE?

It becomes less significant Signup and view all the answers

What is the primary type of atom found in the human body that is crucial for MR imaging?

Hydrogen Signup and view all the answers

What phenomenon occurs when protons align with an external magnetic field in MR imaging?

Precession Signup and view all the answers

Which subatomic particle has a positive charge and participates in creating a magnetic field in MR imaging?

Proton Signup and view all the answers

What occurs to protons when they are placed in the presence of an external magnetic field?

They align parallel or anti-parallel to the magnetic field. Signup and view all the answers

What part of the atom contains most of its mass?

The nucleus Signup and view all the answers

What is the term for the amount of times a proton rotates around its axis in MR imaging?

Precession Frequency Signup and view all the answers

Which of these elements is NOT among the most abundant in the human body that contributes to MR imaging?

Aluminum Signup and view all the answers

What describes the movement of the axis of rotation of a proton when it aligns with the magnetic field?

Precession Signup and view all the answers

What phenomenon allows protons to exchange energy with the RF pulse?

Resonance Signup and view all the answers

What is the primary effect of the RF pulse on protons?

Cause precession and elevate energy level Signup and view all the answers

What kind of transformation is used to convert MR signals into MR images?

Fourier Transformation Signup and view all the answers

Which gradient is responsible for determining the slice position in MRI?

Slice selection gradient Signup and view all the answers

What does the strength of the MR signal depend on?

The magnitude of the transverse magnetization Signup and view all the answers

How does slice thickness relate to the bandwidth of the RF pulse?

Thicker slices correspond to wider bandwidth Signup and view all the answers

What term describes the combination of three magnetic fields aligned along the X, Y, and Z axes in MRI?

Gradient fields Signup and view all the answers

Which gradient is applied to localize the point in a slice from where the signal originates?

Phase encoding gradient Signup and view all the answers

What does TI in an Inversion Recovery (IR) sequence primarily influence?

Image contrast Signup and view all the answers

How does T1 affect the brightness of tissues in T1 weighted images?

Short T1 tissues regain maximum signal faster Signup and view all the answers

What is the significance of using a short TR in T1-weighted imaging?

It maximizes the difference in signal intensity due to T1 differences Signup and view all the answers

What happens to TM immediately after the RF pulse is turned off in a T2 weighted image?

It begins to decrease in magnitude Signup and view all the answers

Which tissue type will retain its signal for a longer time in T2 weighted images?

Tissue with long T2 Signup and view all the answers

To make an image T2 weighted, what adjustment must be made?

Increasing the TE duration Signup and view all the answers

What is observed in T1 weighted images when using a long TR?

Lower overall image brightness Signup and view all the answers

What effect does dephasing have on the signal received in T2 weighted sequences?

Gradually reduces signal intensity Signup and view all the answers

What is the primary purpose of longitudinal relaxation time (T1)?

To indicate the time taken for magnetization to recover to 63% of its original value Signup and view all the answers

Which term describes the process by which transverse magnetization decreases in magnitude?

Transverse relaxation Signup and view all the answers

What distinguishes T2* relaxation from T2 relaxation?

T2* is influenced by external magnetic field inhomogeneity. Signup and view all the answers

What does the Larmor’s equation express?

The relationship between precession frequency and strength of external magnetic field. Signup and view all the answers

Which axis is the external magnetic field conventionally directed along?

Z-axis Signup and view all the answers

What occurs to transverse magnetization immediately after a 90-degree RF pulse?

It rotates out of phase at Larmor frequency. Signup and view all the answers

Why is longitudinal magnetization not directly measurable?

It must be converted to transverse magnetization for measurement. Signup and view all the answers

What does the term Time to Repeat (TR) refer to in the context of RF pulses?

The interval between the start of one RF pulse and the start of the next RF pulse Signup and view all the answers

In the absence of external magnetic field inhomogeneity, what happens to the transverse magnetization in a spin-echo sequence?

It is preserved due to the application of 180-degree RF pulses. Signup and view all the answers

What happens to protons when they align under the influence of an external magnetic field?

They align parallel and antiparallel to the magnetic field. Signup and view all the answers

What effect does the application of a radiofrequency (RF) pulse have on protons?

It causes some protons to precess antiparallel to the magnetic field. Signup and view all the answers

What does the transverse relaxation time (T2) indicate?

The time duration required for transverse magnetization to reduce to its original value Signup and view all the answers

What typically causes the decay of transverse magnetization (TM) in T2* relaxation?

The combination of spin-spin relaxation and magnetic field inhomogeneity Signup and view all the answers

What is created when the forces of protons on positive and negative sides cancel each other?

Longitudinal magnetization. Signup and view all the answers

How does stronger external magnetic field affect precession frequency?

It increases the precession frequency. Signup and view all the answers

What is the result of tilting the magnetization into the transverse plane?

It allows for the measurement of magnetic resonance signals. Signup and view all the answers

Study Notes

Principles of MRI

MRI uses a strong magnetic field and radio waves to create detailed images of the body's internal structures.
Atoms in the body, particularly hydrogen atoms, have tiny magnets (protons).
These protons can be aligned by a strong magnetic field.
A radio frequency (RF) pulse is used to tilt the protons from their aligned state.

Atomic Structure

All matter is composed of atoms.
Atoms are made up of a nucleus (containing protons and neutrons) and orbiting electrons.
Hydrogen is the most abundant atom in the human body, found in water and fat.
Protons have positive electrical charges.
Neutrons have no electrical charge.

Motion of Atoms

Protons, when spinning, also have a magnetic dipole moment. This means they behave like tiny magnets.
The spinning axis, or the spin, can be aligned "up" or "down". It is important to note here that a spin is a quantized quantity
The axis of rotation (spin) precesses (moves in a cone-like fashion) around the axis of the external magnetic field.

Protons in MRI Imaging

Protons have a positive electrical charge.
Protons spin and have a tiny magnetic field (magnetic dipole moment).
When an external magnetic field is applied, protons align either parallel or anti-parallel to the field.
The spinning axis of a proton precesses around the external magnetic field's direction.

Longitudinal Magnetization

When placed in an external magnetic field, protons tend to align parallel to the field.
More protons align with the magnetic field than against it resulting in a net magnetization vector known as longitudinal magnetization
The net magnetization vector forms along the direction of the main magnetic field.

Transverse Magnetization

A radiofrequency (RF) pulse is used to tip the aligned protons to a transverse plane (perpendicular to the main magnetic field). These are not technically transverse magnetization; the result is rather a tipping vector of longitudinal magnetization components
This tipping creates transverse magnetization.
This transverse magnetization rotates, constantly creating an electromagnetic current that the MRI machine detects.

MR Signal

Transverse magnetization produces an electric current detectable by the MRI machine.
The strength of the electric current is related to the magnitude of the transverse magnetization.
The rotating precessing protons produce an electromagnetic signal for the MRI device to measure.

Localization of the Signal

Three extra magnetic fields (gradient fields) are used to pinpoint the source within the body.
These additional fields are carefully shaped and alter the magnetic field strength in specific locations.
Each area of body has its own magnetic field
Gradient fields help to localize signals from specific locations.

Slice Selection Gradient

Determines which region of the body to image
Varying magnetic field strength creates distinct layers (i.e slices) along the body.
This is a crucial component for making cross-sectional or slice images as required for MRI procedures.

Phase Encoding & Frequency Encoding Gradients

Fine-tuning the process, the location of the part of the image/slice to be obtained
Accurately pinpoint a spot in the given slice/layer.

Basic Four Steps of MRI Imaging

Patient placement in the magnet, aligning protons in body
Application of an RF pulse to tip the protons, creating transverse magnetization.
Detection of the electric current produced by the tipped protons.
Computer processing of the signals to create precise images.

T1, T2 Relaxations and Image Weighting

Relaxation times (T1 and T2) indicate how quickly protons return to their initial equilibrium state after an RF pulse.
T1 relaxation relates to how quickly the longitudinal magnetization recovers.
T2 relaxation describes how quickly the transverse magnetization decays (decreases).
These time values impact image generation and help distinguish between different tissues.

T1 Weighted Image

Images that emphasize differences in T1 relaxation times.
T1 values vary between tissue types (fat, water, cartilage, etc.)
Short T1 values translate into brighter signals.

T2 Weighted Image

Images showing differences in T2 relaxation time.
T2 values change among tissue types (fat, water, cartilage, etc).
Long T2 times usually appear brighter on the image.

Proton Density (PD) Image

Images focus on differences in the concentration of hydrogen protons in different tissues.
Used for differentiating tissues without relying too much on T1 or T2 differences.

TR and TE Values

TR: Time to repeat (time between RF pulse sequences).
TE: Time to echo (time it takes for the signal to return to the receiver after an RF pulse).
These values are adjusted to obtain optimized images.

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Description

Test your understanding of MRI physics, focusing on T2-weighted imaging and proton density concepts. This quiz covers topics such as signal differences, TE, TR effects, and the role of protons in MR imaging. Ideal for students learning about medical imaging techniques.