"Part III – Magnetic Resonance Imaging Physics"
Having reached this third level of the examination process of the American Board of Medical Physics, the candidate has demonstrated acceptable educational credentials and an adequate fund of knowledge. This oral examination is designed to determine whether this knowledge can be effectively applied in the clinic. Although the applicant may not have direct experience with the latest MRI technology, it is expected that an understanding of all types of commercially available MRI systems and their roles in clinical practice can be demonstrated.
The successful candidate will convince the examination panel that she/he has sufficient clinical experience to identify and describe the various types of common MRI examinations, the MR imaging pulse sequences and ancillary equipment required for each. The applicant should also demonstrate understanding of and experience with the medical physics activities necessary to support such clinical MRI activity. The successful candidate will be familiar with the different types of apparatus employed, the necessary MRI physics measurements, support activities and the normal range of performance and standards associated with these activities.
The seven content categories, listed in the Part II Study Guide shall be addressed:
1. Basic Physics of Nuclear Magnetic Resonance
2. Magnetic Resonance Imaging Theory and Reconstruction
3. Magnetic Resonance Image Characteristics and Artifacts
4. Advanced MR Imaging Techniques & System Features
5. Contrast Enhancement, MR Angiography & Cardiac MRI
6. Magnetic Resonance Technology and Equipment Quality Control
7. Site Planning and Safety of Magnetic Resonance Examinations
Basic MR physics includes descriptions of the nuclear magnetic resonance phenomonon, relaxation description, mechanisms and measurement, and basic NMR experiments. Magnetic resonance imaging theory includes processes in Fourier imaging, understand of the Fourier transform, and imaging pulse sequence basics. MR image characteristics and artifacts includes contrast and signal-to-noise manipulation, 3D imaging, and common MRI artifacts. Advanced MRI includes gradient echoes, diffusion and MR physics, MR spectroscopy, BOLD contrast and advanced data reconstruction strategies. Both endogenous and exogenous methods for contrast enhancement may be discussed as well as all methods for MRA and cardiac MRI available on current clinical MRI systems. Magnetic resonance technology and QC includes the magnet, gradient and RF subsystems and methods for measuring performance of the MRI system. Site planning and safety includes biological effects , FDA-imposed equipment limitations and policies, procedures and restrictions regarding the safe operation of MRI equipment.
The following are examples of the types of questions you may expect. A comprehensive discussion rather than a precise answer is expected.
In addition, you can expect to be asked for more specific information on various MRI pulse sequences and imaging techniques, classical MR image artifacts and appropriate MRI examinations for certain patient conditions. A modest experience with other clinical imaging modalities and their relationship to the clinical utility of MRI will be expected.
Sample Oral Examination Questions
1. Describe the various processes that control the frequency, phase and amplitude of the NMR signal using the rotting frame model.
2. You are asked to develop a T2-weighted imaging protocol using fast spin echoes, to replace a conventional spin echo protocol and improve imaging speed. Explain your strategy for increasing imaging speed without sacrificing significant image quality.
3. What are the major origins of the statistical noise seen in MR images? Is the statistical character of noise in MRI the same as it is in x-ray imaging and nuclear medicine? What strategies can be employed to reduce the contribution of statistical noise to the image?
4. What are some differences between various MRI pulse sequences that fall in the general class of gradient echo (or fast field echo)?
5. Describe and explain the quality control tests listed in the ACR magnetic resonance imaging quality control manual. Suggest possible causes of failure for each test.
6. Explain in detail the differences between T2 and T2* contrast and give examples of situations in which one or the other is employed to clinical advantage.
7. Explain what SAR is and how it is determined. Is parameter regulated by the government? If so, then in what way? In general, how does SAR change with magnetic field strength? Why do MRI systems manufactured by different companies seem to have greater or lesser SAR limitations on their pulse sequences?
8. Explain the relationship between MR image resolution and the Lorentzian line width of water. Under what conditions is this the dominant contributor to blurry images?
9. What are eddy currents? What is their origin? How do they degrade MR image quality? What are the common strategies used to suppress them?
10. Explain which factors dominantly influence image resolution in conventional (spin-warp) imaging, echo planar imaging and fast spin echo imaging.
11. Discuss the relative merits and disadvantages of respiratory triggering, navigator echo view rejection and breath-hold imaging for cardiac MRI.
12. Major distortion is often seen in the region of the frontal lobe of the brain in echo-planar imaging. What is the cause of this artifact? Why is this not a problem in spin echo, fast spin echo and fast gradient echo imaging sequences?