The diagnosis of TOS in patients with the typical clinical presentation relies on the demonstration of the predisposing soft tissue abnormalities or compression of the brachial plexus, subclavian artery or subclavian vein in the thoracic outlet. As outlined above, this is exceedingly difficult in the clinical evaluation of these patients. Unfortunately, at surgery, up to one third of patients have no predisposing anatomic abnormalities (43). In the absence of such abnormalities, the success rate of surgical decompression is low (53).
Therefore, accurate and reliable imaging of the anatomy of the thoracic outlet, visualization of the predisposing anatomic abnormalities, and demonstration of the compression of nerves, arteries, and veins is paramount for the accurate diagnosis of these patients.
Many studies have been published regarding the effectiveness of ultrasound (71-76), CT scanning (68, 77-85) or MRI scanning (46, 47, 86-103) in the evaluation of thoracic outlet anatomy, and in the diagnosis of TOS.
Ultrasound is excellent for the evaluation of blood flow in the arteries and veins of the thoracic outlet, produces no ionizing radiation, and can measure dynamic changes or compression of the blood vessels in real time. Additionally, it can be used to evaluate these blood vessels with the patient's upper extremities maintained in any position. A few reports have demonstrated the use of ultrasound in evaluating the anatomy of the thoracic outlet and the brachial plexus, but ultrasound is unable to image through bony structures, and the position of the clavicle causes significant limitation of its use in patients with TOS. Since the vast majority of cases of TOS are neurogenic, ultrasound is not suitable at present as a first-line diagnostic tool.
CT scanning is excellent for evaluation of the bony structures of the thoracic outlet, and can be a very rapid scanning technique. However, CT scanning utilizes ionizing radiation, requires iodinated contrast material for the evaluation of blood vessels, and does not differentiate soft tissues as well as MRI. Since many cases of TOS are due to soft tissue abnormalities rather than to bony abnormalities, CT scanning is likely to lack sensitivity and specificity.
MRI scanning is excellent for demonstrating soft tissue detail including the muscles, nerves, blood vessels and fatty areas of the thoracic outlet, produces no ionizing radiation, utilizes gadolinium contrast material for the evaluation of blood vessels (which has considerably fewer adverse effects than does iodinated contrast material), and creates images in multiple planes. However, MRI scanning requires more time than does CT scanning, and bony structures are less obvious on MRI than they are on CT. In experienced hands, MRI defines all of the vital structures of the thoracic outlet, and is the best modality for ruling out spinal stenosis and neural foraminal stenosis, which are important in the differential diagnosis in these patients. The medical literature on MRI continues to expand. Most recently, a review article describing the use of MRI for the diagnosis of thoracic outlet syndrome has been approved for Continuing Medical Education by the Radiologic Society of North America, the largest professional organization of radiologists in the world (104).