MRI of the lung (2/3). Why … when … how?

The prerequisites for the introduction of a new technique into the clinical setting are practicality of the application as well as robustness and reproducibility of image quality and diagnostic accuracy. The practicality of an MRI technique is further enhanced by a set of readily prepared protocols customised for specific clinical questions and available for standard scanners [1.5 Tesla (T)]. Preferably, additional features complicating the procedure such as the need for ECG-triggering or contrast material administration would be avoided. Furthermore, solutions for typical problems such as patients not being able to hold their breath or young children should be offered. In practice, it has been useful to design a common basic protocol trunk to cover the majority of expected clinical problems [1]. This basic protocol will be extended for specific questions such as the staging of malignancy, evaluation of pulmonary vessels and perfusion, etc. Furthermore, if the use of MRI is intended for emergency conditions, such as acute pulmonary embolism, fast and efficient procedures are required with in-room times that make it possible to squeeze the exam into a full MR schedule during the day [5]. Given the high investment into state-of-the-art MRI equipment and the running costs for maintenance and personnel, the key parameter for cost-effectiveness of the method is in-room time. Based on the currently available standards, in-room times of 15 min for the basic protocol or an emergency examination were considered desirable [1]. Additional protocol extensions, e.g. contrast-enhanced series, to detect tumour necrosis and pleural reaction/carcinosis or dynamic contrast-enhanced MRI and visualisation of respiratory motion contribute functional information should not exceed further 5-15 min in addition to the time needed for the basic protocol. To match this frame of expectancies as far as possible was the aim of the presented consensus on lung MRI protocol suggestions. Tables 1, 2, 3, 4 provide a systematic overview over indications for lung MRI and imaging protocol set-ups as well as a list of abbreviations/glossary of terms in order to facilitate comparisons and cross-vendor correlation throughout the manuscript.

Pathological conditions in the lung related to changes of lung structure result either in an increase of lung density (collapse of lung with atelectasis, fluid accumulation inside the alveolar space and/or the lung interstitium, fluid and cell accumulation in infiltrates or growth of soft tissue resulting in nodules and masses) or decrease of lung density (over-inflation due to air-trapping, enlargement of air spaces in emphysema) [6‐10]. Given the well-known limitations of MRI in visualising low-proton-density structures with high susceptibility artefacts at tissue-air interfaces, any pathology resulting in an increase of lung density (and consequently proton density) should be easy to detect, while a decrease of lung density will remain more challenging to visualise with MRI [8]. However, most conditions resulting in destruction and over-inflation of lung parenchyma, such as in emphysema, are associated with characteristic changes in the remaining lung parenchyma (fibrosis, septae, parenchymal distortion, bronchial wall thickening, etc.). Hence morphological MRI can still provide a correct diagnosis based on widened spaces of missing signal due to over-inflation and associated findings with increased signal intensity [11]. A direct visualisation of the airspace remains the domain of sophisticated imaging technology in specialised centres (see airways and lung ventilation disorders). Furthermore, changes in lung perfusion (hypoxic vasoconstriction, pulmonary embolism, congestion due to heart failure) result in changes of lung signal on MRI [7]. Non-enhanced MRI can directly visualise the vessel tree to detect macro-vascular pathology.

More than in X-ray or CT, image quality in MRI depends on patient compliance. As discussed above, it can be expected that some sequences may produce unsatisfactory results in individual patients. Therefore, the use of fast imaging protocols is advocated, with additional sequences to allow for with a certain degree of redundancy to compensate for any failed acquisitions [1]. Excellent results will be obtained in young, compliant subjects with good breath-hold capability. In contrast to this, it may become most difficult to obtain diagnostic image quality in non-compliant, obese patients unable to hold their breath or unable to understand and follow the breathing commands. For the latter, parts of the protocol can be acquired in free breathing, either due to their fast acquisition scheme or with respiration gated or triggered acquisition modes. Respiration-triggered versions of the T2-weighted fast spin echo sequences are available for uncooperative patients who have difficulties with breath-holding (their application increases the total in-room time by 10 min). Technicians will test patient compliance early on in the study, and can adjust the sequences accordingly. However, it needs to be emphasised that coaching of patients is very important to gain most information from any MRI scan. Respiration belts may be helpful, since this allows for monitoring patient compliance even during image acquisition [1].

As discussed in the previous article of this series in detail [MRI of the lung (1/3)], the following protocol suggestions can be transferred from 1.5- to 3-T MR scanners [17]. While most sequences, in particular 3D gradient echo techniques, profit from a higher lesion-to-background contrast to a certain degree, artefacts may deteriorate the image quality of steady state gradient echo sequences [17, 21, 77]. Overall, the changes of image quality with transfer of the concept to 3 T are acceptable or even positive for most sequence types.

Based on the published experience and matching the above-stated expectancies, Table 1 summarises a selection of available sequences and their potential diagnostic yield for lung MRI. For the overview of concept, details on sequence technology and parameters are left out here, since dedicated protocols are described in the literature and may vary in between different vendors [88‐91]. Adjusted to the size of the patient, the fields of view (FOVs) would be typically 450-500 mm in coronal and approximately 400 mm in transverse acquisitions with matrices of 256-384 pixels (for triggered fast spin echo series up to 512) resulting in pixel sizes smaller than 1.8 × 1.8 mm. Slice thicknesses for the 2D acquisitions would range from 4 to 6 mm. Three-dimensional acquisitions for imaging lung morphology in transverse orientation would use slice thicknesses of 4 mm or less, pulmonary angiography in coronal orientation 2 mm or less [88]. This is in line with the QA guidelines for thoracic MRI published by the German Board of Medicine [92].

A basic or general study combines T1-weighted and T2-weighted images based on gradient echo and fast spin echo sequences, respectively [90]. T1-GRE sequences are available as 3D acquisitions and may, therefore, be limited to a volume acquisition of the chest in one breath-hold. Fast T2-weighted spin echo sequences should cover at least two planes, e.g. with a half Fourier breath-hold acquisition in coronal and a transverse orientation. This results in a high sensitivity of the protocol for infiltrates and small nodular lesions. To improve the sensitivity of the protocol for mediastinal lymph nodes and bone lesions (e.g. metastases), a STIR or fat saturated T2-fast spin echo sequence should be added. Since rib metastases are easier to detect on transverse slices, this orientation should be preferred (Fig. 8). The list of “must” sequences (in the table indicated by G for “general protocol”) is concluded with a coronal steady-state free precession sequence in free breathing, which contributes to a high sensitivity for central pulmonary embolism and gross cardiac or respiratory dysfunction. As an option (but at the cost of additional imaging time exceeding the intended limitation to 15 min), motion-compensated T2-weighted fast spin echo sequences can be added to improve the depiction of masses with chest-wall invasion [1, 5].

With this selection of non-contrast-enhanced sequences, the study covers some of the most common potential findings, including pneumonia, atelectasis, pulmonary nodules or masses, mediastinal masses (lymphoma, goiter, cyst, thymoma) and acute pulmonary embolism [5]. Depending on the initial findings following the basic protocol, additional contrast-enhanced acquisitions may be required, which would use the same type of volume interpolated 3D-GRE sequence, but now with fat saturation to improve the visibility of contrast-enhanced tissues and mediastinal lymph nodes (T “tumour protocol”). Although the 3D sequences cover the whole chest, in-plane resolution is optimised in either transverse or coronal section (Fig. 9). Since acquisition times are just one breath-hold, it appears to be feasible to acquire the 3D GRE studies in at least transverse and coronal planes. In particular for the staging of lung cancer, it might be helpful for the detection of small lymph nodes metastases to add a diffusion-weighted sequence as well. Both sequences extend the total imaging time by approximately 5 min [1].

The available options for imaging disorders of lung vasculature comprise three components: First of all, the free breathing steady state gradient echo (SSFP-GRE) study which is also part of the general protocol. Then two variations of 3D gradient echo based contrast enhanced MRA: (1) a time-resolved, low spatial resolution acquisition for first pass perfusion imaging and (2) a high spatial resolution acquisition for a breath-hold angiogram (Fig. 10). Depending on the performance of the MR scanner, the dynamic study produces a comprehensive lung perfusion study with excellent temporal resolution and at the same time serves for the determination of optimum contrast bolus timing for the acquisition of the high resolution angiogram. The “vessel protocol” (V in Table 1) optimises the sensitivity for acute and chronic pulmonary embolism, arterio-venous (AV) malformation (e.g. Osler’s disease), lung sequestration, pulmonary arterial aneurysm, abnormalities of pulmonary venous drainage and any other pathology of lung vasculature [5]. As in other parts of the protocol recommendations, it is suggested to combine different fast imaging sequences to increase sensitivity and specificity of the examination [93]. Therefore, Kluge et al. [94] suggested combinations of different available MRI techniques for the detection of pulmonary embolism [5]. The lung vessel imaging branch of the protocol tree can be used just for a study of vascular pathology, e.g. in suspected acute pulmonary embolism or in combination with the tumour protocol for the comprehensive evaluation of a central mass with vessel invasion.

These protocol suggestions cover all expected clinical questions with different branches of the protocol tree for specific problems. They are a good point to start from and allow users to make up their own branches based on the suggested components.