Unit
- Nuclear magnetic resonance basics: MRI relies on the interaction between hydrogen protons and a strong external magnetic field. When radiofrequency pulses are applied, protons absorb energy and later release it as signals that are reconstructed into images. Understanding this principle helps operators grasp why patient positioning and field uniformity matter.
- Magnetic field gradients: Three orthogonal gradient coils (x, y, z) encode spatial information by varying the magnetic field linearly across the bore. Gradient strength and slew rate affect slice thickness, field of view, and image resolution. Proper gradient calibration reduces artifacts and ensures accurate anatomical localization.
- Radiofrequency coils and signal reception: RF coils transmit energy and receive the induced MR signal. Different coil designs (volume, surface, phased-array) balance signal-to-noise ratio and coverage. Selecting the appropriate coil for each anatomy improves image quality and reduces scan time.
- Pulse sequences: Spin echo, gradient echo, and inversion recovery sequences produce varying tissue contrasts. Key parameters like repetition time (TR), echo time (TE), and flip angle govern T1, T2, and proton-density weighting. Sequence selection determines diagnostic value and scan duration.
- K-space and image reconstruction: MR data is acquired in the spatial frequency domain (k-space). Trajectories such as Cartesian, radial, or spiral influence acquisition speed and artifact patterns. Fourier transformation converts k-space data into a visual image; undersampling can accelerate scans but introduces aliasing.
- Magnetic field strength considerations: Common field strengths are 1.5T and 3T, with higher fields offering better signal-to-noise and spectral resolution but increased susceptibility artifacts and safety constraints. Operators must adjust protocols and safety checks based on the specific magnet type.
- Common artifacts and corrections: Motion, chemical shift, susceptibility, and aliasing artifacts degrade image quality. Techniques like respiratory gating, fat suppression, and oversampling mitigate these issues. Recognizing artifact patterns enables technologists to repeat sequences efficiently.
- Safety implications of physics: The static magnetic field, gradient switching, and RF power all pose risks. Projectile effects, peripheral nerve stimulation, and tissue heating require continuous vigilance. Understanding the physical principles behind safety limits helps staff enforce protocols.
- Operational impact of physics knowledge: When operators understand how sequence parameters affect image quality and scan time, they can optimize workflow without compromising diagnostic value. This foundation supports troubleshooting, protocol customization, and communication with radiologists.
- Shimming and field homogeneity: Magnetic field inhomogeneities cause image distortions and signal loss. Active shimming uses additional coils to correct static field variations, while passive shimming involves placing ferromagnetic pieces. Routine shimming ensures consistent fat suppression and reduces susceptibility artifacts, directly impacting diagnostic quality.
- Parallel imaging and acceleration: Techniques like GRAPPA and SENSE use multiple receiver coils to undersample k-space, reducing scan time. The acceleration factor R trades speed for signal-to-noise ratio. Operators must select appropriate acceleration to balance time savings with acceptable image quality, especially in breath-hold or dynamic studies.
- Specific absorption rate and safety: SAR measures RF energy absorbed by tissue, proportional to field strength, flip angle, and duty cycle. High SAR can cause tissue heating, particularly at 3T. Technologists monitor SAR limits in real time, adjusting sequence parameters to stay within FDA guidelines while maintaining diagnostic contrast.
- Flow and motion compensation strategies: Blood flow and patient motion create ghosts and blurring. Techniques such as gradient moment nulling, cardiac gating, and navigator echoes compensate for periodic motion. Understanding these methods helps operators choose appropriate sequences for vascular or abdominal imaging without prolonging exams.
Terminology
- TR
- Repetition time, the interval between successive RF pulses in a pulse sequence.
- TE
- Echo time, the time between the RF pulse and the peak of the signal echo.
- SNR
- Signal-to-noise ratio, a measure of image quality comparing signal to background noise.
Quick Revision
- MRI uses nuclear magnetic resonance of hydrogen protons in a magnetic field.
- Gradients encode spatial information; RF coils transmit and receive signals.
- Pulse sequences (TR, TE, flip angle) determine tissue contrast and scan time.
- K-space filling affects speed and artifacts; reconstruction uses Fourier transform.
- Field strength influences SNR, safety, and protocol adjustments.