Abstract:
A method and apparatus for obtaining the absolute value of intracranial pressure in a non-invasive manner is described by using an ultrasonic Doppler measuring device which detects the intracranial and extracranial blood flow velocities of the intracranial and extracranial segments of the ophthalmic artery. The eye in which the blood flow is monitored is subjected to an external pressure, sufficient to equalize the intracranial and extracranial angle-independent blood flow factors calculated from the intracranial velocity signal and extracranial velocity signal. The absolute value of the intracranial pressure is identified as that external pressure at which such equalization occur

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a method and apparatus for ultrasonically determining the absolute value of intracranial pressure and more specifically relates to a method and apparatus for determining the intracranial pressure using ultrasonic measurements of the velocity of blood flow through an ophthalmic artery. 
     BACKGROUND OF THE INVENTION 
     This invention is an extension and improvement of our previously invented method and apparatus U.S. Pat. No. 5,951,477 for single or single repeatable absolute intracranial pressure (ICP) value measurement and diagnosing of brain pathologies based on such measurements. This document is incorporated by reference in the present application. 
     An apparatus for determining the pressure and flow inside the ophthalmic artery is described in U.S. Pat. No. 4,907,595 to Strauss. The apparatus uses a rigid chamber that can be affixed and sealed over the human eye so that it can be pressurized to apply an external pressure against the eyeball. An ultrasonic transducer is also mounted to the chamber and oriented to transmit ultrasonic pulses for a Doppler type measurement of the flow inside the ophthalmic artery (OA). The apparatus operates by enabling an operator to increase the pressure to such a level that the blood flow through the OA ceases. The pressure at which this occurs is then an indication of the pressure inside the OA. Typically, the pressure at which this event occurs is in the range of about 170 mmHg. 
     A problem associated with an apparatus as described in the &#39;595 Patent is that the pressure necessary to obtain the desired measurement is so high that it generally exceeds maximum recommended pressures by a significant amount. When such device is then used for an extended time, tissue damage can arise and may result in an increase in the intracranial pressure, ICP, to unacceptable levels. 
     Another ultrasonic device for determining changes in intracranial pressure in a patient&#39;s skull is described in U.S. Pat. No. 5,117,835 to Mick. Such device involves placing a pair of ultrasonic transducers against the skull and storing received vibration signals. U.S. Pat. No. 4,984,567 to Kageyama et al. describes an apparatus for measuring ICP with an ultrasonic transducer by analyzing the acoustic reflections caused by ultrasonic pulses. Other patents related to ultrasonic measuring of either intracranial pressure or other physiological features are U.S. Pat. No. 4,204,547 to Allocca, U.S. Pat. No. 4,930,513 to Mayo et al., U.S. Pat. No. 5,016,641 to Schwartz, and U.S. Pat. No. 5,040,540 to Sackner. 
     None of these prior art teachings provide a clear description for obtaining a non-equivocal indication of the absolute value of intracranial pressure (aICP). The measurements tend to be obscured by noise arising from uncertainties in the measurements and by numerous influential factors, such as arterial blood pressure, cerebrovascular autoregulation state, individuality of anatomy, and patient&#39;s physiology and pathophysiology. Such influential factors cannot be eliminated by calibration of the “individual patient—non-invasive ICP meter” system because the non-invasive “golden standard” absolute ICP meter does not exist. Thus, there is a need for the capability to derive a measurement of a person&#39;s aICP in a safe, accurate and non-invasive manner that can be implemented with reasonable reliability and without the necessity for calibration. 
     SUMMARY OF THE INVENTION 
     With an apparatus in accordance with the invention, one can derive an indication of the absolute value of pressure inside a skull (intracranial pressure or ICP) in a non-invasive manner. This indication is obtained using an ultrasonic Doppler measuring technique that is applied through the eye of a person and to the ophthalmic artery (OA) in a safe manner. 
     This is achieved in accordance with one technique in accordance with the invention, by pressurizing a chamber which is in sealing engagement with a perimeter around an eye, and by using an ultrasonic Doppler measuring device, which is mounted to the chamber, to measure the intracranial and extracranial blood velocities (VI and VE, respectively) of intracranial and extracranial segments of the ophthalmic artery. Velocity parameters representative of or derived from these velocity measurements, VI and VE, are then compared, and the difference between these representative parameters, their difference, ΔV, is identified. ΔV is then used to control the pressure in the chamber. When the pressure in the chamber causes ΔV to approach a desired minimum value close to zero, that pressure becomes an indication of the non-invasively derived intracranial pressure (nICP). 
     The technique of the invention can be implemented in a variety of different manners, such as with a manual increase and control over the pressure to be applied to the chamber while monitoring the parameters representative of intracranial and extracranial velocity signals determined with the ultrasonic Doppler device. When these representative parameters appear substantially the same, the applied pressure at which this occurs is then used to determine the intracranial pressure. 
     Alternatively, with the ultrasonic Doppler velocity measuring technique of this invention, the ophthalmic artery velocity difference measurement, ΔV, can be used to directly control the pressure in the chamber by applying the signal to a pump. A pressure signal indicative of the pressure in the chamber can be used to store a signal in suitable memory and for display to indicate the nICP. 
     A further aspect of the invention enables a measurement of the dynamic characteristics of blood flow velocity in the intracranial and extracranial OA segments of which pulsatility is an example but not an exclusive embodiment. 
     It is, therefore, an object of the invention to provide an apparatus for determining the absolute value intracranial pressure (aICP) using a non-invasive ultrasonic technique (nICP). The aICP value (in mmHg or other pressure units) only can be used for traumatic brain injury or other brain pathology treatment decision making. It was impossible to measure the aICP non-invasively until now. 
     It is still a further object of the invention to obtain a measurement of the ICP of a patient in a safe and dependable manner. 
     Another advantage of the invention is the possibility to measure nICP absolute values in the injured and healthy hemispheres of the brain separately using the ophthalmic arteries of both eyes of the patient. 
     Also, another advantage of the invention is the independence of measurement results from many influential factors such as arterial blood pressure, diameter of the OA, cerebrovascular autoregulation state, and hydrodynamic resistances of the ocular and other distant vessels. The invention achieves this advantage by not using the measured absolute values of blood flow velocities in the intracranial and extracranial OA segments (IOA and EOA respectively). Instead, it uses just the comparison of such velocities or associated pulsatility indices or other parameters of dynamic blood flow to find the “balance point”—the point at which a summary blood flow parameter describing IOA hemodynamics is equal to the summary blood flow parameter describing EOA hemodynamics. It is at the balance point that ICP is equal to the extracranially applied pressure inside the pressure chamber. Such comparison is both accurate and not in need of an independent calibration. 
     A further advantage of the invention is the ability to make non-invasive absolute ICP value measurements without the necessity to calibrate the “individual patient—non-invasive ICP meter” system. The calibration problem is solved when the proposed method uses the balance of two pressures: ICP and extracranially applied pressure to the human eye and intraorbital tissues. Intracranial and extracranial segments of OA are used as natural “scales” for ICP and extracranial pressure balancing. 
     A still further advantage of the proposed invention is the high accuracy of non-invasive absolute ICP measurement which is acceptable for clinical practice. 
     These and other advantages and objects of the invention can be understood from the following description of several embodiments in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic and block diagram illustrating an apparatus with a one-dimensional (1D) scanning ultrasonic transducer array in accordance with the invention. 
         FIG. 1B-1C  are schematic views illustrating cases when a concave two-dimensional (2D) ultrasonic transducer array is used. 
         FIG. 2  is a detailed schematic and block diagram view of an apparatus in accordance with the invention. 
         FIG. 3  is a plot illustrating the spatial scan of blood flow velocity in the OA by using an ultrasonic transducer array. 
         FIG. 4A-4C  are perspective views illustrating the mounting of a head frame of the apparatus, in accordance with the invention, to the skull of a patient. 
         FIG. 5  is a flow chart for a method of using the apparatus shown in  FIG. 1A-1C  for measuring the absolute value of intracranial pressure. 
         FIG. 6-13  are plots illustrating the steps of adjusting the ultrasonic transducer in order to get Doppler velocity signals from the IOA and EOA simultaneously. The device of the invention is working in scan mode for these tasks. 
         FIG. 14  is a view of the software window that shows the procedures of averaging the spectrogram across multiple heart cycles from the IOA and EOA simultaneously. The device of the invention is working in spectral measurement mode for this task. 
         FIG. 15  is a view of the software window that shows the procedures for the calculation of aICP according to the measured data and to the method of the invention. 
         FIG. 16  is a plot illustrating the results of serial non-invasive aICP measurements of the same healthy volunteer obtained with an apparatus in accordance with the invention. 
         FIG. 17  shows the comparison of non-invasive aICP measurement results obtained with a phase contrast MRI apparatus (See Alperin et al., MRI study of cerebral blood flow and CSF flow dynamics in an upright posture: the effect of posture on the intracranial compliance and pressure,  Acta Neurochirurgica Supplementum  2005; 95: 177-181; Alperin et al., Relationship between total cerebral blood flow and ICP measured noninvasively with dynamic MRI technique in healthy subjects,  Acta Neurochirurgica Supplementum  2005; 95: 191-193) and with apparatus in accordance with the invention. These results also are listed in Table 1 below. 
         FIG. 18A-18B  show measured typical dependence of the pulsatility index in OA on the measurement depth with an apparatus in accordance with the invention, without and with, respectively, an external pressure applied to the eye. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With an apparatus in accordance with the invention, the ICP inside a person&#39;s head can be determined from an observation of the blood velocities inside the OA. This involves an ultrasonic apparatus which senses the response of the blood flow to a pressure “challenge” applied to the tissues around the eye. The pressure challenge is accomplished by a pneumatic or fluid-control device, which can apply a slight pressure to the eye. The pressure is applied to the eye to the necessary level for equilibrating parameters representative of the intracranial and extracranial blood flows in the OA leading to the eye. The possibility of this type of measurement has been demonstrated with the analysis presented in our previous patent U.S. Pat. No. 5,951,477. 
     With reference to  FIGS. 1 ,  2 , and  3 , an apparatus  20  is shown to practice the measurement of the ICP as described above. The head frame  22  of the apparatus is mountable to the head of a person so that an eye engaging inflatable device  28  can apply a slight pressure against the eyelid  23  ( FIGS. 4A-4C ). Suitable braces and positioning bands  24 ,  26  are used to hold the head frame  22  in place ( FIGS. 4A-4C ). The inflatable device is formed of a suitable soft material such as rubber to form an inflatable chamber  28  ( FIGS. 4A and 4B ). Chamber  28  is approximately annular in shape so as to enable an ultrasonic transducer  30  to be mounted against an inner flexible membrane  32  and enable a pressurization of the chamber by a pump  34  ( FIGS. 1A-1C ).  FIG. 1A  shows ultrasonic transducer array  30  which can be a 1D transducer array and can scan electronically the OA in one plane. In  FIGS. 1B and 1C , a 2D ultrasonic transducer array  30  is able to perform OA spatial scan in multiple planes. These planes may be parallel or perpendicular to the “boresite” ultrasound beam axis. The distances from the ultrasonic transducer to the internal carotid artery, IOA, and EOA are marked by D, D I  and D E  respectively ( FIG. 1A-1C ). 
     The inner flexible membrane  32  conforms to the shape of the eye  35  as illustrated in  FIG. 1A  and in such manner as to enable the pressure from the inflation of chamber  28  to provide a slight pressurization of the tissues around the eye. These tissues are contiguous with the tissues in the posterior portion of the eye socket, so the applied pressure is effective there as well. This results in a pressurization of the extracranial ophthalmic artery  36 . The OA originates from the siphon of the internal carotid artery ICA  41  inside the cranium  40  and passes through the optic nerve canal  42  to the eye  35  ( FIGS. 1A-1C ). 
     The preferred embodiment of this invention is shown in  FIG. 1  and  FIG. 2 , and is comprised of an apparatus  20  consisting of: an orbital Doppler velocity meter  1 , a pulse wave spectrogram processing unit  2 , a pressure control unit  3 , and an absolute ICP calculation unit  4 . 
     The orbital Doppler velocity meter  1  controls ultrasonic transducer  30  which can be a 1D transducer array ( FIG. 1A ) or a 2D transducer array ( FIGS. 1B and 1C ). This meter has the ability to steer the ultrasound beam relative to the transducer bore site axis. This steering is done electronically within angle ranges from 0 to 8 degrees in one plane for the 1D array embodiment ( FIG. 1A ) and within a solid angle about the transducer bore site for the 2 D array ( FIGS. 1B and 1C ). The orbital Doppler velocity meter  1  can work in two modes:
         Scan mode (adjustment) is used to search for Doppler velocity signals from IOA and EOA. In this mode, the scan to locate blood flow in the vicinity of the optical canal is done by pointing the ultrasound beam in a series of different directions and sampling the Doppler signal acquired at multiple depths along the beam (each such direction is called a “look” or “look direction”). The look direction is adjusted electronically.   Spectral measurement mode is used when Doppler velocity signals from IOA and EOA have been found and the beam directions (“looks”) and depths associated with, respectively, the intracranial and extracranial ophthalmic arteries, are known. In this mode, the transducers are operated in an alternating (pulse by pulse) fashion. This operation is referred to as “multiplexing” the two beam directions and is done in order to receive Doppler velocity signals from the IOA and EOA separately. These signals are then demodulated and used to derive spectrograms characterizing blood flow at the locations of the IOA and EOA.       

     The orbital Doppler velocity meter  1  consists of: transmitter  1 . 1 , receiver  1 . 3 , beam forming circuit  1 . 4 , digital signal processing DSP N-channels  1 . 5 , and units for data processing in scan mode  1 . 6  and spectral mode  1 . 7  ( FIG. 2 ). The beam forming circuit  1 . 4  can be applied for 1D or 2D scan respectively for cases in  FIG. 1A  and  FIG. 1B . 
     The transmitter  1 . 1  generates electrical signals to excite ultrasonic transducer array  30 , which can be a 1D or 2D transducer array. Each electrical signal is delayed in beam forming unit  1 . 4  in order to steer the diagram of ultrasonic transducer at required directions (for 1D or 2D scan). The steering angle is set from steering vectors  1 . 62  when apparatus  20  is working in scan mode or from steering vectors  1 . 72  when apparatus  20  is working in spectral mode. 
     The Receiver  1 . 3  is put in a low-gain state during transmission of an ultrasonic pulse, and then into a high gain state while listening for echoes. The received signals from each element of ultrasonic transducer array  30  are processed in an N-channel DSP unit  1 . 5 . The number N of DSP channels is equal to the number of elements in the ultrasonic transducer array  30 . In DSP channels  1 . 5 , the received signal is sampled in digitization unit  1 . 52 , and demodulated in demodulation unit  1 . 53  to get a demodulated digital Doppler signal. After demodulation, the signal is decimated with decimation unit  1 . 54  and filtered with clutter filter  1 . 55 . One skilled in the art will appreciate that digitization, demodulation and decimation are applied to echo data in the “RF” domain, typically across one pulse period, while clutter rejection is applied in the “baseband” domain, across multiple pulse periods. Further, clutter rejection can be applied before or after beam forming, if both are linear processes. 
     When orbital Doppler velocity meter  1  is working in scan mode, the demodulated and filtered Doppler signal is directed with mode selector  1 . 56  into FFT unit  1 . 57  to calculate the spectrum of this signal. In the next steps, this signal is processed in scan mode processing unit  1 . 6  to reconstruct a spatial image of the Doppler signal intensity distribution in a spatial 3D rendering. In this rendering, the Doppler signal intensity is colored according to signal intensity and plotted based on spatial position. In one embodiment the X-axis is transducer steering angle in degrees while the Y-axis is depth in mm. The color in the image reflects the Doppler signal intensity of blood flow in the eye artery and the spatial location of this artery. 
     The scan mode processing unit  1 . 6  consists of: beam forming unit  1 . 61 , steering vectors  1 . 62 , power meter  1 . 63 , Colormap unit  1 . 64 , gain and range control unit  1 . 65 , scan mode image  1 . 66 , and cursor former unit  1 . 67 . With cursor former  1 . 67 , the operator (or the system in an automatic detection mode) is enabled to select and fix two spatial points in the display of the spatial Doppler signal intensity versus spatial position. By placing cursors at the points where Doppler velocity signals indicate blood flow in IOA and EOA, the transducer steering parameters (angle and depth) will be fixed to get Doppler signals only from those selected segments when apparatus  20  is switched in spectral mode. The fixed transducer steering parameters (angle and depth) are then converted into steering vectors  1 . 72 . 
     When the orbital Doppler velocity meter  1  is working in spectral mode, the transducer steering vectors  1 . 72  are utilized in a “multiplexed operation”—pulses aimed at the selected segment of the IOA are alternated on a pulse-by-pulse basis with pulses aimed at the selected segment of the EOA. In this mode, the demodulated and filtered Doppler signals from the DSP channels  1 . 5  are directed with mode selector  1 . 56  into spectral mode processing unit  1 . 7 . This unit processes only two Doppler signals from the IOA and EOA in order to get velocity spectrogram image  1 . 77 . The spectral mode processing unit  1 . 7  consists of: beam forming unit  1 . 71 , steering vectors  1 . 72 , FFT calculation unit  1 . 73 , amplitude meter  1 . 74 , Colormap unit  1 . 75 , gain and range control unit  1 . 76 , and velocity spectrogram image unit  1 . 77 . 
       FIG. 3  illustrates two embodiments for visualization of blood flow signals in the OA with the orbital Doppler velocity meter operating in scan mode. In one embodiment, the ultrasonic transducer  30 , which can be a 1D or 2D transducer, performs a scan inclusive of the optical canal, based acquiring Doppler data from a series of different directions (“looks”) and depth ranges according to steering vectors  1 . 62 , in order to get a spatial image depicting location of blood flow. Received information regarding the spatial distribution of blood flow can be represented in color maps at different depth frames ( FIG. 3   a ). Each depth frame represents a different distance from the ultrasonic transducer (depth); the color intensity in the depth frame reflects the blood flow signals while the coordinates of the colored spot in the depth frame reflects the transducer steering angle at which the blood flow in the eye artery is detected ( FIGS. 3   a  and  3   b ). The example in  FIG. 3   b  shows that at fixed depth in frame  7  blood flow is detected in the EOA. 
     A second embodiment may be used in what may be a simpler technique for simultaneous visualization of blood flow velocity in the IOA and EOA. In this second embodiment, the spatial planes of scanning are made based on rotation of a planar scanning region about the bore site axis of the transducer ( FIG. 3   c ). The rotational method of acquiring the set of scan planes can be implemented by using a 2D transducer array with electronic steering across two spatial angles, or can be implemented with a linear array capable of one scan plane, combined with mechanical rotation of that array. Either method is capable of accomplishing the scan depicted in  FIG. 3   c . Whichever method is used, the rotation of scanning planes is performed until blood flow velocity signal is detected from both IOA and EOA simultaneously from different depths ( FIG. 3   d ). Note that this technique may be facilitated by first locating the internal carotid artery and manipulating the transducer until the associated flow signal is on the bore site axis, and then performing the rotational scan. The visual information regarding blood flow spatial distribution may be plotted in a spatial 3D image, or a series of 2D images, but is not restricted to these approaches. 
     In the eye, blood flow velocities are typically low and difficult to characterize because of poor signal-to-noise ratio (SNR). This is further complicated because ultrasonic Doppler devices as a rule must use very low power in the eye, which contributes to the low SNR. To overcome this disadvantage, the present invention provides significant improvement in SNR of the ophthalmic artery signal by averaging multiple heart cycles after cross-correlation (time) alignment of the set of spectrograms representative of the multiple heart cycles. 
     The pulse wave spectrogram processing unit  2  performs calculation of a coherently averaged full heart cycle blood flow velocity spectrogram and maximum velocity envelope from the set of spectrograms representative of the multiple heart cycles ( FIG. 2 ). After segmentation of the velocity spectrogram derived from a particular vessel location into separate spectrograms, one for each heart cycle, and synchronization of the heart cycles in these separate spectrograms via maximum-correlation of the Doppler shift signals, the coherent averaging in this step can be applied to obtain the maximum velocity envelope by either taking an average of the synchronized envelopes of the individual heart cycle spectrograms, or the envelope of the average of the synchronized spectrograms. This technique may be included to accomplish significant improvement in the accuracy and resolution of the blood flow maximum velocity envelope. 
     In order to apply an external pressure on the eyelid, the pressure control unit  3  drives pump  34  and reads data from digital manometer  90  ( FIGS. 1A-1C  and  2 ). The absolute ICP calculation unit  4  performs the processing of all measurement data in order to determine the aICP. 
     The ultrasonic transducer  30  can be a 1D or 2D array transducer from which an ultrasonic beam can be electronically steered in order to enable the system to direct its ultrasonic acoustic pulses concurrently at both intracranial and extracranial segments  46 ,  48  of the ophthalmic artery  36 . Whichever type of transducer, it is helpful that the transducer&#39;s central axis, or “bore site”,  44 , is first aligned to the optical canal and directed to view the IOA  46  and internal carotid artery (ICA)  41  ( FIG. 1A ). This alignment is accomplished by positioning an ultrasonic transducer on the eyelid according to known a priori information about human skull geometry. The EOA is then found by electronically adjusting the angle of the transducer scan plane. This results in the ability to electronically steer the transducer beam so as to direct its ultrasonic pulses at the intracranial and extracranial segments  46 ,  48  of the ophthalmic artery  36  ( FIGS. 1A-1C ). Concurrent observation of blood flow in the intracranial and extracranial ophthalmic arteries is thereby accomplished. The signal location technique described in this paragraph is part of the preferred embodiment for this invention, but one skilled in the art will appreciate that this is one technique to improve signal quality and ease of acquisition, and that the underlying invention is not restricted to its inclusion. 
     In the operation of apparatus  20 , it is desirable that an initial alignment mode be undertaken to assure that the transmitter pulses from the transducer  30  are properly directed at both the intracranial and extracranial segments  46 ,  48  of the ophthalmic artery  36  ( FIGS. 1A-1C ). This involves adjustments in the angle phi between the bore site axis  44  of the ultrasonic transducer  30  and the alignment axis  96  of the ophthalmic artery passage  42  ( FIG. 1A ). Such adjustment can be done with the alignment screws  98 . 1 ,  98 . 2  and  98 . 3  or with such other suitable frame affixed between the band  26  and the transducer  30  in  FIG. 4 . 
     As described above, one advantage of the present invention is the independence of measurement results from many influential factors such as arterial blood pressure, diameter of the OA, cerebrovascular autoregulation state, and hydrodynamic resistances of the ocular and other distant vessels. A unique and critical advantage of the invention is the ability to make non-invasive absolute ICP value measurements without the necessity to calibrate the non-invasive ICP meter system. The invention achieves these advantages by not using the measured absolute values of blood flow velocities in the intracranial and extracranial OA segments (IOA and EOA respectively). Instead, it uses just the comparison of such velocities or associated pulsatility indices or other parameters representive of blood flow dynamics, to find the “balance point”—the point at which a summary blood flow parameter describing IOA hemodynamics is equal to the summary blood flow parameter describing EOA hemodynamics. At the balance point, the ICP is determined and is equal to the extracranially applied pressure inside the pressure chamber. Such comparison is accurate and independent of the influential factors noted above since it is always find this balance point regardless of these factors. 
     A necessary property of the parameters representative of blood flow dynamics above is that they are independent of different angles at which Doppler blood flow velocities are measured in the IOA and EOA. Therefore when the blood flow pulsation parameters are measured, angle-independent blood flow factors are calculated. In one embodiment, these blood flow pulsation parameters are peak systolic velocity (VS) and end diastolic velocity (VD). Other measurement points of the blood flow envelope within one heart cycle may be used to calculate an angle-independent blood flow factor. The angle-independent blood flow factor in one embodiment is the pulsatility index, which is calculated for measurements in IOA and EOA:
 
 PI   IOA =2*( VS   IOA   −VD   IOA )/( VS   IOA   +VD   IOA ),
 
 PI   EOA =2*( VS   EOA   −VD   EOA )/( VS   EOA   +VD   EOA ).
 
     One skilled in the art will appreciate that any other index of blood flow velocity pulsation which is not influenced by the OA insonation angle can also be used (e.g., resistivity index, any non-standard index which uses more than two measurement points of the blood flow envelope within one heart pulse, etc.). 
     The “balance point” noted above, at which parameters representative of blood flow are equal in the EOA and the IOA, is accomplished when:
 
PI IOA =PI EOA ,
 
or
 
 PI   IOA   /PI   EOA =1.
 
     Pulsatility index is a highly vulnerable metric in that it takes two points out of an entire cardiac cycle of information—velocity envelope values at peak systole and diastole—and constructs an index. Using averaged heart cycle blood flow velocity spectrograms (as described above) greatly reduced the uncertainty associated with each of these two points. Due to the improvement in accuracy and precision of the envelope function from using the averaged heart cycle spectrograms, the calculation of the pulsatility index as used in the ICP determination is in turn of higher accuracy and precision. 
     The flow chart of apparatus  20  with reference to  FIGS. 1 ,  2 ,  3  and  4  is shown in  FIG. 5 . 
     The steps to measure non-invasive intracranial pressure (nICP) are now enumerated. There are two primary aspects to this measurement: scan mode and spectral mode. Scan mode is comprised of steps # 1 - 6  below, and spectral mode is comprised of the remaining steps. 
     Step # 1 : Software initialization of scan mode. This mode allows for the operator to align the ultrasonic transducer in the following sequence. 
     Step # 2 : Head frame with ultrasonic transducer is placed on patient and acoustic contact between ultrasonic transducer and eyelid is established with coupling gel or acoustically similar material. 
     Step # 3 : Transducer is fixed in the head frame according to a priori known angles and positions that align the transducer central axis to the optical canal. This alignment is most successful when the blood flow signal can be observed in the internal carotid artery, ICA. The distance between the ultrasonic transducer to ICA is a priori known to be in range of depth from 65-75 mm ( FIG. 6 ). 
     Step # 4 : The steering of the ultrasonic transducer is manipulated in order to visualize the blood flow signal from the IOA. The depth of the IOA signal is between 5 and 6 mm less than the distance from transducer to the ICA ( FIG. 6 ). 
     Step # 5 : For a 1D transducer, it is rotated around its axis until the signal from EOA appears. The depth of the EOA is approximately 5 to 7 mm less than the distance from transducer to the IOA. Both signals from the intracranial and extracranial segments of ophthalmic artery (IOA, EOA) must be clearly seen in the software window while in scan mode ( FIGS. 7-13 ). If the 2D array embodiment is utilized, then the manual steering described above can be accomplished electronically. 
     Figures  FIG. 6-13  show the software windows when the apparatus is working in scan mode (also known as adjustment mode) and the transducer is rotated around its axis:
         In  FIG. 6 , the transducer rotation angle is 0 degrees at which the velocity Doppler signal from the ICA is seen at depth ˜72 mm and the signal from the IOA is seen at depth ˜50-60 mm, as shown in the left image.   In  FIGS. 7-8 , the transducer rotation angles are 20 and 40 degrees, respectively, at which the velocity Doppler signals are seen simultaneously from the IOA and EOA in the left image: IOA (depth ˜54 mm) and EOA (depth ˜46 mm).       

     In  FIGS. 9-10 , the transducer rotation angles are 60 and 120 degrees, respectively, at which the only velocity Doppler signals seen are from the IOA (depth ˜54 mm). The signal from the EOA is weak and unsuitable for measurement. 
     In  FIGS. 11-12 , the transducer rotation angles are 210 and 230 degrees, respectively, at which the velocity Doppler signals are seen again from the IOA and EOA in left the image: IOA (depth ˜54-60 mm) and EOA (˜45-48 mm). 
     In  FIG. 13 , the transducer rotation angle is 260 degrees at which the velocity Doppler signals are seen only from the IOA (depth ˜54 mm). The signal from the EOA is again weak and unsuitable for measurement. 
     In scan mode  FIGS. 6-13 , the Doppler echo signals shown are obtained by systematically steering the ultrasound beam in a B-mode style planar region. Note that in the images on the left, the Y-axis is depth in mm, and the X-axis is the transducer steering angle in degrees. The spectral mode signals are shown in the images on the right; the top and bottom right images are Doppler velocity signals from locations designated by markers  2  and  1  respectively (in m/s). The markers are located in the scan mode image on the left. 
     In  FIGS. 6-13 , it is shown that by turning the ultrasonic transducer around its axis, we always obtain a signal from the IOA. This means that an a priori angle and position of the transducer&#39;s positioning is set properly and confirmed empirically (i.e., the transducer central axis is aligned to the optical nerve canal). The signal from the EOA appears only at angles 20-40 degrees and 210-230 degrees. This is consistent with the fact that rotation of the scan plane by 180 degrees will produce the same scan plane. 
     Step # 6 : The angles and depths at which selections are made for sampling velocity Doppler signals (spectral mode on the right side of these images) are fixed by manually placing markers in the software window—by pointing and clicking the mouse—when the apparatus is working in scan mode (adjustment). 
     After the transducer is positioned to obtain velocity Doppler signals from two different depths and directions (“looks”), the apparatus is put in measurement mode, also referred to as “spectral mode”, in which the transducer is working by alternating its pulsing activity on a pulse-by-pulse basis between two fixed angular steering directions. In the next series of steps # 7 - 14  are the procedures for measuring absolute value of intracranial pressure. 
     Step # 7 : A known external pressure on eyelid is applied by inflating pressure chamber  28  by pump  34  ( FIGS. 1A-1C  and  2 ). Using manometer  90 , the pressure within chamber  28  is measured and used by the apparatus software to control pump  34 . The measured pressure value is transferred from manometer  90  and stored for each measurement cycle (pressure is varied across measurement cycles). 
     Step # 8 : When required pressure is set and stabilized, the software makes Doppler spectral measurements in which the velocity signals are collected and analyzed from the IOA and EOA segment locations. 
     Step # 9 : Doppler velocity signals measured in the IOA and EOA segments are demodulated and used to form a spectrogram representative of blood flow at each location. 
     Step # 10 : Spectrograms of velocity signals at the IOA and EOA locations are parsed into separate heart cycles, which are synchronized and coherently averaged to form a separate IOA composite heart cycle spectrogram and an EOA composite heart cycle spectrogram ( FIG. 14 ). 
     Step # 11 : The peak velocity envelopes for IOA and EOA composite heart cycle spectrograms are calculated ( FIG. 14 ). 
     Step # 12 : The parameters representative of velocity signals in the composite heart cycle spectrograms of the IOA and EOA (VS for peak systolic velocity and VD for end diastolic velocity) are calculated from maximum flow velocity envelopes derived from these composite spectrograms. 
     Step # 13 : Angle independent factors such as pulsatility indexes are calculated from measured velocity signals separately for IOA and EOA composite spectrograms. 
     Step # 14 : The algorithm now repeats measurements of angle independent factors at different pressures applied to the eye by performing steps # 7 - 13  for each different externally applied pressure. The externally applied pressure varies by adjusting the inflation pressure of the chamber placed adjacent to the eye. The external pressure is changed within desired range by increasing it, for example, from 0 mmHg to 30 mmHg in increments such as 5 mmHg. At each fixed pressure, the measured velocity parameters in the IOA and EOA are stored for further processing. 
     Step # 15 : When the measurement of velocity parameters in the IOA and EOA is completed, the calculation of aICP is performed. The ICP is the pressure that achieves the “balanced point” where the calculated parameter representative of IOA blood flow is equal to the calculated parameter representative of EOA blood flow ( FIG. 15 ). 
     The result of non-invasive absolute ICP value measurements with an apparatus in accordance with the invention is shown in  FIG. 16 . Serial non-invasive absolute ICP value measurements have been performed on the same healthy volunteer. The measurements are conducted with 15 minute breaks between two consequent measurements. The conclusion is that the standard deviation (SD=1.7 mmHg) is very low and is interpretable as a physiological variance of aICP combined with the absolute error of non-invasive absolute ICP measurement. The absolute error of ICP measurement is lower than +/−2.0 mmHg ( FIG. 16 ). The error +/−2.0 mmHg is a nominal error of existing invasive absolute ICP meters. 
     In  FIG. 17 , non-invasive absolute ICP measurement results obtained using phase contrast MRI apparatus (See Alperin et al., MRI study of cerebral blood flow and CSF flow dynamics in an upright posture: the effect of posture on the intracranial compliance and pressure,  Acta Neurochirurgica Supplementum  2005; 95: 177-181; Alperin et al., Relationship between total cerebral blood flow and ICP measured noninvasively with dynamic MRI technique in healthy subjects,  Acta Neurochirurgica Supplementum  2005; 95: 191-193) are compared with results obtained using an apparatus in accordance with the invention on a group of healthy volunteers. Forty-two healthy volunteers were studied in supine and sitting body positions using a proposed apparatus. Three different ways of transcranial Doppler (TCD) signal analysis were used. These results are also listed in Table 1 below. The good agreement between experimental aICP measurement data using MRI and data using the proposed apparatus is evidence that the proposed method and apparatus are of high accuracy and do not require calibration. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 MRI 
                   
                 Vittamed 
                   
               
             
          
           
               
                   
                   
                 Mean ICP, 
                 SD, 
                 Mean ICP, 
                 SD, 
               
               
                   
                 POSITION 
                 mmHg 
                 mmHg 
                 mmHg 
                 mmHg 
               
               
                   
                   
               
             
          
           
               
                   
                 SUPINE 
                 9.6 
                 3.6 
                 10.7 
                 3.7 
               
               
                   
                   
                 10.6 
                 3.0 
                 10.3 
                 3.6 
               
               
                   
                   
                   
                   
                 10.6 
                 3.4 
               
               
                   
                 SITTING 
                 4.5 
                 1.8 
                 4.2 
                 3.2 
               
               
                   
                   
                   
                   
                 4.8 
                 3.1 
               
               
                   
                   
                   
                   
                 4.5 
                 3.5 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 18  shows the measured typical dependence of the pulsatility index in the IOA and EOA of the ophthalmic artery. The measurements were performed with the apparatus in accordance with the invention on healthy volunteers in a supine body position for which normal aICP is close to 10 mmHg (see Table 1). 
       FIG. 18A  shows the mean value and standard deviation of measured pulsatility indexes in the IOA and EOA when the external pressure applied in the pressure chamber (Pe) is 0 mmHg.  FIG. 18B  also shows the same parameters when Pe is 10 mmHg and when Pe≈aICP. 
     The experimental results shown in  FIG. 18  are evidence that the achievable uncertainty U of aICP measurement by proposed method and apparatus is low enough (U&lt;+/−2.0 mmHg) and acceptable for different clinical applications. 
     It should be understood that the foregoing is illustrative and not limiting, and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, reference should be made primarily to the accompanying claims, rather than the foregoing specification, to determine the scope of the invention.