Subclavian steal phenomenon (SSP) refers to subclavian artery steno-occlusive disease proximal to the origin of the vertebral artery and is associated with flow reversal in the vertebral artery. While Contorni first described retrograde flow in the vertebral artery in 1960,  Reivich in 1961 first recognized the association between this phenomenon and neurologic symptoms.  Fisher dubbed this combination of retrograde vertebral flow and neurologic symptoms subclavian steal syndrome (SSS), suggesting that blood is stolen by the ipsilateral vertebral artery from the contralateral vertebral artery. It was later suggested that such "steal" may cause brainstem ischemia and stroke, either continuously or secondary to arm exercise. 
See the images below of subclavian steal syndrome.
The term SSS should be reserved for retrograde vertebral artery flow associated with transient neurologic symptoms related to cerebral ischemia. SSP refers to retrograde flow in the vertebral artery only. First diagnosed angiographically in the early 1960s, SSS is now most commonly diagnosed during Doppler ultrasound (US) examination of the neck arteries. [4, 5, 6, 7]
Color Doppler US is the preferred examination for subclavian steal syndrome, but it is operator dependent. In addition, direct examination of the proximal subclavian artery is compromised by the overlying clavicle, ribs, and sternum.
Most commonly, subclavian artery steno-occlusive disease is not associated with symptoms and therefore can be managed conservatively with attention to diabetes and hyperlipidemia management, cessation of smoking, and antiplatelet therapy. When symptomatic, coincident significant carotid system stenoses should be excluded. Endovascular stenting and extrathoracic surgical bypass of subclavian steno-occlusive disease are safe and lasting therapeutic options. 
Contrast-enhanced CT multidetector scans (with 3-D reconstructions techniques) are replacing conventional catheter angiography in the diagnosis of subclavian steal steno-occlusive disease.  With contrast enhancement, particularly using test bolus or bolus tracking techniques, the degree of subclavian steno-occlusion, mural thrombus, ulceration, and arterial wall calcification can be evaluated. [10, 11, 12]
Calcified plaque (which may have a density similar to contrast) has the potential to compromise estimates of the degree of stenosis and occlusion. Atheroma-related calcification projected over the proximal subclavian artery may be apparent. The absence of calcification does not rule out subclavian steno-occlusive disease, nor does the presence of calcification confirm it. [11, 12]
Magnetic Resonance Imaging
Contrast enhanced 3-D MRA after localizing 2-dimensional time-of-flight can confirm subclavian steal phenomenon (SSP).  Phase-contrast MRA measures vertebral artery flow direction and velocity. MRA techniques do not require iodinated contrast (and its accompanying nephrotoxic and allergic side effects), but they can be time consuming. Lack of widespread availability, claustrophobia, and contraindications to MR (eg, pacemakers) limit MRA use (see the images below).
With head and neck coils and high-gradient MRI technology, gadolinium-enhanced MRA enables accurate depiction of proximal subclavian artery steno-occlusive disease.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
Subclavian steal phenomenon (SSP) most commonly is diagnosed incidentally during carotid and vertebral artery color Doppler US.
Transcranial color Doppler US enables examination of the basilar artery and arteries of the circle of Willis, characterizing collateral pathways. [4, 5] Flow reversal in the basilar artery is more likely when subclavian steal is symptomatic, particularly when carotid disease is present and is a potential alternative source of symptoms (see the images below).
The earliest waveform change associated with subclavian artery stenosis is a transient decrease in ipsilateral vertebral artery midsystolic velocity. Demonstration of ipsilateral vertebral artery flow reversal with a parvus tardus waveform in the ipsilateral subclavian artery confirms the diagnosis of SSP. A parvus tardus waveform (prolonged systolic acceleration time with decreased peak systolic velocity) in the distal subclavian artery and brachial artery is expected with severe subclavian stenosis or occlusion; a monophasic waveform also replaces the usual triphasic Doppler signal in these vessels. US examination can document other arterial disease. Investigate further if symptoms are atypical, suggesting a different intracranial or cardiac etiology.
Although total vertebral artery reversal of flow correlates with severe subclavian artery stenosis or occlusion, it does not indicate the presence of related neurologic symptoms.
With progressive stenosis of the subclavian artery, midsystolic velocity slows further, then reverses direction until both systolic and diastolic flow are reversed. Reactive hyperemia provoked by blood pressure cuff maneuver and arm exercise exaggerates this waveform change (subclavian steal test).
A rare finding is vertebral artery flow reversal related to proximal vertebral artery occlusion (spinal artery steal) but without subclavian disease. In this syndrome, posterior fossa symptoms can occur with a normal subclavian artery, normal upper extremity blood pressures, and normal triphasic subclavian and brachial Doppler waveforms.
Lack of vertebral artery flow reversal can occur with severe left steno-occlusive subclavian disease when the left vertebral artery originates directly from the aortic arch, a variant that occurs in 6% of individuals and accounts for 14% of all arch vessel anomalies.
Conventional film-screen or digital subtraction angiography is the traditional test used for subclavian steal diagnosis but is now reserved for the time of endovascular intervention. 
Arch aortography shows subclavian stenosis with antegrade vertebral flow in more minor subclavian stenosis.
In progressive subclavian steno-occlusion, the filling of the vertebral artery becomes less obvious, until eventually, it fills in a retrograde fashion on more delayed imaging (see the images below).
Evaluation should detail the presence of concomitant disease in carotid, cerebral, and vertebrobasilar systems (see the images below).
Misregistration and motion artifacts can compromise digital subtraction arteriography; if postprocessing does not overcome these problems, repeat contrast injection.
Spinal artery steal can cause vertebral artery flow reversal without subclavian steno-occlusive disease. Subclavian steno-occlusive disease distal to the vertebral artery origin or left subclavian steno-occlusive disease associated with left vertebral origin directly from the aorta cannot be associated with vertebral artery flow reversal.