The current status and further prospects for lung magnetic resonance imaging in pediatric radiology

Many children need regular lung imaging. Developments in MRI make this a viable radiation-free alternative for pediatric chest imaging. Special considerations in pediatrics have led to the development of these techniques, which could subsequently influence adult practice.

In principle, a morphological quality standard in MRI has been attainable for several years, and this in many cases is in no way inferior to pulmonary CT [1]. In addition, lung MRI offers the possibility of functional assessment regarding ventilation and perfusion [2], which can only be achieved with functional diagnostics or nuclear medical methods at this point. Functional lung MRI does not yet play a role in clinical routine [3, 4]. The exclusively morphological evaluation of the lung with conventional MR proton imaging has become the method of first choice at many pediatric radiology facilities when sectional imaging of the lung is necessary [5, 6].

Although the radiation exposure of the thoracic CT with new CT detector technology is really fairly low, any radiation exposure to children should be avoided if possible because radiation-sensitive organs (breasts, thyroid gland) are exposed. In addition, the tissue differentiation of some lung processes is better possible with MRI than CT. This is shown later in this pictorial review. The use of gadolinium (which some people have reservations against) is usually not necessary in lung MRI but makes co-evaluation of the mediastinum possible, whereas in CT a contrast medium has to be used.

In this paper we discuss what is done better — or differently — at hospitals that incorporate lung MRI from institutions that continue to perform only pulmonary CT. Technical aspects of lung imaging are therefore discussed. Subsequently, representative morphological lung MRI examinations are shown in this pictorial essay. The image quality presented, which is now standard, is likely to convince even skeptics of the diagnostic potency of lung MRI.

Only if you succeed in convincing the clinical partners of the unfamiliar image impression of the MRI and if you succeed in gaining understanding for the fact that about 10% of MRIs of the lung are diagnostically insufficient, will the chest MRI become established as routine lung diagnostics. The main point for the use of pulmonary MRI is the elimination of the high radiation exposure of lung CT. One CT is equal to to 100–200 chest radiographs. Depending on the age of the patient, the diagnostic-induced triggering of a malignant tumor occurs statistically after approximately 1,000 thoracic CT examinations. Pediatric pulmonologists rarely ignore this argument. About 80–90% of all CT examinations of the lung could be replaced by a lung MRI examination with good diagnostic validity — probably even more in the future. A second point is the fact that, in certain diseases, the lung MRI is not only of equal value to CT, but in individual cases MRI can even be superior. Additional information — such as the detection of pus in pneumonia with abscess formation [7] or signal-reduced lymph nodes in sarcoidosis [8] — makes MR diagnostics interesting for pediatric pulmonologists.

It is essential to identify the so-called MR-plus pathologies together with the referring partner. MR-plus pathologies are diseases that are particularly well described on MRI because many protons accumulate in the pathological lung process [9]. These include inflammation-caused accumulation of fluids in the alveolar space as well as all infiltrative processes, tissue proliferation and lung metastases.

This implies, according to current knowledge, that so-called MR-minus pathologies are not to be examined primarily with MRI, but preferentially by CT. These include diseases with a loss of protons, such as emphysema, little cysts and fibrosis at an early stage. According to current knowledge, the search for metastases in osteosarcoma also belongs in CT because these metastases typically calcify at an early stage and thus elude MRI diagnostics.

Although cysts and pulmonary fibrosis are generally counted as MR-minus pathology, discrete findings can now also be made using increasingly better MR examination techniques or special techniques. One should be aware, however, that the presentation and interpretation of such discrete findings on MRI is not as easy as with the changes usually found in MR-plus pathology. This means that the boundaries have become blurred: “In-between pathologies” could be called diseases that, depending on the referrer’s familiarity and the radiologist’s expertise, can be diagnosed at MRI too. These include lung cysts and congenital pulmonary airway malformation (CPAM) Types 1 and 2 [10].

A good MRI examination has the following criteria as minimum requirements:

In accordance with these quality criteria, the radiologist — regardless of the diagnostic assessment — must first formally evaluate the lung MRI examination according to a three-category scheme: Category A, the examination was technically successful without any relevant movement artifacts and the question can be answered; Category B, the examination shows movement artifacts but allows for a sufficiently reliable answer to the clinical question; or Category C, the examination shows movement artifacts or dorsal dystelectases from sedation such that a diagnostic evaluation is not possible.

Only Categories A and B examinations should be evaluated radiologically (Fig. 1; Table 1). In contrast, Category C examinations have to be formally classified as diagnostically inadequate and a timely CT examination should be organized by the pediatric radiologist, preferably on the same day. It should not be the task of the clinical colleague to commission the CT examination in the case of a non-diagnostic lung MRI. At the end of the day the clinician needs an image-based diagnosis, regardless of the method. This formalized approach will earn you the trust of your clinical colleagues.

Disagreement on the optimal sequences for a diagnostically sufficient depiction of pathological lung findings is regarded as one of the causes for the still-low prevalence of lung MRI examinations. It is also evidence that there is not the optimal sequence. Sequences that are adapted to different and essential questions have been published in various reviews and consensus papers (Table 1) [11].

In addition, there are several very detailed compilations with suitable lung MRI sequences, of which the publication of Ohno et al. [12] on nodule detection and the publication of Baez et al. [13] on pediatric radiology list the MR parameters in detail. However, the problem with such compilations is that the different sequences are presented in detail regarding their parameters. However, with this variety of sequences, it remains open which of the sequences mentioned should then be used in the daily routine to provide the best diagnostic results. Therefore, next we describe some basic considerations about sequence selection that might be helpful. The focus is on the evaluation of lung parenchyma and the examination of small children up to 8 years of age.

These diseases typically show findings that are easy to diagnose on MRI. Assuming an adequate examination technique, MRI diagnostics is on a par with CT for these diseases. This includes all lung diseases with alveolar exudation and infiltration by infections [24, 25], tissue proliferation such as tumors or metastases [22], malformations such as sequesters [26] and of course cystic fibrosis, which is particularly important in pediatric pulmonology [27].

The lung diseases in Figs. 20, 21, 22, 23, 24, 25 and 26 represent examples of MR-minus pathologies with regard to the difficult representability on MRI. If fibrosis, cysts, CPAM Types I and II and emphysema are suspected, lung CT might still be indicated as first-line diagnostic test in children [28]. Although MR findings are often very discreet in such MR-minus pathology, MRI can provide helpful diagnostic information in addition to the CT [29].

The use of T1-weighted UTE sequences for lung imaging was described in 2007 by Togao et al. [30]. But only now this idea is being taken up again and technically implemented, after the device manufacturers were able to realize extremely short echo times. Several working groups have since reported on the successful introduction of this 3-D sequence as a non-contrast-enhanced sequence with T1 weighting [31, 32]. The sequences suitable for T1-W 3-D imaging have a high resolution in the submillimeter range and are therefore also suitable for reconstructions in all three planes because of their isovoxel-geometry of 0.86 mm (Fig. 27). The sequence is used without contrast medium. It is self-gated to the respiratory phase and is additionally performed with a special spiral K-space readout. This further reduces movement artifacts [19]. The T1-W UTE sequences are available as 3-D sequences in reconstructible isovoxel geometry or as 2-D sequences, which can be executed in 13 s (slice width 2.5 mm) and also as so-called Zero Echo Time sequences. Technical background of these three lung sequences can be found in an excellent review by Wielpütz et al. 2019 [33].

Very promising results of dynamic regional ventilation and perfusion mapping using phase-resolved functional lung (PREFUL) MRI (Fig. 28) [4, 34‐36], a further development of Fourier decomposition MRI [37], have been shown recently. This technique holds the promise to mature into a patient-friendly MRI spirometry test, with novel clinically relevant information to guide clinical decision-making and improve patient monitoring [38‐40]. PREFUL MRI typically uses standard 1.5-T or 3-T MRI equipment and is based on a routine gradient echo fast low-angle shot (FLASH) sequence. PREFUL is well suited especially for children, because it is a free-breathing exam without the need for intravenous contrast agent and has a relatively short examination time (about 1 min per coronal 2-D slice, 15-mm slice thickness). The ventilation, perfusion and dynamic flow-volume loop maps are reconstructed entirely after the image acquisition using complex registration and post-processing algorithms, which are currently available as a research tool.

The correct indication and thus the preselection of children with a so-called MR-plus pathology are crucial for successful MRI diagnostics of the lungs. T2-weighted turbo spin-echo sequences remain the workhorse of lung imaging in pediatric radiology. However, this only applies if excellent respiratory triggering is achieved, regardless of which of the various trigger methods is used. In the near future, however, T1-weighted UTE sequence is expected to take a dominant position. Because of the current device developments with ultrashort echo times (in the microsecond range), their clinical use is just ahead. The functional evaluations of ventilation and perfusion are also in the preclinical stage and are likely to be an important addition to morphological diagnostics for specific questions.