An innovative method using a two-depth transorbital doppler (TDTD) of intracranial pressure quantitative absolute (ICP) value measurement relies on the same fundamental principle that is used to measure blood pressure with a sphygmomanometer. A sphygmomanometer works using a pressure balance principle - an air-filled pressure cuff wrapped around the arm compresses the brachial artery to a point where blood can no longer flow. Externally applied pressure is equal to systolic blood pressure in this case. The examiner slowly releases the air from the cuff and uses a stethoscope to listen for the return of blood flow. At the pressure balance point, where the pressure in the cuff equals systolic artery pressure, a ‘whooshing’ noise can be heard as blood flows through the artery again. Pressure balance based a non-invasive blood pressure meter does not need a patient-specific calibration.

The TDTD method uses Doppler ultrasound to translate the pressure balance principle of blood pressure measurement with a sphygmomanometer to the measurement of ICP. The ophthalmic artery (OA), a unique vessel with intracranial and extracranial segments, is used as a pressure sensor and as a natural pair of scales for absolute ICP value in mmHg or mmH2O measurement. Blood flow in the intracranial OA segment is affected by intracranial pressure, while flow in the extracranial (intraorbital) OA segment is influenced by the externally applied pressure (Pe) to the eyeball and orbital tissues.

As with a sphygmomanometer, a special pressure cuff is used - in this case, to compress the tissues surrounding the eyeball and also intraorbital tissues surrounding the extracranial segment of OA. External pressure changes the characteristics of blood flowing from inside the skull cavity into the eye socket. In place of the stethoscope, a Doppler ultrasound beam measures the blood flow pulsations in intracranial and extracranial segments of the ophthalmic artery. The non-invasive ICP meter based on this method gradually increases the pressure over the eyeball and intraorbital tissues so that the blood flow pulsation parameters in two sections of the OA are equal. At this pressure balance point, the applied external pressure (Pe) equals the intracranial pressure (ICP).

This measurement method eliminates the main limiting problem of all other non-successful approaches to non-invasive ICP measurement, primarily the individual patient calibration problem. Direct comparison of arterial blood pressure (ABP) and externally applied pressure is the basic arterial blood pressure measurement principle, which eliminates the need for individual calibration. The same calibration-free fundamental principle is used in the TDTD non-invasive ICP absolute value measurement method.

The mean value of OA blood flow, its systolic and diastolic values, pulsatility and other indexes are almost the same in both OA segments in the point of balance when ICP equals Pe. As a result of that, all individual influential factors (ABP, cerebrovascular auto-regulation impairment, individual pathophysiological state of patience, individual diameter, and anatomy of OA, hydrodynamic resistance of eyeball vessels, etc.) do not influence the balance of ICP equaling Pe and, as a consequence, such natural “scales” do not need calibration.

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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 transducer's 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 pathological masses on the path of the ultrasound wave, or by brain masses shifts.
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.
The method claims that ICP can be inferred from the thickness of the dura mater that is estimated from interference echoes of an 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. This method also needs a calibration to the individual patients.
Michaeli proposed that ICP should 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 because of the impossibility of the calibration to the individual patient.

Method and device for non-invasive (NI) ICP measurement according to inventions of David Michaeli MD, PhD, based to the TRA (tissue resonance analysis) have 2 options: (1) The qualitative method makes an evaluation of mild (10-20mm.Hg), moderate (20-40) and severe (above 40mmHg) ICP elevation. These method is using NI, long-term recording of ICP waves patterns, Like Lundsbergs ICP waves. (2) Quantitative measurement OF ICP WAVES with special ICP formulation in mm.Hg., Developed new device and method for calibration of ICP for each patient; see patent description.
More recently, multivariate methods have been proposed that derive ICP by c…

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The TCD measures the velocity of blood flow through the major intracranial vessels by emitting a high frequency (>2MHz) wave from an ultrasound probe and detecting a frequency shift between the incident and reflected wave which directly correlates with the speed of the blood (the so-called Doppler effect). The measurement is taken over the regions of the skull with thinner walls (temporal region, back of the head, or through the eye), as the bones strongly attenuate the transmission of the ultrasound at these frequencies. TCD is primarily a technique for diagnosing various intracranial vascular disorders such as emboli, stenosis, or vasospasm, and can be used to identify patients who are at risk of developing cerebral ischemia in early phases of traumatic brain injury or stroke.

ICP can be estimated from the TCD measurements because it impedes the blood flow and consequently decreases the velocity of blood flow. Besides the mean velocity, pulsatility index (which is the difference between peak systolic and end diastolic velocity, divided by mean flow velocity), a fraction of the cycle in systole and slopes of the TCD waveforms have been correlated with ICP. The estimates are however insufficiently accurate with the margin of error of ±10 - 15 mmHg.

Physiosonics, Inc. used transcranial Doppler ultrasound to measure ICP indirectly by assessing the elasticity of the biological material in a defined part of the brain. However, the elasticity in the brain is highly dependent on many other variable individual factors apart from ICP, including arterial blood pressure, state of cerebral blood flow auto-regulation, and the level of edema. Therefore, this approach would require calibration and expert positioning.