Anatomy
The brachial plexus stems from the cervical spinal cord and extends to the axillary region to form the nerves that control movement in the upper limbs [1]: The brachial plexus is divided into roots, trunks, divisions, cords and branches (Fig. 1)
- 5 roots: formed from cervical nerves (C5 to C8) and the first thoracic nerve (T1)
- 3 trunks: ventral rami of roots form superior (C5, C6), middle (C7) and inferior (C8, T1) trunk
- 6 divisions: each trunk splits into two forming anterior divisions of superior, middle and inferior and posterior divisions of superior, middle and inferior trunks
- 3 cords: divisions regroup to form three cords; posterior (formed from post. divisions of C5-C8, T1), lateral (formed form anterior divisions and superior and middle trunks (C5-C7)) and medial cord (is a continuation of the anterior division of the lower trunk (C8, T1))
- 5 branches: musculocutaneous nerve, axillary nerve, median, ulnar and radial nerve
In the course of the brachial plexus there are several clinically relevant spaces which serve as the landmarks for the brachial plexus pathology [2]:
- the interscalene triangle: bordered by the anterior and middle scalene muscles on each side and the first rib inferiorly
- the costoclavicular space: a triangular space bounded anteriorly by the inner half of the clavicle, underlying the subclavian muscle and the costoclavicular ligament, and posteromedially by the first rib and insertion of the anterior and the middle scalene muscle
- the retropectoralis minor space (subcoracoid tunnel): crossing down the clavicle, the nerve cords enter the retropectoralis minor space.
MRI
MRI is the imaging modality of choice for the diagnosis of brachial plexus pathology. It provides a clear structural analysis of the brachial plexus, neuronal integrity as well as the analysis of the surrounding structures. The normal appearance of nerves on the T1-weighed image is a hypointense appearance of fascicles that are surrounded by hyperintense connective tissue due to its fat content.
For the imaging of the brachial plexus, various imaging strategies are proposed in the literature with no consensus on a universal protocol [3]. To display the brachial plexus anatomy a T1-weighed sequence in all three planes (axial, coronal and sagittal) is performed, which is coupled with various sequences depending on different pathologies (Fig. 2)[4]. Most commonly used sequences in combination with T1 weighed sequence are [4]:
- Post-gadolinium-enhanced T1-weighed sequence with fat suppression (indications are after surgery and presence of known or suspected mass)
- T2-weighed sequences (i.e. DIXON & STIR); can reveal enlargement and hyperintense signal of nerve roots (due to excessive water content)
- 3D DESS (double echo steady-state) or MENSA (GE)
- DTI with tractography; is used for depicting tissue anisotropy and providing information about orientation and architectural organization of tissue i.e. nerve fibers. The use of tractography is limited to the evaluation of large nerve bundles.
STANDARD MRI PROTOCOL
Table 1
In our center, we use the 3.0T MRI scanner. We use the following protocol:
Table 2
Ultrasound
With the development of new high-resolution equipment, ultrasound has gained more importance in the evaluation of normal anatomy and pathology of the brachial plexus [5]. A nerve can be evaluated in the short or long axis but due to their complex anatomy, the short axis enables a better display of the course (Fig. 3). The nerves of the brachial plexus on the ultrasound have a characteristic appearance of a fascicular pattern, the hypoechoic fascicles of a nerve are surrounded by hyperechoic connective tissue and collagen (Fig. 4 and Fig. 5).
As opposed to the nerves of the brachial plexus the roots, trunks and cords do not display the same characteristic fascicular pattern. They appear as homogenous, hypoechoic structures, tubular in longitudinal slices and oval in axial slices [6].
Image comparison - from roots to branches
Ultrasound is gaining recognition and producing comparable results to MRI, but both imaging techniques have their intrinsic advantages and limitations.
Although many centers routinely perform MRI to evaluate peripheral nerve disease, the US offers several benefits over MRI. US is cheaper and faster to perform than MRI, and can also be used to image patients who are not eligible for the MRI [2]. Also, the advantages of the US are low economic costs and real-time contact with the patient during examination, which has also led to the development of a variety of US-guided interventional procedures [7]. An often-cited drawback of the US is that the modality is operator dependent and requires time to learn [1].
In recent years the US has become a broadly available bedside tool that has excellent sensitivity and a more flexible field of view. By enabling real-time dynamic evaluation of peripheral nerves (i.e. suspected entrapment), an entire nerve can be quickly evaluated with the US, whereas the MRI is limited by the coil and coverage constraints. Also, comparison with the contralateral side can easily be made with the US as opposed to the MRI.
When comparing the obtained images, the MRI of the brachial plexus shows more anatomical details (bone, structures inside the vertebral canal) and has better contrast differences between the nerves and the surrounding fat compared to the US and is at the same time superior for the evaluation of the deep structures and also tissue characterization using multi-sequence analysis and intravenous contrast. On the MRI, the nerve fascicle is also the smallest unit that can be depicted similarly to the US. With MRI, fascicles can be identified in large nerves, but not invariably in small distal branches [1]. On the other hand, the ultrasound has an advantage in the imaging of the superficial tissues.
When evaluating the brachial plexus with the US and MRI at the level of the nerve roots, we can evaluate only one root at a time with the US and only from the level of foramina onward. The MRI, on the other hand, has a clear advantage over the US as one image can entail all of the roots branching at the spinal cord as they are exiting the vertebral canal through the foramina ( Fig. 6, Fig. 7, Fig. 8). The trunks of the brachial plexus are evaluated at the interscalene triangle and can represent an imaging challenge as they are tightly surrounded with other structures which can sometimes make the differentiation difficult on the MRI. On the contrary, the trunks depictions on the US are straightforward when excluding the anatomical limitations (Fig. 9, Fig. 10, Fig. 11). The divisions and partly cords evaluation at the supraclavicular region of the brachial plexus demonstrates also anatomical limitations when performing US analysis, as opposed to the MRI that offers a good anatomical display ( Fig. 12, Fig. 13, Fig. 14). The brachial plexus at the level of cords can be very well imaged using both the MRI and the US (Fig. 15, Fig. 16, Fig. 17). However, the only disadvantage in performing the US evaluation represents the inability to image the patients who are unable to raise their arm.