Ultrasound Time of the Flight Techniques
The majority of patented methods for noninvasive monitoring of ICP are based on an assumption that changes in ICP affect the physical dimensions and/or acoustic properties of the cranial vault or intracranial structures (dura, brain tissue, brain ventricles, and/or intracranial vessels). The common drawback of all these methods is that they measure only relative changes of ICP as referenced to a baseline measurement during which absolute ICP is known, i.e. the ultrasound readouts need to be calibrated on each subject against an invasive measurement. Ultrasound ‘time of the flight’ methods for non-invasive ICP monitoring have not been extensively validated and currently the majority of them do not seem to be accurate enough for a routine clinical use. Their original formulations usually do not specify locations for the transducers placement, and do not address how the intentional or accidental use of different locations and/or angles of the transducers will affect the reliability of ICP estimates. It has also remained unexplored how the measurements are affected by the presence of intracranial pathologic masses on the path of the ultrasound wave, or by brain masses shifts. 1
Dimensions of the cranium or its structures are determined with the ultrasound “time-of-the-flight” technique that measures the transit time of an ultrasound wave and its (potentially multiple) echoes on their path through the cranium and calculates the corresponding distance(s) using known ultrasound propagation velocities in different tissues (e.g. bone, brain, or fluid). Unfortunately, a reproducible quantitative relationship between the diameter of the cranium and ICP could not be established because ICP-induced changes in the cranium diameter are very small compared to the inter-individual variability of skull sizes, shapes and thicknesses 2.
Thickness of the dura
The method 3 claims that ICP can be inferred from the thickness of the dura mater that is estimated from interference echoes of ultrasonic wave. The utility of the method was successfully confirmed on four healthy subjects and four patients with intracranial hypertension, but larger validation studies have never been conducted as the method failed to attract enough interest among clinicians.
Michaeli 4 proposed that ICP be inferred from the magnitude and shape of pulsations of the third ventricle synchronous with the cardiac cycle or respiration, where the pulsations are measured along the propagation axis of an ultrasound wave. The method so far has not been independently validated, and the author provides no exact data from which one could estimate the accuracy of the method. However, the discussion in the body of the patent document suggests that the method is able to distinguish among three ranges of ICP (<20, 20-40 and >40mmHg) but cannot provide an exact value of ICP within the range.
Brain parenchyma tissue
More recently, multivariate methods have been proposed that derive ICP by combining the transit times with measured acoustic impedance, resonant frequency and ultrasound velocity 5, or with dispersion of the ultrasound wave on its way through the brain parenchyma 6.
Ultrasound ICP monitors based on the latter approach, which were developed at Vittamed Technologijos (Kaunas, Lithuania), have showed an impressive agreement with invasively measured ICP, with an average difference of only 2-3 mmHg in a small clinical population 7. However Vittamed Technologijos time-of-flight technologies were developed further for other applications of neuromonitoring technologies (including cerebral Autoregulation and Cerebral Compliance) but not for non-invasive ICP assesment.
1 Popovic et al. Noninvasive Monitoring of Intracranial Pressure, Recent Patents on Biomedical Engineering 2009, 2, 165-179
2 Petkus V, Ragauskas A, Jurkonis R. Investigation of intracranial media ultrasonic monitoring model. Ultrasonics 2002; 40: 829-833.
3 Kageyama, N., Kuchiwaki, H., Ito, J., Sakuma, N., Ogura, Y., Minimiyama, F.: US4971061 (1990).
4 Michaeli, D.: WO00068647 (2000).
5 Bridger et. al. US5919144 (1999).
6 Ragauskas A, A., Daubaris, G.: US5388583 (1995).
7 Ragauskas A, Daubaris G, Ragaisis V, Petkus V. Implementation of non-invasive brain physiological monitoring concepts. Med Eng Phys 2003; 25(8): 667- 678.