MR imaging of the chest: A practical approach at 1.5 T

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Abstract

Magnetic resonance imaging (MRI) is capable of imaging infiltrative lung diseases as well as solid lung pathologies with high sensitivity.

The broad use of lung MRI was limited by the long study time as well as its sensitivity to motion and susceptibility artifacts. These disadvantages were overcome by the utilisation of new techniques such as parallel imaging. This article aims to propose a standard MR imaging protocol at 1.5 T and presents a spectrum of indications.

The standard protocol comprises non-contrast-enhanced sequences. Following a GRE localizer (2D-FLASH), a coronal T2w single-shot half-Fourier TSE (HASTE) sequence with a high sensitivity for infiltrates and a transversal T1w 3D-GRE (VIBE) sequence with a high sensitivity for small lesions are acquired in a single breath hold. Afterwards, a coronal steady-state free precession sequence (TrueFISP) in free breathing is obtained. This sequence has a high sensitivity for central pulmonary embolism. Distinct cardiac dysfunctions as well as an impairment of the breathing mechanism are visible. The last step of the basic protocol is a transversal T2w-STIR (T2-TIRM) in a multi-breath holds technique to visualize enlarged lymph nodes as well as skeletal lesions. The in-room time is approximately 15 min.

The extended protocol comprises contrast-enhanced sequences (3D-GRE sequence (VIBE) after contrast media; about five additional minutes). Indications are tumorous lesions, unclear (malignant) pleural effusions and inflammatory diseases (vaskulitis). A perfusion analysis can be achieved using a 3D-GRE in shared echo-technique (TREAT) with a high temporal resolution. This protocol can be completed using a MR-angiography (3D-FLASH) with high spatial resolution. The in-room time for the complete protocol is approximately 30 min.

Introduction

An excellent soft tissue contrast and the capacity to produce cross-sectional images or three-dimensional data sets in different orientations designate MRI as an ideal tool to evaluate chest wall masses or tumors of the mediastinum [1], [2]. It is widely accepted for these indications, and protocols for routine applications are readily available on most scanners. The value of MRI for imaging the heart and large vessels is well accepted [3], [4]. However, the largest organ of the thorax, the lung, is usually not investigated with MRI in the first place. Acute and chronic diseases of the lung and its malignancies, most importantly bronchial carcinoma, are responsible for high morbidity and mortality worldwide. The lung is also the major target for metastases and reacts to malfunction of other organs, e.g., lung edema due to heart failure. Thus, imaging of the lung by means of plain film radiography and computed tomography (CT) is among the most frequent radiological procedures. These techniques are cheap, robust and well accepted. With the advent of multidetector CT, even former advantages of MRI in imaging the chest wall and mediastinum, such as multiplanar image acquisition, are no longer clear-cut. However, X-ray and CT are criticized for their radiation exposure, in particular when used for frequent follow-up examinations, in pregnancy, or in children. A non-invasive radiation-free method for follow-up of lung diseases during clinical trials or physiologic research is desirable. Therefore, the development of MR techniques for lung imaging has continued, and as a result, we can nowadays recommend a comprehensive routine imaging protocol for the whole chest including lung parenchymal diseases.

Section snippets

General requirements for imaging thoracic organs

The major problem in imaging thoracic organs is the continuous motion of all components induced by heart pulsation and respiration. Both are most prominent in the lower and anterior sections of the chest. Technical difficulties in overcoming these effects are one major reason why MRI of the chest was limited to the posterior chest wall and the thoracic outlet for a long time. Both locations are relatively static and can be examined with classical T1- and T2-weighted spin-echo and fast spin-echo

Basic MR sequences for imaging of the chest at 1.5 T

The technical needs for MRI of the lung parenchyma are fast sequences, preferably for breath hold imaging with reasonable high spatial resolution, and short echo time (TE), to receive as much lung signal as possible within the short interval before signal decay.

Lung parenchyma

Ex vivo experiments and in vivo experience have shown that solid lung pathology can be well detected with fast T1-weighted gradient echo sequences (T1-GRE). For lung nodules larger than 4–5 mm, 3D gradient echo sequences reach the detection rates of conventional helical CT with a single row detector technique. Only recently could it be shown that 3D sequences are superior to 2D techniques [9]. Modern scanners with parallel imaging technique are capable of acquiring 3D-data sets of the whole

Pulmonary vasculature

In addition to morphologic imaging of the lung parenchyma, pulmonary magnetic resonance angiography (MRA) has been developed as a non-invasive imaging tool for the assessment of pulmonary vascular diseases. While in the early phase pulmonary MRA relied on black blood and time-of-flight approaches, contrast-enhanced MRA (CEMRA) is nowadays considered the method of choice for pulmonary MRA. The major advantage is its ability to image complex and slow flow with different spatial orientations,

Pulmonary perfusion

MR perfusion imaging can be accomplished using contrast-enhanced perfusion MRI. The basic principle of contrast-enhanced perfusion MRI is a dynamic acquisition following an intravenous bolus injection of a paramagnetic contrast agent. Perfusion MRI of the lung requires a high temporal resolution in order to visualize the peak enhancement of the lung parenchyma. Consequently, contrast-enhanced perfusion MRI uses T1-weighted ultra-short TR and TE gradient echo MRI [18]. Depending on the spatial

Mediastinum

The mediastinum contains the heart and the large vessels, the trachea, the esophagus, neural structures, and the lymphatic tissues and thoracic duct. Typical indications for cross-sectional imaging of the mediastinum are masses originating from the present structures. Size and position of a tumor can be assessed with MRI as well as with CT. Both modalities contribute to the characterization of tumors, e.g., with the detection of fat or calcifications inside a teratoma. In this case, CT has a

Proposal for a comprehensive standard protocol

The following recommendations for a lung imaging protocol refer to common MR sequence components of current standard installations. The necessary hardware is available at most sites. However, the recommended protocols of Table 1 are based on parallel acquisition techniques, but multi-breath hold acquisitions can be used instead.

Preparation for the examination includes providing the patient with instructions for the breathing maneuvers, the application of a respiratory belt and selection of a

Extensions to the standard protocol

Post-contrast scans markedly improve the diagnostic yield of 3D-GRE sequences by clearer depiction of vessels, hilar structures and pleural enhancement. Parenchymal disease and solid pathologies are also enhanced. Thus, a study to exclude pulmonary malignancies, e.g., for staging purposes, should usually comprise contrast-enhanced series, preferably with a fat-saturated 3D-GRE sequence. Contrast-enhancement is also necessary in case of pleural processes (empyema, abscess, metastatic spread of

Clinical applications for MRI of the chest

A list of potential indications for morphologic MR imaging of the lung is given in Table 2. These are covered with the above-described standard protocol. High quality MRI can provide significant input into clinical decision-making in a lot of pulmonary diseases such as bronchogenic carcinoma, malignant pleural mesothelioma, pulmonary arterial hypertension, acute pulmonary embolism, airway diseases such as cystic fibrosis, interstitial lung disease, and pneumonia. Besides the lung parenchyma and

Conclusion

Suitable protocols for simple and fast MRI studies for the lung can readily be set up at almost any state of the art 1.5 T scanner. Since the requirements can be matched with current equipment at most sites, this valuable technique will no longer be limited to specialized institutions. This opens the perspective to use MRI of the lung as a valuable alternative to X-ray and computed tomography for scientific purposes, examinations for medical opinions, frequent follow up studies in young

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