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Missy Rajewski
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Lung magnetic resonance imaging (MRI) using conventional sequences is limited due to strong signal loss by susceptibility effects of aerated lung. Our aim is to assess lung signal intensity in children on ultrashort echo-time (UTE) and zero echo-time (ZTE) sequences. We hypothesize that lung signal intensity can be correlated to lung physical density.
Ultrashort echo-time and zero echo-time sequences are designed to capture the rapid decaying T2* signal of tissues like the lung or bone. Both sequences have been implemented for lung morphology and pathology [11,12,13,14,15,16], and ultrashort echo-time sequences have been recently used for COVID-19 patients [17,18,19].
Three-dimensional ultrashort echo-time Cones (3D UTE Cones) is a sequence which covers the k-space using multiple twisting radial spokes that form conical surfaces. Therefore, it is a non-cartesian data acquisition method. Cones achieves echo times down to 30 µs [21] and thus is an excellent candidate sequence for lung density imaging. 3D UTE Cones provides higher signal-to-noise ratio than the plain projection reconstruction because it samples the k-space more efficiently and leads to shorter scan times as well. The length of the spokes is operator-adjusted by adding more twisting. The spokes that are longer, have a longer readout so they cover the k-space faster achieving even shorter scan times [21]. The filling of the k-space is realised through the utilization of many size-varying cones that are made up of a different number of spokes.
Respiratory motion-resolved four-dimensional zero echo-time (4D ZTE) [22] is a zero echo-time sequence that is binning the respiratory cycle into different data sets that can be reconstructed separately, so motion is the fourth dimension. The final number or reconstructions depends on the number of the bins that may vary from 4 up to 8. Zero echo-time can achieve even shorter echo-times, namely, near 0, because it employs a continuously switched-on reading gradient during radiofrequency excitation [23,24,25]. 4D ZTE is a non-cartesian data acquisition scheme using radial spokes as trajectory to fill the k-space. The scan time of zero echo-time is faster than the ultrashort echo-time due to the gradients being constantly on, leading to shorter repetition times and silent scans, rendering the sequence clinically more favourable.
Comparison of the ultrashort echo-time (left column) and zero echo-time sequence (right column) depending on age in patients without lung pathology. a Axial slices at the hilum level of a 1-year-old boy and b, 4-year-old boy c and d and 17-year-old teenager e and f reveal signal change in lung parenchyma between 1 and 4 years of age for both sequences
The anterior, middle, and posterior lung-to-background ratio curves in relation to patient age follow the same curve as for the whole lung. For both sequences, the posterior lung-to-background ratio was higher compared to the anterior and middle lung-to-background ratio for all ages and was highest for newborns. We found a rapid decrease in lung-to-background ratio until the age of 2 years. The age dependency is shown in Fig. 4a for the ultrashort echo-time sequence and Fig. 4b for the zero echo-time sequence.
Lung-to-background signal intensity ratio curve depending on age and gravity gradient. Regional lung to background ratio for anterior, middle, and posterior areas for a ultrashort echo-time and b zero echo-time for controls related to age
The values of signal-to-noise ratio for ultrashort echo-time and zero echo-time sequences decreased from 17 and 14 in newborns to 4.5 and 5.2 in young adults. Furthermore, the contrast-to-noise ratio varied from 9.6 to 1.7 for the ultrashort echo-time sequence and from 7.8 to 1.6 for the zero echo-time sequence.
We could demonstrate in this study that lung signal intensities can be measured by using a zero echo-time sequence in pediatric patients. The sequence is able to detect age-dependent changes similarly to ultrashort echo-time sequences. Both sequences can visualize lung parenchyma with its rapid signal decay due to its very short T2* time in contrast to conventional MR sequences that are based on proton imaging.
With improvements in MRI hardware and software, particularly with the introduction of non-Cartesian data acquisition schemes, echo times near zero can now be used for image acquisition either as ultrashort echo-time or as zero echo-time acquisitions. Based on their acquisition scheme both sequences are three-dimensional [5]. Zero echo-times sequences have the advantage of minimal gradient switching and, therefore, allow for silent lung imaging compared to UTE. Therefore, ZTE may be more clinically useful and comfortable for the patients supposing that delivers similar results to UTE.
Both sequences revealed an increased posterior lung-to-background ratio compared with the anterior lung-to-background ratio for all patients of the normal cohort, independent of age. These findings are in line with previously described CT results in the literature [36,37,38,39]. The zero echo-time sequence had increased posterior-to-anterior values compared to the ultrashort echo-time sequence. This could be caused by noisier signal intensities in the anterior region in the zero echo-time sequence compared to the ultrashort echo-time sequence. The fact that both sequences were able to depict the anteroposterior gravity gradient of the lung density supports our hypothesis of the correlation between the lung signal intensity measured by MRI and the physiological lung parenchymal density. The signal-to-noise ratio and contrast-to-noise ratio of the posterior lung-to-background ratio was higher compared to the anterior and middle lung-to-background ratios for both sequences for all patients. The ultrashort echo-time sequence had higher signal-to-noise and contrast-to-noise ratios for newborns while zero echo-time provided slightly higher signal-to-noise ratio for the older patients. This could probably be caused by lower noise of the ultrashort echo-time sequence in newborns and lower noise of the zero echo-time sequences in older children.
The ultrashort echo-time sequence is a highly sensitive method that may also be valuable for the multiple follow-up imaging examinations required for lung patients during treatment [26]. Both sequences have been investigated in recent studies by Bae et al. [22, 35]. The four-dimensional zero echo-time sequence provided images of lung parenchyma with improved signal-to-noise ratio and contrast-to-noise ratio compared with its three-dimensional variant. In addition, the zero echo-time sequence had higher signal-to-noise and contrast-to-noise ratios than the ultrashort echo-time sequence for adult patients, similarly to our findings.
The captured signal is proportional to the voxel size. Both sequences had comparable in plane-resolution but different slice thickness. The ultrashort echo-time sequence had a thicker slice thickness and could, therefore, have theoretically between 8% (with 2 mm slice thickness) up to 62% (with 3 mm slice thickness) higher signal. The zero echo-time sequence has shorter echo-times leading to higher signal as well. However, the sequences are different, and thus, other parameters may have an impact on the signal.
For reasons of radiation exposure, we did not compare our MRI results with CT lung density measurements. However, the lung-to-background ratio curves for both ultrashort echo-time and zero echo-time sequences follow the characteristics of the CT lung density curves in the literature.
Ultrashort and zero echo-time sequences are promising within the scope of modern lung imaging as a radiation free imaging method which is particularly important in children. Both sequences are possible candidates for MR quantification of normal and pathological lung.
Ultrashort echo-time and zero echo-time sequences are able to assess lung density by capturing higher lung signal intensity in comparison to the background air. Based on their efficiency in visualizing and quantification of lung parenchyma, they may further be clinically utilized for lung MRI in children without exposing the patients to unnecessary radiation exposure.
Base Reflectivity (N0R, N1R, N2R, N3R/19 and N0Z/20)A display of echo intensity measured in decibels relative to Z (dBZ). Scientists use these products to detect precipitation, evaluate storm structure, locate boundaries, and determine hail potential. Four low elevation angles are available, with specific elevation angles depending on the scanning mode of the Radar. Sixteen possible data levels are also available.
Composite Reflectivity displays the maximum reflectivity from all scanned heights above the ground during the volume scan. These products reveal the highest reflectivities in all echoes, examine storm structure features, and determine the intensity of storms.
Low/Mid/High Layer Composite Reflectivity is a display of maximum reflectivity for three different height ranges within the volume scan. Use this product to reveal the highest reflectivities in all echoes, examine storm structure features, and determine the intensity of storms. The NLA/67 product is similar to NLL/65, but edited to remove contamination from anomalous propagation.
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