When I was introduced to cerebrovascular physiology a couple years ago, the first piece of equipment I’ve worked with was transcranial Doppler. After having learned the technique to search flow velocity in the middle cerebral artery, the following was pretty clear to me: 1) a good quality Doppler signal was critical for subsequent interpretation and, 2) you first use a hand-held probe to search for the artery of interest and then switch to a frame-held probe (see figure below) to monitor, usually for a continuous period of time (for example, continuous infusion of vasopressors or endurance exercise), blood flow velocity.
That frame-held probe is appealing since, among other things, you need to keep the same angle of insonation throughout the study protocol in order to have reliable blood flow velocity data (because there is no way to know that angle of insonation with transcranial Doppler) especially in situations where the head is moving during data acquisition.
So, in the beginning of my training, the utilization of a frame-held probe was the way to go in cerebrovascular research since I had only worked with healthy volunteers/patients without head injuries…but what about these individuals for whom these head frames are most likely not tolerated ? Could we use hand-held probes and still be confident in our data acquisition ? What about stability and reliability of hand-held probes?
The main objective of an interesting study recently published by Saeed et al. was to compare cerebral blood flow velocity and cerebral autoregulation using a hand-held vs. frame-held probe.
Cerebral blood flow velocity (in the middle cerebral artery) was thus monitored with a frame-held probe or held in position by one investigator for 5 minutes (for each method) in 11 heatlhy volunteer. Hemodynamics was also monitored and cerebral autoregulation calculated for each period. These measurements were repeating in a subsequent visit. A total of 3 frame-held and 2 hand-held measurements were performed on both left and right sides at each visit.
The main findings of this study were:
1) no significant differences in cerebral blood flow velocity and cerebral autoregulation between the methods;
2) no significant difference between repeated measures on separate days for both cerebral blood flow velocity and cerebral autoregulation.
As highlighted by the authors:
(…) this is the first study to report that HH measurements of CBFv and ARI estimates are not statistically different from FH, which has important implications for bedside estimates of CBFv and ARI, particularly in traumatic brain injury (TBI) patients and neonates, where CBFv and ARI measurements form an important part of clinical assessment.
(HH: Hand-held; CBFv: cerebral blood flow velocity; ARI: cerebral autoregulation; FH: frame-held)
These results are very interesting and promising if one wants to investigate cerebral blood flow velocity with transcranial Doppler in patients who cannot tolerate a head frame. However, we have to note that a short period of recording was investigated in this study. It will be important to replicate this protocol with longer recording periods.
Saeed NP, Panerai RB, Robinson TG. Are hand-held TCD measurements acceptable for estimates of CBFv? Ultrasound Med Biol 2012 38(10):1839-44