Questions & explanations
1. What is magnetization transfer contrast (MTC) in MRI?
Magnetization transfer contrast (MTC) is an MRI technique that creates image contrast based on the interaction between free water protons and protons bound to macromolecules. In tissues, there are two pools: free water (mobile) and bound (restricted) protons. An off-resonance radiofrequency pulse selectively saturates the bound pool, and this saturation transfers to the free pool via chemical exchange, reducing its signal. The amount of signal reduction depends on the tissue's macromolecular content. MTC helps highlight tissues with high protein content, such as brain white matter, and suppresses background signal from blood or cerebrospinal fluid.
2. Compare the artifacts seen in Cartesian versus spiral k-space trajectories.
Cartesian sampling can produce ghosting artifacts from motion or flow, appearing as repeated copies of the object. Spiral sampling, which follows a curved path from center outward, is more robust to motion but can cause blurring if there are off-resonance effects (e.g., from fat or field inhomogeneity). Spiral also has a characteristic 'spiral' artifact from gradient imperfections. Cartesian images may have truncation (Gibbs) artifacts at sharp edges due to finite sampling. Both can suffer from aliasing if the field of view is too small, but the pattern differs: Cartesian aliasing appears as wrap-around, while spiral aliasing is more diffuse.
3. Give an example of a clinical scenario where spiral k-space trajectory is preferred.
Spiral trajectories are often used in functional MRI (fMRI) because they can acquire whole-brain images very quickly, improving temporal resolution. The rapid coverage reduces motion artifacts and allows better detection of brain activation. Spiral is also used in cardiac imaging for real-time cine, where speed is crucial to capture heart motion. Additionally, spiral sequences can achieve very short echo times, making them useful for imaging tissues with short T2*, such as in susceptibility-weighted imaging or for detecting hemorrhage. The trade-off is increased sensitivity to off-resonance blurring, which requires careful shimming.
4. Compare MTC with T1-weighted contrast in terms of what they highlight.
T1-weighted contrast depends on the longitudinal relaxation time of free water, highlighting differences in tissue composition like fat (bright) and fluid (dark). MTC, on the other hand, highlights differences in macromolecular content, independent of T1. For example, in the brain, white matter appears brighter than gray matter on T1-weighted images, but on MT-weighted images, white matter appears darker due to stronger MT saturation. MTC can suppress signal from blood and CSF, making it useful for angiography or perfusion imaging. Combining both contrasts provides complementary information about tissue structure and pathology.
5. How does the undersampling pattern in compressed sensing differ from that in parallel imaging, and why?
In compressed sensing, the undersampling pattern is typically random or pseudo-random (e.g., Poisson disk) to make aliasing artifacts incoherent, meaning they appear as noise-like rather than structured. In parallel imaging, the undersampling is usually regular (e.g., skipping every other line) because the reconstruction relies on coil sensitivity patterns to unfold coherent aliasing. The random pattern in compressed sensing ensures that the artifacts are spread out and can be suppressed by the sparsity constraint. Parallel imaging's regular pattern would cause structured aliasing that is harder to remove with sparsity alone.
6. How does a low-field MRI system reduce power consumption compared to a standard high-field system?
Low-field MRI systems operate at magnetic field strengths below 0.5 Tesla, much lower than the 1.5 or 3 Tesla of standard machines. Lower field requires less electrical power to generate the main magnetic field and reduces the need for heavy shielding. They often use permanent magnets instead of superconducting electromagnets, which eliminates the need for liquid helium cooling and the associated power for refrigeration. This makes them more energy-efficient and suitable for areas with unstable electricity. Additionally, lower field strengths produce weaker signals, but modern signal processing compensates for this.
7. Explain the difference between GRAPPA and SENSE in how they reconstruct undersampled data.
SENSE (Sensitivity Encoding) reconstructs images by first performing a direct Fourier transform on the undersampled data, which yields aliased images from each coil. Then, using known coil sensitivity maps, it unfolds the aliasing by solving a linear equation per pixel. GRAPPA (Generalized Autocalibrating Partially Parallel Acquisition) instead fills in the missing k-space lines directly. It uses a set of fully sampled autocalibration lines (ACS) to compute weights that estimate the missing data from neighboring acquired lines. The fully sampled k-space is then Fourier transformed to produce the final image.
8. What is window width and window level in digital radiography image processing?
Window width and window level are tools to adjust the contrast and brightness of a digital X-ray image. Window width controls the range of gray shades displayed: a narrow width shows high contrast but fewer details, while a wide width shows low contrast but more details. Window level sets the center of that range, determining which tissue brightness is in the middle. For example, to see lung details, you use a wide window width and a low window level. To see bone details, you use a narrow window width and a high level. These adjustments help radiologists see different structures without repeating the X-ray.
9. What is a potential artifact in MTC imaging and how can it be minimized?
A common artifact in MTC is direct saturation of free water protons by the off-resonance pulse, which reduces signal even in tissues with low macromolecular content. This can mimic MT effect and lead to misinterpretation. To minimize it, the off-resonance pulse is applied at a frequency far from the free water resonance (e.g., 1-2 kHz off) and with a narrow bandwidth. Additionally, using a high flip angle and short duration reduces direct saturation. Proper calibration and sequence parameters ensure that the signal change is truly due to MT exchange. Careful choice of the saturation pulse shape also helps.
10. Compare strain elastography and shear wave elastography.
Strain elastography applies a gentle push with the ultrasound probe to compress the tissue. It compares the tissue deformation before and after compression to estimate relative stiffness. It gives a qualitative map (e.g., softer vs. harder). Shear wave elastography uses a push from acoustic radiation force to create a shear wave and measures its speed to give a quantitative stiffness value in numbers. Shear wave is more operator-independent and reproducible. For example, in breast imaging, strain elastography helps distinguish benign from malignant lumps, while shear wave provides exact stiffness numbers.
11. Compare the training needed for operators of a standard MRI versus a simplified low-field MRI.
Standard MRI operators require extensive training in physics, safety, and sequence optimization, often taking months or years. Simplified low-field MRI systems are designed with automated protocols and user-friendly interfaces that minimize operator decisions. For example, the system may have preset scan parameters for common exams, reducing the need to understand complex adjustments. Training can be shortened to a few weeks, focusing on patient positioning, safety checks, and basic troubleshooting. This allows local healthcare workers with limited prior imaging experience to perform scans effectively.
12. How does the choice of target material affect the X-ray spectrum in mammography?
Different target materials produce X-ray spectra with different energies and intensities. Molybdenum gives a spectrum with strong characteristic peaks at low energies (around 17-20 keV), which are good for contrast but may not penetrate dense breasts well. Rhodium has characteristic peaks at slightly higher energies (around 20-23 keV), so it can penetrate denser tissue better. Tungsten produces a continuous spectrum with higher energy X-rays, so it needs a filter to remove low-energy X-rays that would increase dose without helping image quality. The choice affects both image contrast and patient dose.