MRI physics and real-time application Flashcards
Explain how an MRI works.
- The water in our bodies is magnetic (or actually parts of the hydrogen atoms of the watermolecules are magnetic).
- The MRI contains an unified magnet that is able to create a magnetic field around the patient.
- Normally, the water molecules within our body are arranged randomly and when the magnetic field is applied, most of these molecules will move at the same frequency. There are also water molecules that do not move, called lower-energy water molecules.
- For these lower-energy water molecules, radiowaves are applied that resonate with the magnetic field. The lower-energy molecules will absorb the transmitted energy of the radiowaves, which causes them to also move with the magnetic field.
- When the radiowaves stop, the lower-energy water molecules release the energy they have absorbed and return to their original position (relaxation)
- This movement is detected by the MRI and the information is send to the computer.
- By taking images of the body in each section of the magnetic field, the machine is able to produce a 3D image.
What is the definition of the following in regard to MRI:
- magnetic moment
- precession
- resonance
- magnetic moment → molecules with an odd number of protons/neutrons (i.e. dipoles) will have a property of magnetic moment. This means that when an external magnetic field is applied, the dipole molecules will align with the field.
- precession → besides this magnetic moment, the application of an external magnetic field will produce a secondary spin/wobble of nuclei around the main or static magnetic field (field B). Note: the earth also has a magnetic field, so e.g. water molecules and specifically the hydrogens atoms are always precessing.
- resonance → oscillating response over a narrow range of frequencies to the external input of energy.
True or false
The frequency of the precession of the water molecules is proportional to B (magnetic field).
- fprecession = constant B
True
How is resonance of the water molecules evoked inside the MRI?
By inducing a short radiowave (RF pulse) with the same frequency as fprecession.
Explain why resonance is important for the MRI.
So resonance is the oscillating response over a narrow range of frequencies to the external input of energy. This narrow range of frequencies is used by the gradient of the MRI to adust the magnetic field into smaller sections at different magnetic strenghts to isolate certain body parts.
In other words:
Resonance enables the protons to absorb enough energy from the RF pulse to rotate their axes away from the B0 field, so that the MRI scanner can measure it.1
What is the result of varying the sequence of radiofrequency (RF) pulses applied and collected?
Diferent types of images can be created.
- What is the repetition time (TR)?
- What is time to echo (TE)?
- TR → the amount of time between successive RF pulse sequences applied to the same slice.
- TE → the time between the delivery of the RF pulse and the receipt of the echo signal.
- What is the result of a long repetition time?
- What is the result of a short repetition time?
- What is the result of a long echo time?
- What is the result of a short echo time?
- Long TR → it allows the protons in all of the tissues to relax back into alignment with the main magnetic field.
- Short TR → results in protons of some tissues not having fully relaxed back into alignment before the next measurement is made, decreasing the signal from this tissue.
- Long TE → results in reduced signal in tissues like white and gray matter since the protons are more likely to become out of phase. . Protons in a fluid will remain in phase for a longer time since they are not constrained by structures such as axons and neurons.
- Short TE → results in the reduction of the amount of dephasing that can occur in tissue like white and gray matter.
Answer the following questions:
- What is the primary source of energy input for MRI?
- How is the rotating magnetic field generated?
- Precession of an individual nuclei is not the same as nuclear resonance, explain why precession of the net magnetization (M) is a manifestation of resonance.
- The primary source of energy input for MRI is the rotating magnetic field (B1)
- The magnetic field is generated by passing alternating current through a nearby radiofrequency (RF) coil.
- This is because M is not an individual spin but the sum of many. Macroscopic precession of M ony occurs when the spin system is moved out its equilibrium condition by input of energy. As M returns to equilibrium through magnetic relaxation, this energy is transferred out of the spin system once again.
The MR signal is an electromagnetic wave, which can be used to localized anything and in any plane through the body. However, all hydrogen nuclei are the same.
So how is it then possible that an MRI contains information about different body tissues?
Because different tissues have different relaxation times (T1 and T2)
- What is the T1 relaxation time?
- What is the T2 relaxation time?
- T1 relaxation time → i.e. longitudinal relaxation time → the measure of how quickly the net magnetization vector (the summation of all the magnetic moments of the individual hydrogen nuclei) recovers to its ground state in the direction of B0. The return of excited nuclei from the high energy state to the low energy or ground state is associated with loss of energy to the surrounding nuclei.
- T2 relaxation time → i.e. transverse relaxation time → the time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other (see picture).
- What is a T1-weighted image?
- What is a T2-weighted image?
- T1-weighted image is the image generated by the MRI as a result of the T1-weighted sequence highlighting differences in the T1-relaxation times of tissues.
- T2-weighted image is the image generated by the MRI as a result of the T2-weighted sequence highlighting the differences on the T2-relaxation time of tissues.
- How are T1-weighted images produced?
- How are T2-weighted images produced?
- T1 → By using a short TE and TR time. Here, the contrast and brightness of the image are predominantly determined by T1 properties of tissue.
- T2 → By using longer TE and TR times. Here, the contrast and brightness are predominantly determined by T2 properties of tissue.
Why does fat appear bright and water appear dark on a T1-weighted image?
- Fat quickly realigns its longitudinal magnetization with B0.
- Water has much slower longitudinal magnetization realignment after an RF pulse and therefore also has less transverse magnetization after an RF pulse. Therefore, water has low signal and appears dark.
An acute infarction in the anterior wall is recognized as bright on a T2-weighted image. What does this mean?
- Prolonged T2
- Oedema
- Inflammation
