Multi-channel non-invasive tissue oximeter

Abstract

A method and apparatus for spectrophotometric in vivo monitoring of blood metabolites such as hemoglobin oxygen concentration at a plurality of different areas or regions on the same organ or test site on an ongoing basis, by applying a plurality of spectrophotometric sensors to a test subject at each of a corresponding plurality of testing sites and coupling each such sensor to a control and processing station, operating each of said sensors to spectrophotometrically irradiate a particular region within the test subject; detecting and receiving the light energy resulting from said spectrophotometric irradiation for each such region and conveying corresponding signals to said control and processing station, analyzing said conveyed signals to determine preselected blood metabolite data, and visually displaying the data so determined for each of a plurality of said areas or regions in a comparative manner.

Description

andAs shown in FIGS. 11 and 12, a first emitter 624, a second emitter 626, a first detector 628, and a second detector 630 are placed over a first tissue region 632. The first emitter 624 is adapted to emit a first light into the first tissue region 632 and the second emitter 626 is adapted to emit a second light into the first tissue region 632. The first detector 628 is located a first distance 634, also referred to as the first line 634, from the first emitter 624 and is located a second distance 636, also referred to as the second line 636, from the second emitter 626. As shown in these figures, the second distance 636 is greater than the first distance 634. The second detector 630 is located a third distance 638, also referred to as the third line 638, from the first emitter 624 and is located a fourth distance 640, also referred to as the fourth line 640, from the second emitter 626. As shown in these figures, the fourth distance 640 is less than the third distance 638. The first emitter 624 is closer to the first detector 628 than the second detector 630, and the second emitter 626 is closer to the second detector 630 than the first detector 628. The third distance 638 is longer than the first distance 634 and is longer than the fourth distance 640. The second distance 636 is approximately equal to the third distance 638. The first distance 634 is approximately equal to the fourth distance 640.

As further shown in FIGS. 11 and 12, the first emitter 624, the second emitter 626, the first detector 628 and the second detector 630 are aligned within the cross-sectional plane. In addition, the second line 636 defined between the center of the first detector 628 and the center of the second emitter 626 partially overlaps with the third line 638 defined between the center of the second detector 630 and the center of the first emitter 624.

Referring now to FIG. 11, a third emitter 724, a fourth emitter 726, a third detector 728, and a fourth detector 730 are placed over a second tissue region 732. The third emitter 724 is adapted to emit a third light into the second tissue region 732 and the fourth emitter 726 is adapted to emit a fourth light into the second tissue region 732. The third detector 728 is located a fifth distance 734, also referred to as the fifth line 734, from the third emitter 724 and is located a sixth distance 736, also referred to as the sixth line 736, from the second emitter 726. The second detector 730 is located a seventh distance 738, also referred to as the seventh line 738, from the third emitter 724 and is located an eighth distance 740, also referred to as the eighth line 740, from the fourth emitter 726. As also shown in FIG. 11, the third emitter 724 is closer to the third detector 728 than the fourth detector 730, and the fourth emitter 726 is closer to the fourth detector 730 than the third detector 728. The fifth distance 734 is less than the seventh distance 738. The eighth distance 740 is less than the sixth distance 736.

As shown in FIGS. 13 and 14, the first detector 628 is adapted to detect the first light propagated over a first mean path 664 through the first tissue region 632 and to detect the second light propagated over a second mean path 666 through the first tissue region 632. The second mean path 666 has a length 667 greater than a length 665 of the first mean path 664. The second detector 630 is adapted to detect the first light propagated over a third mean path 668 through the first tissue region 632 and is adapted to detect the second light propagated over a fourth mean path 670 through the first tissue region 632. The fourth mean path 670 has a length 671 less than the length 669 of the third mean path 668. The length 665 of the first mean path 664 is substantially equivalent to the length 671 of the fourth mean path 670 and the length 669 of the third mean path 668 is substantially equivalent to the length 667 of the second mean path 666. The length 665 of the first mean path 664 is less than the length 669 of the third mean path 668 and the length 671 of the fourth mean path 670 is less than the length 667 of the second mean path 666. The second mean path 666 and the third mean 668 path overlap at a location 672 below a tissue surface of the tissue region 632. In addition, along a line 674 orthogonal to the surface of the tissue between the first detector 628 and the second detector 630, the third mean path 668 lies farther from the tissue surface than the second mean path 666. The second mean path 666 lies substantially as far from a tissue surface as the third mean path 668 at approximately a midpoint 676 between the first detector 628 and the second detector 630.

As further shown in FIGS. 13 and 14, the first emitter 624 and the first detector 628 form a first near coupling. The second detector 630 is located farther from the first emitter 624 than the first detector 628 to form a first far coupling. The second emitter 626 and the first detector 628 form a second far coupling. The second detector 630 is located closer to the second emitter 626 than the first detector 628 to form a second near coupling. The first emitter 624 is adapted to transmit the first light along the first mean path 664 through a first section 680 of the first tissue region 632. The second emitter 626 is adapted to transmit the second light along the second mean path 666 through the first section 680 of the first tissue region 632 and the fourth mean path 670 through a second section 682 of the first tissue region 632. The first emitter is adapted to transmit the first light along the third mean path 668 through the second section 682 of the first tissue region 632. The first emitter 624 and the second emitter 626 are further adapted to transmit the first light and the second light along the third mean path 668 and second mean path 666, respectively, through a third section 684 of the first tissue region 632 and to transmit the first light and the second light along the first mean path 664 and the fourth mean path 670, respectively, that substantially avoid the third section 684 of the first tissue region 632.

As shown in FIG. 13, the third detector 728 is adapted to detect the third light propagated over a fifth mean path 764 through the second tissue region 732. The third detector 728 is adapted to detect the fourth light propagated over a sixth mean path 766 through the second tissue region 732. The fourth detector 730 is adapted to detect the third light propagated over a seventh mean path 768 through the second tissue region 732. The fourth detector 730 is adapted to detect the fourth light propagated over an eighth mean path 770 through the second tissue region 732. The length 769 of the seventh mean path 768 is greater than the length 765 of the fifth mean path 764 and the length 767 of the sixth mean path 766 is greater than the length 771 of the eighth mean path 770.

As shown in FIG. 15, a first transmitter 724 (previously referred to as the third emitter 724 during the discussion of FIGS. 11 and 13 above), a first detector 826, a second detector 828, and a third detector 830 are placed over a first region of tissue 732 (previously referred to as the second tissue region 732 during the discussion of FIGS. 11 and 13 above). The first transmitter 724 is adapted to transmit light into the first region of tissue 732. The first detector 826 forms a near detector grouping with the first transmitter 724. The second detector 828 and the third detector 830 are located farther from the first transmitter 724 than the first detector 826 to form far detector groupings. As also shown in FIG. 15, a line 840 passing through a midpoint of the first transmitter 724 and a midpoint of the first detector 826 is spaced apart from a midpoint of the second detector 828 and a midpoint of the third detector 830. In addition, the line 840 defined between a center of the first transmitter 724 and the center of the first detector 826 forms an acute angle 842 with a line 844 defined between the center of the transmitter 724 and a center of the second detector 828. The line 840 defined between the center of the first transmitter 724 and the center of the first detector 826 forms a second acute angle 846 with a line 848 defined between the center of the transmitter 724 and a center of the third detector 830, with the second acute angle 846 substantially similar to the first acute angle 842.

As further shown in FIG. 15, a second transmitter 624 (previously referred to as the first emitter 624 during the discussion of FIGS. 11-14 above), a fourth detector 628 (previously referred to as the first detector 628 during the discussion of FIGS. 11-14 above), a fifth detector 928, and a sixth detector 930 are placed over a second region of tissue 632 (previously referred to as the first tissue region 632 during the discussion of FIGS. 11-14 above). The fourth detector 628 forms a near detector grouping with the second transmitter 624. The fifth detector 928 and the sixth detector 930 are each located farther from the second transmitter 624 than the fourth detector 628 to form far detector groupings. As shown in FIG. 15, the distance 940 between the first transmitter 724 and the first detector 826 is approximately equal to the distance 942 between the second transmitter 624 and the fourth detector 628.

As will be understood, the foregoing disclosure and attached drawings are directed to a single preferred embodiment of the invention for purposes of illustration; however, it should be understood that variations and modifications of this particular embodiment may well occur to those skilled in the art after considering this disclosure, and that all such variations etc., should be considered an integral part of the underlying invention, especially in regard to particular shapes, configurations, component choices and variations in structural and system features. Accordingly, it is to be understood that the particular components and structures, etc. shown in the drawings and described above are merely for illustrative purposes and should not be used to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.

Claims

1. A method for comparative spectrophotometric in vivo monitoring and display of selected blood metabolites present in a plurality of different internal regions of the same test subject on a continuing and substantially concurrent basis, comprising the steps of:

applying separate spectrophotometric sensors to a test subject at each of a plurality of separate testing sites and coupling each of said sensors to a control and processing station;

operating a selected number of said sensors on a substantially concurrent basis to spectrophotometrically irradiate at least two separate internal regions of the test subject during a common time interval, each of said regions being associated with a different of said testing sites;

separately detecting and receiving light energy resulting from said spectrophotometric irradiation for each of said at least two separate internal regions, and conveying separate sets of signals to said control and processing station which correspond to the separately detected light energy from said at least two separate internal regions;

separately and concurrently analyzing said conveyed separate sets of signals to separately determine quantified data representative of a blood metabolite in each of said at least two separate internal regions; and

concurrently visually displaying said separately determined quantified data for each of said at least two separate internal regions for direct concurrent mutual comparison, wherein said sensors are applied to a head of the test subject and are used to monitor two mutually separate regions within a brain of the test subject.

2. The method of claim 1, wherein said step of analyzing comprises quantitative determination of blood oxygenation levels within each of said at least two separate internal regions.

3. The method of claim 2, wherein said analyzing step includes producing separate quantitative value determinations for hemoglobin oxygen saturation for each of said at least two separate internal regions.

4. The method of claim 3, wherein said analyzing step includes production of ongoing graphical traces representing a plurality of said quantitative value determinations made at successive points in time.

5. The method of claim 4 including the step of visually displaying a plurality of said graphical traces at substantially the same time and in predetermined relationship to one another to facilitate rapid and accurate visual comparison.

6. The method of claim 5, including the step of visually displaying a plurality of said quantitative value determinations at substantially the same time and in predetermined relationship to one another to facilitate rapid and accurate visual comparison.

7. The method of claim 3, including the step of visually displaying a plurality of said quantitative value determinations at substantially the same time and in predetermined relationship to one another to facilitate rapid and accurate visual comparison.

8. The method of claim 1, wherein said metabolite comprises hemoglobin oxygen.

9. The method of claim 1, wherein said sensors are positioned in locations proximate to different brain hemispheres and said two mutually separate regions are located in a different brain hemisphere.

10. The method of claim 9, wherein said metabolite comprises cerebral blood hemoglobin oxygenation.

11. An apparatus for concurrent comparative spectrophotometric in vivo monitoring of selected blood metabolites present in each of a plurality of different internal regions on a continuing basis, comprising:

a plurality of spectrophotometric sensors, each attachable to a test subject at different test locations and adapted to separately but concurrently spectrophotometrically irradiate at least two different internal regions within the test subject associated with each of said test locations;

a controller and circuitry coupling each of said sensors to said controller for separately and individually but concurrently operating certain of said sensors to spectrophotometrically irradiate each of said different internal regions within the test subject associated with each of said test locations;

said sensors each further adapted to receive light energy resulting from the separate spectrophotometric irradiation of said sensors' associated one of said at least two different internal regions on a substantially concurrent basis with other said sensors, and to produce separate signals corresponding to the light energy received, said circuitry acting to convey said separate signals to said controller for separate analytic processing;

said controller adapted to analytically process said conveyed signals separately and determine separate quantified blood metabolite data therefrom for each of said sensors' and said sensors associated one of said at least two different internal regions; and

a visual display coupled to said controller and adapted to separately but concurrently display the quantified blood metabolite data determined for each of said sensors in a mutually-comparative manner, wherein said sensors are adapted to be applied to a head of the test subject and to monitor a brain of the test subject.

12. The apparatus of claim 11, wherein said controller is adapted to analyze said data to quantitatively determine blood oxygenation within said at least two different internal regions.

13. The apparatus of claim 12, wherein said controller is adapted to produce separate numeric value designations for hemoglobin oxygen saturation for said at least two different internal regions.

14. The apparatus of claim 13, wherein said controller and said display are adapted to produce ongoing graphical traces representing a plurality of said numeric value designations for the same region taken over a period of time.

15. The apparatus of claim 14, wherein said controller and said display are adapted to visually display at least two of said graphical traces on a substantially concurrent basis and in predetermined relationship to one another to facilitate rapid and accurate visual comparison.

16. The apparatus of claim 15, wherein said controller and said display are adapted to visually display at least two of said numeric value designations as well as at least two of said graphical traces on a substantially concurrent basis and in proximity to one another to facilitate rapid and accurate visual comparison.

17. The apparatus of claim 13, wherein said controller and said display are adapted to visually display at least two of said numeric value designations on a substantially concurrent basis and in predetermined relationship to one another to facilitate rapid and accurate visual comparison.

18. The apparatus of claim 11, wherein said sensors are adapted to provide signals to said controller which comprise at least two separate data sets that cooperatively define at least portions of a particular area within a given one of said at least two different internal regions.

19. The apparatus of claim 18, wherein said data sets provided by said sensors include a first set characterizing a first part of said particular area and a second set characterizing a second part of said particular area.

20. The apparatus of claim 19, wherein said second part of said particular area characterized by said second set includes at least part of said first part of said area.

21. The apparatus of claim 11, wherein said controller is adapted to determine blood oxygenation saturation in said brain.

22. The apparatus of claim 11, wherein at least two of said sensors are adapted to be positioned in locations associated with mutually different hemispheres of the brain and each of said sensors is operable to separately monitor at least portions of each of said different hemispheres.

23. The apparatus of claim 22, wherein said controller is adapted to determine cerebral blood oxygenation saturation within each of said different hemispheres.

24. The apparatus of claim 22, wherein said sensors are adapted to provide signals to said controller which comprise at least two data sets that cooperatively define at least portions of a particular area within the same hemisphere of said brain.

25. The apparatus of claim 11, wherein said sensors are adapted to be applied to the outside periphery of the test subject and to operate non-invasively.

26. A method for concurrent comparative in vivo monitoring of blood metabolites in each of a plurality of different internal regions in a selected test subject, comprising the steps of:

spectrophotometrically irradiating each of a plurality of different testing sites on said test subject;

detecting light energy resulting from said spectrophotometric irradiation of said testing sites, and providing separate sets of signals to a control and processing station which are representative of the light energy received by each of said testing sites and which cooperatively define blood metabolite data for an individual one of at least two different internal regions;

analyzing said separate signals to determine quantified blood metabolite data representative of at least one defined region within said at least one test subject associated with each of at least two different of said testing sites, each said defined region being different from the other; and

concurrently displaying data sets for each of said at least two different internal regions at substantially the same time for direct mutual comparison, wherein said at least two different internal regions are located within different brain hemispheres of said test subject.

27. The method of claim 26, wherein said data sets include a first set which characterizes a first zone within one of said at least two different internal regions and a second set which characterizes a second zone that is at least partially within the same one of said at least two different internal regions.

28. The method of claim 26, wherein said spectrophotometric irradiation comprises application of at least two different wavelengths applied in an alternating sequence of timed pulses, and wherein detection of light energy corresponding to each of said at least two different wavelengths is done on a timed periodic basis using detection periods whose occurrence generally corresponds to that of said applied spectrophotometric irradiation.

29. The method of claim 28, wherein the duration of each of said detection periods is limited to a length which is less than that of each pulse of applied spectrophotometric irradiation.

30. The method of claim 29, wherein the duration of each of said detection periods is less than half that of a pulse of said applied spectrophotometric irradiation.

31. The method of claim 30, wherein a plurality of said detection periods are used during pulses of said applied spectrophotometric irradiation, and a corresponding energy detection occurs during each of a plurality of said detection periods.

32. The method of claim 31, further including the steps of averaging a selected number of energy detection event values to obtain a resultant value therefor, and using said resultant value to compute a metabolite value which is representative thereof.

33. The method of claim 32, wherein said display includes said computed representative metabolite value.

34. The method of claim 33, wherein said display is refreshed periodically by using a sequence of computed representative metabolite values which are based upon and represent the averaged detection event values produced during the different time intervals corresponding to the intervals of said periodic display refreshment.

35. Apparatus for spectrophotometric in vivo monitoring of a selected metabolic condition in each of a plurality of different test subject regions on a substantially concurrent basis, comprising:

a plurality of spectrophotometric emitters, each adapted to separately spectrophotometrically irradiate a designated region within a test subject from a test location on said test subject;

a controller and circuitry coupling each of said emitters to said controller for individually operating selected ones of said emitters to spectrophotometrically irradiate at least two particular regions within the test subject;

a plurality of detectors, each adapted to separately receive light energy resulting from the spectrophotometric irradiation of said at least two particular regions, and to produce at least one separate set of signals for each one of said at least two particular regions; and circuitry acting to convey said at least one separate set of signals to said controller for analytic processing;

said controller adapted to analytically process said at least one separate set of signals to determine separate sets of quantified data representative of a metabolic condition in said at least two particular regions; and

a visual display coupled to said controller and adapted to display separate representations of said separate sets of quantified data for each of said at least two particular regions in a mutually-comparative manner and on a substantially concurrent basis, wherein at least two of said at least two particular regions are located in mutually separate regions of a brain of said test subject.

36. The apparatus of claim 35, wherein said controller includes a computer programmed to analyze said signals to separately determine a blood oxygenation state within each of said at least two particular regions.

37. The apparatus of claim 36, wherein said computer comprises a processor, data buffers, and a timing signal generator, said data buffers adapted to store data representative of said blood oxygenation state and said timing signal generator adapted to control actuation of said emitters and detectors.

38. The apparatus of claim 36, wherein said controller comprises a unitary device which includes said computer and said display.

39. The apparatus of claim 38, wherein said unitary device further includes a keyboard interface to said computer.

40. The apparatus of claim 38, wherein said unitary device further includes a data output interface.

41. The apparatus of claim 40, wherein said unitary device further includes an integral keyboard interface to said computer.

42. The apparatus of claim 38, wherein said display comprises a flat electroluminescent visual display screen.

43. The apparatus of claim 42, wherein said unitary device further includes an integral keyboard interface to said computer.

44. The apparatus of claim 35, wherein at least certain of said detectors and certain of said emitters comprise operational pairs, and said controller is arranged to operate the emitters and detectors of at least certain of said operational pairs in predetermined timed relationship while maintaining the emitters and detectors of other of said operational pairs in a non-operating condition.

45. The apparatus of claim 44, wherein said controller is adapted to sequence the operation of said at least certain of said operational pairs.

46. The apparatus of claim 45, wherein at least one of said operational pairs include a plurality of said detectors arranged at mutually spaced locations which are spaced at differing distances from the emitter of said at least one of said operational pairs.

47. The apparatus of claim 46, wherein said controller is adapted to operate the emitter and a selected number less than all of the detectors of at least one of said operational pairs substantially in unison while holding the other detectors of said at least one of said operational pairs in a non-operating condition, and said controller is further arranged to operate said other detectors substantially in unison with said emitter at another time during which said selected number of said detectors are maintained in a non-operating condition.

48. The apparatus of claim 44, wherein at least one of said operational pairs includes a first detector and a second detector, and wherein the first detector is located nearer the emitter than the second detector to thereby provide near and far detector groupings for said at least one of said operational pairs.

49. The apparatus of claim 48, wherein said controller is adapted to sequence the operation of said at least one of said operational pairs.

50. A regional oximetry system adapted to substantially simultaneously display tissue oxygen saturation measurements for at least two human tissue regions, comprising:

a first sensor including a first emitter, a first near detector, and a first far detector, the first near detector being located closer to the first emitter than the first far detector, the first near detector and the first far detector each being configured to detect a first light transmitted by the first emitter, and the first sensor being configured to generate a first set of data indicative of the detected first light;

a second sensor including a second emitter, a second near detector, and a second far detector, the second near detector being located closer to the second emitter than the second far detector, the second near detector and the second far detector each being configured to detect a second light transmitted by the second emitter, and the second sensor being configured to generate a second set of data indicative of the detected second light; and

an oximeter unit including one or more processors, the oximeter unit being configured to:

operate the first emitter of the first sensor and the second emitter of the second sensor in sequence on a substantially simultaneous basis in a manner that reduces cross-talk between the sensors;

receive the first set of data and the second set of data;

determine a first tissue oxygen saturation measurement corresponding to a first tissue oxygen saturation parameter based on the first set of data;

determine a second tissue oxygen saturation measurement corresponding to a second tissue oxygen saturation parameter based on the second set of data; and

superimpose a first trace indicative of the first tissue oxygen saturation measurement over a time period and a second trace indicative of the second tissue oxygen measurement over the time period on a display.

51. The regional oximetry system of claim 50, wherein the oximeter unit is further configured to display an event marker in conjunction with the superimposed first and second traces.

52. The regional oximetry system of claim 51, wherein the oximeter unit is further configured to overlay the event marker on the superimposed first and second traces.

53. The regional oximetry system of claim 51, wherein the oximeter unit is further configured to align the event marker with a time indicator.

54. The regional oximetry system of claim 53, wherein the oximeter unit is further configured to display the superimposed first and second traces in substantially real time and to maintain a position of the displayed event marker with respect to the superimposed first and second traces and the time indicator over a measurement period of time.

55. The regional oximetry system of claim 51, wherein the event marker includes a circle selectively placed on a display.

56. The regional oximetry system of claim 50, wherein the oximeter unit is further configured to overlay a vertical line on the display to associate an event occurrence with the superimposed first and second traces.

57. The regional oximetry system of claim 50, wherein the oximeter unit is further configured to superimpose the first and second traces on a graph having a vertical scale of oxygen saturation values that range from less than 100% and more than 0%.

58. The regional oximetry system of claim 50, wherein the first light comprises at least four different wavelengths.

59. The regional oximetry system of claim 50, wherein the first emitter of the first sensor forms an emitter-detector pair grouping with a third far detector configured to detect the first light, the third far detector being located farther from the first emitter than the first near detector, the third far detector being configured to generate data indicative of the first light detected by the third far detector, and the oximeter unit being configured to determine the first tissue oxygen saturation measurement using the data indicative of the first light detected by the third far detector.

60. The regional oximetry system of claim 50, wherein the one or more processors of the oximeter unit are coupled to a display generator and to a data output interface.

61. A method for comparatively displaying oxygen saturation measurements of at least two regions of human tissue, the method comprising:

transmitting, with a first emitter, a first light into a first region of tissue of a patient and transmitting, with a second emitter, a second light into a second region of tissue of the patient, the first light being transmitted into the first region of tissue of the patient and the second light being transmitted into the second region of tissue of the patient in sequence on a substantially simultaneous basis;

detecting the first light with a first near detector and with a first far detector, the first near detector being closer to the first emitter than the first far detector;

generating a first set of data indicative of the first light detected with the first near detector and with the first far detector;

determining, using one or more central processing units, a first tissue oxygen saturation measurement based on the first set of data;

detecting the second light with a second near detector and with a second far detector, the second near detector being closer to the second emitter than the second far detector;

generating a second set of data indicative of the second light detected with the second near detector and with the second far detector;

determining, using the one or more central processing units, a second tissue oxygen saturation measurement based on the second set of data; and

superimposing a first trace indicative of the first tissue oxygen saturation measurement over a time period and a second trace indicative of the second tissue oxygen saturation measurement over the time period on a display.

62. The method of claim 61, further comprising a step of detecting the first light with a third far detector that is farther from the first emitter than the first near detector, wherein the step of generating the first set of data includes generating data indicative of the first light detected with the first near detector, the first far detector, and the third far detector.

63. The method of claim 61, wherein the first tissue region of the patient is a tissue region spaced apart from a brain of the patient.

64. The method of claim 63, wherein the first tissue region is a body extremity.

65. The method of claim 61, further comprising a step of displaying an event marker in conjunction with the superimposed first and second traces.

66. The method of claim 65, wherein the event marker is a circular event marker.

67. The method of claim 66, wherein the superimposing step is performed in substantially real time, and the displaying step includes maintaining a position of the displayed event marker with respect to the first trace, the second trace, and a time indicator.

68. The method of claim 61, wherein the step of transmitting the first light into the first region of tissue includes transmitting at least three different wavelengths of light into the first region of tissue.

69. The method of claim 61, wherein the step of transmitting the first light into the first region of tissue includes transmitting the first light through a fiber-optic cable to the first emitter and transmitting the first light into the first region of tissue using the first emitter.

70. The method of claim 61, wherein the step of determining the first tissue oxygen saturation measurement based on the first set of data is performed using a single central processing unit.