Magnetic Resonance Imaging of the Diaphragm: From Normal to Pathologic Findings
The diaphragm is a dome-shaped musculotendinous structure placed between the thorax and the abdominal cavity.
It is considered the main inspiratory muscle, since its contraction causes the enlargement of the chest with consequent pressure lowering and airways gas filling.
Different imaging modalities can be employed for the evaluation of the diaphragm. X-ray plain film still represents the initial imaging step for diaphragmatic pathology, although it can only provide a few morphologic information.[1,3]
Computed tomography (CT)-scan can provide morphological but not functional information about the diaphragm.
On the other hand, conventional fluoroscopy, ultrasound (US), and magnetic resonance (MR) are able to overcome the mere morphologic assessment, extending the evaluation to the diaphragmatic functionality, through a real-time appraisal.[3-5]
Compared to fluoroscopy, the US comes with the advantages of lack of radiation exposure, easy portability, and capability of both morphologic and functional assessment. The main findings quantifiable on the US are diaphragmatic thickness and amplitude of excursion during free or forced breathing.
As well as the US, MR imaging (MRI) is a radiation-free technique that can provide a static or dynamic evaluation with the further benefit of a wider field of view and a more detailed soft tissue characterization.
In particular, the latter considerations are particularly important in the challenging differential diagnosis of lung diseases from diaphragm weakness in patients suffering from respiratory failure.
Beyond the limits of a time-consuming exam and the indispensable patient’s compliance, MRI is currently the most comprehensive imaging modality in the evaluation of diaphragmatic pathologies.
Hence, the aim of this paper is to provide an overview of normal and pathological features of the diaphragm on MRI and, therefore, to demonstrate the usefulness of this technique in different clinical circumstances.
Normal MRI findings
The diaphragm is composed of a central tendon and a peripheral muscular component, both provided of three major openings that allow the passage of vascular (caval and aortic hiatuses) and gastroenteric (esophageal hiatus) structures.
The diaphragm is anterolaterally connected to the sternum, the xiphoid process, and to the last six costal cartilages through muscle bundles (or “diaphragmatic slips”), while posteriorly it is attached to the first lumbar vertebral bodies through two musculotendinous structures (the “crura”).[1,6]
Innervation is provided by the phrenic nerves, originating from nerve roots C3–C5.
Normally the diaphragm looks like a thin band with low signal intensity on both the T1-w and T2-w images.
Beyond the morphologic and structural assessment, the use of dynamic gradient echo recalled acquisitions for the evaluation of diaphragmatic excursion has been longstanding established.
This type of sequences enables to obtain sequential images that can be acquired on the coronal or sagittal planes during real-time breathing. Afterward, the images can be displayed in a cine-loop viewing, thus providing a dynamic report about diaphragmatic motion.
The usual classification includes: Intrapleural (or Bochdalek), mediastinal (or Morgagni), and hiatal herniations: The formers mainly cause lung hypoplasia and mediastinal shift to the contralateral side due to the thoracic herniation of abdominal content; mediastinal hernias occur posteriorly to the sternum, with the involvement of liver and bowel, and are mainly related to cardiac malformations; hiatal hernias arise posteriorly within the mediastinum, usually together with esophageal alterations.[1,9]
The first imaging approach is based on endouterine US.
The injuries of the diaphragm are a relatively rare occurrence in subjects suffering from thoracic-abdominal trauma (0.8–8%) and can be related to blunt or penetrating traumas.
Acquired hiatal hernias in the adult population are caused by an enlargement of the esophageal hiatus in conjunction with the weakness of phrenoesophageal ligaments.
Diaphragmatic anterior or posterior congenital defects account for some cases of herniation.
Additional conditions, such as increased intra-abdominal pressure due to obesity, can further facilitate their onset.
At MRI, these types of hernias are usually detected as incidental findings [Figure 3].
Weakness and paralysis
Temporary or permanent, unilateral or bilateral diaphragmatic functional deficiencies can arise at three levels: The nervous system, the muscle, or the neuromuscular junction.
The causes are several, from injuries to infections, tumors, inherited metabolic, or collagenous diseases.
As a result, weakness or paralysis with impaired excursion and cranial dislocation of the diaphragm can be detected, with consequent lung parenchyma atelectasis and respiratory distress.
A “real-time” imaging of diaphragmatic function can be performed through fluoroscopy, US, and MRI during normal respiration, deep breathing, or sniffing.
The diaphragm can be affected by a plethora of benign or malignant primary tumors.
While benign lesions are usually simple cysts (with bronchogenic or mesothelial origin), the most common benign solid tumor is lipoma that, extremely rarely, can show a malignant evolution into liposarcoma.[8,14]
Benign entities are usually asymptomatic unless their size leads to a “mass-effect,” generally with respiratory impairment.
On the other hand, rhabdomyosarcoma and leiomyosarcoma are the most frequent cancers, both characterized by poor prognosis.
Moreover, the diaphragm can be affected by metastasis from primary tumors, especially breast, ovarian, and thymus, or it can be infiltrated by tumors arising in the adjacent, thoracic, or abdominal structures [Figure 8].[1,8,14]
Cystic echinococcosis (or “hydatid disease”) is considered a separate chapter in the field of cystic lesions. It is generally defined as a zoonotic infection caused by the incidental ingestion of the eggs of a small tapeworm parasite (Echinococcus granulosus), and the involvement of the diaphragm is of rare occurrence.
The most common scenario, due to contiguity, consists of the direct extent from the liver (0.6–16%).
Seldom, the diaphragm can be the primary and only site of the implant of the hydatid cysts (1%), through a vascular or lymphatic spread from the bowel.
From this site, the infection can easily diffuse into the thorax, involving mediastinum, pleura, and lung parenchyma with the formation of a bronchial fistula.
The correct diagnosis of diaphragmatic pathologies can be challenging, especially in the context of an accurate differentiation from respiratory diseases.
Due to the wider availability, CT-scan is generally the first- line imaging study, especially in emergency situations, while the US represents a staple approach for a functional assessment.
Beyond the well-known limitations, MRI is currently the technique that best combines the advantages of CT and US, succeeding in providing the most comprehensive evaluation of the main inspiratory muscle.
Therefore, radiologists and physicians should be aware of the diagnostic possibilities of this safe and valuable technique and confident with the images achievable.