Outline of the module
An MR scanner is a complex instrument employing several electromagnetic fields in order to create and manipulate magnetisation of nuclei in the biological tissue. The main components of an MRI system are the superconducting magnet, the gradient system, the radiofrequency (RF) system including RF coils and the computer system and software. The magnet produces a strong, static field and the radiofrequency transmit and receive coils excite and detect the MR signal. The magnetic field gradients localise the MR signal and the computer system facilitates scanner control, image display and archiving.
This module will provide an introduction into the different hardware types for each of the main components. It will explain what the impact of the different hardware options on MR image quality is as a function of the tissue type imaged, field strength and MR sequences. In addition, students will visit the MR scanner site(s) to have a visual and practical impression on the MR hardware.
Learning objectives
At the end of the module, students will have expert knowledge about the following topics:
• Hardware architecture of an MR scanner
• Design criteria for superconducting magnets to create main magnetic field
• How to manipulate main magnetic field using shimming coils and their characteristics
• Calculate the stringe field of a shielded and unshielded magnet
• Radiofrequency coils and electronics options and the resulting interaction with tissue
• Specific absorption rate (SAR): legal limits and how to calculate them using computer simulations
• Type of MR gradients: head, body and novel systems (e.g. PatLoc)
• Unwanted current generated by the MRI scanner (e.g. eddy currents) and how to compensate them
• Additional optional hardware components to interact with the subject/patient
• Identify the hardware components on a real scanner
Content
Magnetic resonance (MR) is a physical way of interacting with the magnetic moment of atomic nuclei. This method is used in various scientific fields reaching from solid state physics to biology. In biological tissue, typically, the proton magnetic moment is probed using electromagnetic fields. As the frequency of the electromagnetic field is at the radiofrequency (RF) range, MR uses non-ionizing radiation and is, therefore, considered to be a non-invasive technology. With RF pulses, different properties of the biological tissue are measured, including proton density and transverse and longitudinal relaxation times, which are a function of mater density and the local biochemical environment.
To create an MR image, different hardware has to be employed. The most important are: the main magnet, RF electronics and coils, MR gradients and image reconstruction and MR sequence control. This module will give an introduction into these various components of an MR scanner. The main magnetic field is created via a superconductor, which is a niobium-titanium wire with zero conductance below a critical temperature (i.e. 7.7 Kelvin). The homogeneity of the field is achieved using additional so-called shim coils. Thus, an integral part of the main magnet is the cooling system, which typically consists of two stages, one using liquid nitrogen and the other one using liquid helium. The stray magnetic field outside the bore of the magnet is known as the fringe field. Fringe fields can be compensated for by the use of magnetic field shielding which may be active or passive. Passive shielding is the more expensive alternative using iron plates to restrict the field lines. Active shielding partially cancels the field outside the main magnet coils thus reducing the magnitude of the fringe field.
The so-aligned magnetic moments of the protons are excited using RF field. The transmission and reception of the electromagnetic signals is done utilizing dedicated RF coils and electronics (including amplifiers and A-D-converters). For example, special coils exist for imaging different body parts, i.e. knee, spine, head/neck, cardiac coils. An active field of research is to develop optimised coils for each body part to be imaged. The coils can be composed by different sub-coils to allow for parallel reception and transmission, which can be utilised to increase imaging speed, increase SNR, target specific areas etc.
In order to achieve spatial information of the targeted area, the magnetic field has to be varied in a localised manner using addition MR coils. The so-called MR gradients are alterations to the main magnetic field and are generated by coils of wire located within the bore of the magnet through which current is passed. Passage of current through a gradient coil induces a gradient (magnetic) field around it that either subtracts from or adds to the main static magnetic field strength. Thus, the position of a nucleus can be identified according to its local magnetic field (i.e. precessional frequency).
Finally, after the signal is collected, an image has to be created. A computer system with specific software reconstructs the image and allows for an on-line control of the image acquisition.
In summary, this module will give a guide into the complex hardware architecture of an MR scanner and will discuss what the implications are of each hardware for the image formation. In addition, the students will use the MR scanner and will visit an MR scanner sites to visually inspect each hardware components. Finally, they will recognise specific hardware failures and how to handle them, which is a very important qualification for a MR technician, radiologists and MR scientists.
Overview of tasks and lectures
There will be 10 lectures of 2 hours distributed over 5 days.
• Introduction into Hardware architecture of an MR scanner
• MRI sequences and their constraint for specific hardware
• Superconducting magnet designs for different field strengths and bore sizes
• Radiofrequency coils and electronics I (hardware)
• Radiofrequency coils and electronics II (software and computer simulations)
• MR Gradient system: head and body
• Peripheral MR scanner including some stimulation and clinical equipment
• Shimming (active and passive) coils of different orders and shielding (active and passive)
• Advanced, non-typical MRI hardware
• Summary and Introduction of post-module assignments
Position within the programme
This module is related to the modules on MRI physics, coils and sequences. To follow the content of the module, these modules are mandatory. This module will introduce scientist-practitioners to the “inner life” of an MRI scanner and will be very useful for physicists and technicians to identify the sources of hardware problems and how to solve them.
Teaching format
Structure
The module is a one week-long residential module consisting of 10 lectures of 2 hours. Each day, the students will in addition perform data acquisition on the MRI scanners checking for performance of the system located at the UM (3, 7 and 9.4 Tesla), computer simulations of scanner and hardware performance and visual inspection of the hardware of the MRI scanners. Furthermore, the residential part is combined with a preparatory reading phase and post-module marked assignments.
Grading
Passing the module requires an 85% attendance to the lectures and practical sessions, and a satisfactory completion of the practical sessions and the module assignments. The module assignments will be summarised by the students in a written form which will be evaluated by the module coordinator(s).