Transcranial Doppler: An Introduction for Primary Care Physicians

Abstract

Christian Johann Doppler, a mathematician, physician, and astronomer, first described the Doppler principle in 1843. Its use in neurology was first reported in 1982, 1 and since then, its convenience and growing range of uses have made it an attractive tool to evaluate the cerebrovascular tree in patients with neurovascular disorders. Transcranial Doppler (TCD) is a noninvasive ultrasound device that allows real-time evaluation of the intracranial cerebral circulation. It is currently under-utilized in part due to the lack of awareness about the diagnostic usefulness of this test. In this article, we will review TCD testing as a procedure and discuss its clinical applications of special interest to the primary care physician. Examination and Doppler InstrumentUltrasound examination of a vessel by means of TCD is referred to as insonation. The TCD probe is placed over different "acoustic windows" that are specific areas of skull where there is a lack of boney covering or the cranial bone is thin (Figure 1). The transtemporal (temporal) window is used to insonate the middle cerebral artery (MCA), the anterior cerebral artery (ACA), the posterior cerebral artery (PCA), and the terminal portion of the internal carotid artery (TICA), before its bifurcation. The transorbital (orbital) window gives access to insonate the ophthalmic artery (OA) as well as the internal carotid artery at the siphon level. The transforaminal (occipital) window allows insonation of the distal vertebral arteries (VA) and the basilar artery (BA). Finally, the submandibular window allows insonation of the more distal portions of the extracranial internal carotid artery. TCD does not allow direct visualization of insonated vessels like carotid Doplex, rather it is an indirect evaluation by means of an ultrasonic beam of 2 MHz frequency that is produced from piezoelectric crystals that have been stimulated electrically. This beam bounces off the erythrocytes within the insonated artery. The reflected signal is received by the transducer and converted to an electric signal (Figure 2). This information is subtracted from the transmitted signal and then processed to obtain a waveform that allows accurate determination of blood flow velocities, direction of flow, and also allows for certain calculated parameters to be added to the evaluation. The pulsatility index (PI) is one of the useful calculated parameters, and it is considered a reliable marker of resistance distal to the insonated site. It is usually calculated by the Gosling equation. 2 PI ϭ (Peak systolic velocity Ϫ end diastolic velocity)/mean velocity.

The value of transcranial Doppler imaging in children with tuberculous meningitis

“…It can be repeated multiple times and used for serial monitoring. TCDI may provide accurate information on intracranial pressure (ICP) and cerebral blood flow velocities (BFV) which may help guide therapy in critically ill patients [1]. This applies especially to resource-poor countries where expensive monitoring equipment is often not available.…”

“…Transcranial Doppler imaging (TCDI) is a safe, portable, noninvasive, inexpensive and non-ionizing method of assessing cerebral hydrodynamics and haemodynamics [1]. It can be repeated multiple times and used for serial monitoring.…”

Transcranial Doppler measurements in migraine and nitroglycerin headache

“…The lack of significant FV changes during the headache phase of migraine as explored by TCD has been explained by several theories including the various technical limitations of TCD [16]. TCD is a blind procedure and due to that, the angle of insonation is not always optimal, it is operator-dependent and it measures only a short span of cardiac cycles.…”

“…value that has been found to reflect the distensibility of the insonated vessel, and to be indicative of peripheral vascular resistance [14][15][16]. There is a paucity of published data on the PI in migraine.…”

Effects of metformin on middle cerebral artery flow velocity in newly diagnosed type 2 Diabetic patients

“…Transcranial Doppler is useful tool for detection of cerebral vasculopathy. It can detect systolic and diastolic velocities, resistive index and pulsatility index [8][9][10]. Diastolic flow in cerebral arteries is important because the perfusion of tissues occurs during diastole, therefore it is important to maintain adequate diastolic flow to keep adequate cerebral perfusion and overcome any changes in general circulation [11].…”