Troubleshooting and Quality Assurance in Hyperpolarized Xenon Magnetic Resonance Imaging: Tools for High-Quality Image Acquisition

In hyperpolarized xenon MRI, the actual inhaled gas is directly imaged. The gas uniformly distributes itself in lungs of healthy individuals, but in those with lung disease such as asthma, COPD, or cystic fibrosis, it does not. We aim to understand these diseases better using this technology.

X-ray computed tomography, or CT, provides the current clinical standard in pulmonary structural imaging, but involves ionizing radiation, and shows only lung tissue, not the inhaled gas. Nuclear medicine by inhaled or injected radio tracers can also be used to assess pulmonary ventilation and profusion, but is ionizing and relatively low resolution. Xenon MRI, however, provides a high-resolution snapshot of an individual's pulmonary ventilation and gas exchange.

Implementing xenon MRI is uniquely challenging, requiring specialized hardware, xenon imaging coils, and sequence development. The journey doesn't end there. The acquired data must undergo meticulous reconstruction and analysis.

Without proper expertise, these tasks can be daunting, leading to frustrating and expensive outcomes. This protocol offers insight into quality control, troubleshooting, and tools for xenon MRI sites. We explored the setup of hyperpolarizer labs, lean on MRI coils, data acquisition consideration, and image properties.

Our goal is to guide the audience through potential errors, challenges, and occurrences that may impact image quality or outcomes. Troubleshooting xenon MRI issues is necessary to mitigate problems in real life. Given the absence of a dedicated hyperpolarized gas infrastructure and limited support from scanner manufacturers, quality control tasks fall solely on individual labs.

This video aims to equip viewers with practical tips for addressing challenges and ensuring successful data acquisition. To begin, ensure the xenon-129 hyperpolarizer is set up and operational as per the manufacturer's guidelines. Next, use NMR at the hyperpolarization or HP measurement station to conduct T1 relaxation measurements on a representative HP xenon-129 gas sample.

Ensure a direct and efficient route from the xenon collection point to the magnet room designated for imaging to minimize delays during xenon transportation. Measure the initial dose equivalent, or DE, of the hyperpolarized xenon-129 gas before transportation, using the displayed equation. Avoid extraneous radio frequency signals along the route as they can contribute to polarization loss.

After completing the round trip by transporting the gas from the measurement station into the magnet bore, then back along the same route to the polarimetry station, measure DE again to quantify the anticipated signal loss during gas transport. Begin by creating a thermally polarized 129 xenon phantom. To do so, connect a glass pressure vessel to a xenon gas-filled bag, having an appropriate size and volume aligning with the vessel's capacity.

Then submerge the pressure vessel in a small amount of liquid nitrogen to allow xenon diffusion and freezing. Seal the vessel after the xenon has formed frozen snow inside. Allow it to thaw while pressurizing the vessel before calculating the pressure in the vessel.

To detect peak frequency, put the phantom inside the xenon-129 coil, and place it similar to that of a loaded patient. Perform a scan with proton frequency as some scanners may disallow multinuclear scans without an initial proton frequency localizer. Use a broadband transmit pulse, if available, high bandwidth and high resolution readout experiment to accurately detect the xenon frequency peak.

Once a well-defined peak is detected, record the frequency to full precision. Repeat the new experiment at the new frequency with a low bandwidth of about 1000 hertz to maximize signal-to-noise ratio, or SNR, and peak frequency precision. Once a satisfactory high signal peak is detected, save the protocol for future quality control tests.

Use a small amount of hyperpolarized xenon-129, which is well-concentrated and free of oxygen for imaging. Measure the xenon-129 dose equivalent, or DE, accurately immediately prior to imaging. Set the test imaging protocol to reflect desired in vivo parameters as closely as possible.

Acquire and save the image of the xenon bag as a baseline measure of scanner performance. Measure and record the SNR of the acquired images alongside all scan parameters and xenon DE.For measuring alpha, the flip angle, perform a full volume spoiled gradient echo scan in which the field of view is imaged twice in succession using identical sequence parameters. Measure the SNR at the DC offset of the two images, S0 and S1.Count the number of phase encoding steps, n, and calculate the flip angle.

Begin by creating a control noise profile for quality control or QC purposes. Use a specific customized 2D GRE sequence, including a high field of view, to capture the maximum signal from the area, a high bandwidth per pixel to identify nearby noise resonances, and the lowest possible repetition time, or TR, and echo time, or TE.Acquire the QC for noise profile using a xenon vest or a loop coil. Obtain an image with no hyperpolarized xenon-129 sample in the coil.

This image will characterize the noise profile. Examine the acquired noise data, particularly the K-space, for non-Gaussian elements such as spikes, patterns, or discretized or binned values. Create a quantile quantile, or Q-Q plot, by plotting the acquired real or imaginary data against a synthesized Gaussian dataset with identical mean, standard deviation, and vector length, both ordered from the smallest to largest.

Deviations from the line y is equal to x in the Q-Q plot indicate the presence of non-Gaussian components within the acquired data requiring further investigation. Proceed to identify the noise distribution pattern and potential outliers with a suitable plot of choice. To rule out the scanner as a noise source, acquire images using the standard site protocol with various pulse sequence parameters disabled and electronic components powered off.

Refer to the list on the screen for possible noise sources. To eliminate noise sources from the room, use a simple surface loop coil tuned to the xenon-129 frequency to sniff around the magnet room for noise sources. Physically place the xenon coil element near potential problematic devices and run a test sequence to detect amplified noise.

Examine K-space and image data to pinpoint the exact source of coherence noise. If a specified source is identified, attempt to disable it or cover it with aluminum foil, fleshing, or a copper mesh to reduce noise. Rerun the scan after disabling or covering noise sources to see if the noise resolves.

Continue this process until all noise sources are eliminated, leaving only low root-mean-square Gaussian noise. Identify irregular noise as high signal spikes in individual case-based pixels with abnormally high or low signals in the real or imaginary channels. Perform imaging in different phase-encoding orientations, including anterior to posterior, head to foot, and left to right.

Eliminate potential issues with X-Y-or Z-gradients by enabling or disabling individual gradients selectively. Systematically examine the resulting images to identify which specific gradient direction is contributing to the noise. The results of the noise characterization analysis performed on the noise scan demonstrated the impact of both regular and irregular noise on the K-space.

Regular noise led to a continuous pattern in the K-space, while irregular noise resulted in high value outliers in the Q-Q plot. A series of lung images acquired using HPG MRI showed that a distinct bright spot centered in the K-space indicated a clear lung signal with low noise. Conversely, the presence of regular noise was spread throughout the images.

Irregular noise evidently caused high value spikes in the K-space, and resulted in a stripe pattern in image space. A scenario where both regular and irregular noises were present simultaneously also affected the lung image.