GB2429056A - An optical sensing device for locating a vein in a subject - Google Patents

Abstract

A device for locating a vein (31, fig. 3) in a subject (21, fig. 2), the device comprises: a source of collimated or focussed radiation 103 to radiate a point 117 on the subject; and a detector 105 for detecting the radiation scattered from the point, and its surrounding area. An indicating means e.g. a display 192 indicates the size of the detected radiation signal relative to the size of the detected radiation collected from the other points on the subject. The source may be monochromatic, or it may emit wavelength in the range of 400 to 450 nm; 535 to 550 nm; 575 to 585 nm or 700nm upwards. A shield 190 which is made of material opaque to the radiation and has a window 191 may be provided over the detectors 105 to stop stray radiations, and to allow only scattered radiation reaching the detectors. A marking means 113 may be provided to mark the position of a vein derived from the detected radiation. A gain control 196, and suppression control 194 may be used to configure a calibration measurement for the display, and to amplify its full scale deflection.

Description

1 2429056 A Medical Device The present invention is concerned with the

field of medical devices. More specifically, the present invention relates to a device for locating a vein in a subject.

It certain patient groups such as children under six months, obese or elderly people, or people whose veins have collapsed through dehydration or shock, it can be difficult even for highly-trained medical staff to quickly locate a vein suitable for a emergency transfusions of blood or plasma. Delays or errors can be potentially fatal.

WO 2000/080276 describes a device suitable for enhancing the visibility of subcutaneous blood vessels by generating a video image of the body tissue and blood vessels based on reflected infrared light. The system works by illuminating an area of the subject with diffuse infrared light, recording an infrared image, generating a visible version of the same image and projecting that image back onto the subject.

This system has the drawback that it is relatively expensive to produce.

An older, simpler, device was proposed in JP 2161956. This device is a handheld device which emits light in the red to infrared region. The radiation source is non- collimated and radiation is detected via two or more detectors which are intended to be aligned on either side of the vein. LEDs are used to indicate whether or not the device is aligned along a vein.

The above device suffers from the problems that it cannot be effectively used on areas where the thickness or density and hence scattering level of the tissue around the veins is changing and also the use of a non- focused radiation source will cause an inaccurate measurement. Also, there are two additional disadvantages of the above device: using detectors placed either side of the vein will detect a smaller proportion of the injected photons which have undergone multiple scattering events along the line of the vein, and so will give vein contrasts which are lower than the maximum achievable. The second disadvantage is that limbs with a very small radius of curvature (for example, the limbs of a small child) cannot be investigated accurately as one or both of the detectors are liable to be out of contact with the skin, and therefore prone to detecting ambient light which will lower, or even completely obscure, the vein contrast.

There is hence a need for a device which can be used to locate awkward veins and which can be produced cheaply enough to be provided on every drip stand or for every patient who needs to be injected.

The present invention addresses the above problems and, in a first aspect provides a device for locating a vein in a subject, the device comprising: a source of focussed or collimated radiation for irradiating a point on said subject, a detector for detecting radiation from the source which has been scattered from the point and its surrounding area and indicating means for indicating the size of the detected radiation signal relative to the size of the detected radiation collected from other points on the subject.

Blood, particularly Haemoglobin, absorbs radiation at 740 nm, whereas the tissue which typically surrounds a vein strongly scatters radiation of this wavelength. Thus, the amount of scattered light will depend on the amount of blood sampled by the beam.

Hence, the position of veins or arteries can be determined by measuring the amount of scattered radiation from different points on the subject. Penetration into the skin is generally about 10 mm at this wavelength.

Preferably, the indicating means can indicate at the same time the relative sizes of the scattered signals measured by irradiating different points on the subject. For example, the indicating means may comprise a visual display showing the amplitude, power etc. of the detected signal against time or distance moved by the source. Thus, as the device is scanned across the subject, readings from different points on the subject are displayed next to one another. Alternatively, the output may be directed to a plotter for plotting the results on a permanent medium such as paper.

Thus, since the results from different points on the arm can be compared directly with one another, the positions of the veins can be easily determined. Also, the use of relative measurements allows the device to be effectively used regardless of the variations in the scattering characteristics of the skin. For example, the skin at the top of the arm scatters more strongly than the skin at the bottom of the arm. Therefore, a device which provides a single isolated measurement indicating the presence or absence of the vein may work at the bottom of the arm but may not work at the top of the arm.

The present invention does not suffer from this disadvantage.

Preferably, the separation between the point where the radiation enters the subject and the detectors is of the order of the depth of penetration of the radiation into the skin.

This separation ensures that a significant proportion of the photons which reach the detectors will have had the opportunity to interact with a vein if one is present just below the irradiation point.

Preferably, the distance from the point on the subject to the centre point of the detector is 1 to 30 mm, more preferably from 6 to 12 mm and even more preferably from 8 to 10 mm.

In a preferred embodiment, the detector is positioned so that there is no reflective interface, for example, from the device itself overlying the subject in the space between the detectors and the point on said subject. This avoids unintentional reflections between the surface of the subject and the surface of the device contributing to the measured signal, which is more likely to happen if the two surfaces are in proximity.

This may be achieved ensuring that the device does not cover the area between the point and the detector or detectors. For example, where there are two detectors, the two detectors are provided in a line with the point on said subject and the source is set-back from the line.

The source is preferably provided at a distance of I to 100 mm from the line, more preferably 5 to 40mm.

In a particularly preferred embodiment, the detector is provided on a projection extending generally away from said source. Where there are two detectors both are provided on projections. The projections may be posts arranged forward of the source and focusing optics so that the intervening space between the detectors is open The device may further comprise a shield which is opaque to radiation emitted from the source, the shield being provided over the detectors and having a window such that radiation from the source which has been scattered by the subject reaches the detector.

For example, each detector is surrounded on all but the side facing the subject by the shield.

Preferably, the window has a width similar to the width of the veins, when the detectors are aligned along a vein. Equal to or less than the width of a vein is required for optimum contrast. This window is preferably of width 0.5 to 5mm, more preferably 1 to 3 mm. The dimensions of the detector active area may be larger than the dimensions of the window.

Preferably, a monochromatic source is used. Preferably, the source is a laser or a focussed LED.

The source preferably emits in the ranges from 400 to 450 nm and/or 535 to 550 nm and/or 575 to 585 nm and/or 700nm up to 2jim.

The indicating means preferably comprises a display and further comprises gain and suppression means to allow the measured signals for a subject to be fitted to the full scale deflection of the display. This allows the device to be used over a range of patients as the maximum signal level and contrast will vary for patient to patient. By setting the suppression and gain of the display to be calibrated for each subject, , it is possible to obtain at relative measurement for each point measured on the subject.

In a preferred embodiment, the suppression means are configured to process a calibration measurement and display the calibration measurement at a predetermined value on the display. Preferably, the calibration measurement will be taken where the signal from a vein is not expected. The predetermined value is preferably a value between 65% to 95% of the maximum deflection of the display, more preferably between 75% and 85% of the maximum deflection. The suppression means may be configured to process a calibration measurement in response to an activation signal generated when an activation switch, button or the like is depressed.

The gain means may be configured to amplify the measured signal so that the full scale deflection of the display represents a predetermined percentage change in the signal.

Preferably, this predetermined percentage will be a value in the range from 45% to 75%, more preferably a value in the range from 55% to 65%.

The gain means may also preferably comprise means to manually set the percentage change which is represented by the full scale deflection.

The indicating means is preferably an LCD screen, an array of LEDs etc. Once the position of a vein has been determined, the position may be marked by using a pen, pencil, inkjet head or the like or the patient may be directly injected with the device in position. To aid this operation, the device preferably comprises a marking means such as a holder for a pen, pencil, hypodermic needle or the like. The holder is preferably fixed with respect to the point on the patient which is measured by the device. In a preferred embodiment, the source creates a point of light on the arm of the subject and the holding means is positioned so that marking or injection takes place at this point.

More preferably, the marking means comprises an antiseptic, this avoids the need for the area to be swabbed prior to injection and also helps minimise cross contamination.

Also, preferably, the marking means is configured to provide marks which indicate the area to be injected as opposed to marking the area itself. For example, the marking means may mark a circle around the area to be injected. Marking the actual area where the skin is to be broken is undesirable in some cases.

Preferably, the part of the device which is to be in contact with the skin will be sealed against air and moisture penetration, so that it may be wiped clean with antiseptic or other disinfectant media between application of the device to different patients.

Alternatively, a cheap disposable end-piece may be employed on the end of the device, to be discarded for another when the device is used on a new patient.

The device may also further comprise a filter positioned to filter out radiation reaching said detector. As the body is a natural source of some near infrared emissions, the use of a narrowband filter ensures that the correct radiation i.e. that emitted from the source (and not the subject) is detected by the detectors.

In a further preferred embodiment, the source is configured to emit a modulated signal and the detector comprises a lock-in amplifier in order to amplify signals received with the given modulation. This allows a higher signal to noise ratio to be detected.

Further, the device preferably comprises means to vary the focus of the radiation from the source on the skin. By varying the focus of the radiation, it is possible to determine the depth of the veins under the skin.

In order to improve the signal to noise ratio, it is preferable if the device is moved so that the detector and source are aligned along the vein. In a preferred embodiment, the device comprises two detectors aligned either side of the source. In a further embodiment, the detector is in the form of an annular ring arranged around the source.

The device may comprise means to scan the point across the subject. In this specific embodiment, the device is preferably anchored to the subject by means of either a strap or cuff. In a preferred embodiment, the source and detector are fixed to one another and are scanned together. When the device is scanned across the subject, the device may moved twice across the subject. During the first pass, the device may be used to collect data and during the second pass the position of the veins identified during the first pass may be marked.

In a second aspect, the present invention provides a method for locating the position of veins or the like in a subject, the method comprising: irradiating a point on the subject with a source of focussed or collimated radiation; detecting radiation scattered from said the point and its surrounding area with a detector; and indicating the size of the detected radiation signal from the area relative to the size of the detected radiation from other points on the subject.

The present invention will now be described with reference to the following embodiments in which: Figure 1 A is a schematic of the underneath of the head of a vein locator in accordance with an embodiment to the present invention, Figure lB is a front elevational view of the head of Figure 18 and Figure IC is a right-side elevational view of the head of Figure 1A; Figure 2 is a schematic of the head of Figure 1A placed on the ann of a subject; Figure 3A is schematic of a subjects arm in cross-section with the head in position directly over a vein, Figure 3B is a schematic cross-section of the subjects arm with the device in a position where it is not directly over a vein and Figure 3c is a perspective view of the cross section of figure 3A; Figure 4a is a plot of the photocurrent output by the head of the vein locator as it is first moved across the skin of the forearm to indicate the positions of veins, then held stationary on the arm to demonstrate the constant signal level in the absence of movement, figure 4b is a plot of the photocurrent output by the head of the vein locator as it is moved forwards and backwards over the same vein four times, the x-axis of the plot represents time in arbitrary units; Figure 5A is a schematic of the underneath of the head of a vein locator in accordance with a further embodiment of the present invention, Figure 5B is a front elevation view of the head of Figure 5A and Figure 5C is a right-side elevation view of the head of Figure 5A; Figure 6 is a head in accordance with a further embodiment of the present invention showing a variation on the design of Figure 1 where the marker is positioned along the elongate axis of the head; Figure 7 is a detector head in accordance with a further embodiment of the present invention having an annular photodetector arranged around the light source; and Figure 8 shows a device in accordance with an embodiment of the present invention where the head is mounted on a cuff or strap. and Figure 9 shows an embodiment of the present invention, in which the detectors, source, power supply, signal display and electronics are all integrated into one convenient hand- held unit. The detectors are positioned forward of the source on two posts.

The device will first be described with reference to the specific head shown in Figure 1.

Figure 1A illustrates the underneath of a head 1. The underneath of the head 1 is placed facing and preferably in contact with the subject. The head 1 is substantially elongate in this plane and comprises a source 3 provided between photodetectors 5 and 7 such that photodetectors 5 and 7 lie on either side of source 3 along elongate axis 9.

In this particular embodiment, source 3 is an NIR laser. Any laser source may be used providing that it emits radiation at a frequency which is absorbed by blood. In the specific embodiment of Figure 1A, the laser emits at 740nm.

The detectors 5 and 7 are silicon photodetectors in this specific embodiment. These photodetectors emit a current when radiation of the desired wavelength impinges on the detector. Any detector may be used which detects radiation at the suitable wavelength.

The source 3 and detectors 5, 7 are provided within head casing 11. Head casing 11 comprises moulded plastic with a polycarbonate window. Mounting sleeve 13 is provided on a long side of head 1 level with source 3. Mounting sleeve 13 is for holding a needle or marker pen.

The distance between the mid point of one detector 5 or 7 and the source 3 is typically between 6 to 12 mm. In other words, the distance between the mid-points of the two detectors 5, 7 is between 12mm and 24 mm. The optimum separation of the detectors 5 and 7 depends on the penetration range of the radiation into the skin of the subject. The interaction of the radiation with the subject will be described in more detail with reference to figures 3a, 3b and 3c.

Figure lB shows a front elevation view of the head of Figure 1A. To avoid uimecessary repetition, like reference numerals are used to denote like features. It can be seen from the elevation view that source 3 is set slightly back from the edge 15 of detector head 1.

Radiation from source 3 is emitted in a focussed beam 16 along optical axis 17 of the head 1. The beam 16 is focussed by focussing element, e.g. lens 18, provided on the lower edge of head 1. Detectors 5, 7 are provided at the edge 15 of head 1. Mounting sleeve 13 is provided on the front side of head 1.

Figure IC is right-side elevation view of the head of Figures 1A and B. To avoid unnecessary repetition, like reference numerals will be used to denote like features.

Mounting sleeve 13 is a substantially cylindrical sleeve which is configured to hold a needle or marker pen.

The mounting sleeve 13 is positioned so that its elongate axis intersects both the lower edge 15 of head 1 and the optical axis 17 of the head 1. Radiation is emitted from the source 3 along optical axis 17 and forms a spot on the skin of a subject. Thus, if a hypodermic needle or marker pen is provided within the sleeve 13, it can either mark or inject at a point of the intersection between the optical axis 17 and lower edge 15.

Hence, if the lower edge 15 is placed in contact with a subject, a mark (or injection) can be made exactly at the point where a measurement is taken.

In the case where a hypodermic needle is to be used, mounting sleeve 13 is covered by a disposable polymer membrane 19.

Figure 2 schematically illustrates the head 1 in position on the arm 21 of a subject. The head us aligned with its elongate axis 9 substantially parallel to the long axis of arm 21. Veins are known to run substantially along the long axis of arm 21. Thus, the detectors 5, 7 (Figure 1) and source 3 (Figure 1) are provided in a line parallel to the direction of the veins in the arm.

The head 1 is connected via a cable 23 to a signal monitor 25 a laser driver 27 and power source 29. The cable 23 may have multiple wires within the cable. The laser driver provides a signal for source 3 (Figure 1) which is located in head 1.

In another preferred embodiment, the incoming laser light and the scattered photons could be conducted, respectively, to and/or from the patient's arm by optical fibres contained in cable 23. The light source and/or the detectors may then be situated outside of the head, and the head may be commensurately more compact.

Signal monitor 25 receives an electrical signal from detectors 5, 7 (Figure 1). The signal monitor may further comprise a display device such as an oscilloscope or may have a screen to indicate an output reading. Alternatively, the signal monitor may output to a plotter. The power for the system is provided by a 5 volt unit which may be mains or battery operated. Optionally, the laser driver may have means to put a modulation signal on the source output. The signal monitor 25 may then further comprise a device configured to amplify or otherwise pick-up this modulated signal, for example, a lock-in amplifier (not shown). The use of a lock-in amplifier substantially increases the signal to noise ratio.

Figures 3A and 3D schematically show the operation of the device. The head 1 is placed on the subject's arm. The head is arranged so that the source 3 and detectors 5 and 7 are provided in line arranged approximately along the direction of the veins.

As previously described (with reference to Figure 1), source 3 emits a beam of collimated radiation 16. When light source 3 is moved so that it is directly over vein 31 in subject's arm 21, the blood in vein 31 absorbs the radiation and hence little light is scattered and detectors 5, 7 register a low signal.

When head 1 is moved so that source 3 is not above vein 31 as shown in Figure 3b, the collimated beam 16 does not immediately impinge a vein and more radiation is scattered from other tissue in the arm. Thus detectors 5, 7 receive a substantially larger signal and the detector output is high. Hence, by measuring the output from the detectors and determining whether or not it is high or low, the positions of veins can be determined.

Figure 3c is a perspective of the arm 21 of figure 3a with the head in position above a vein 31. The focussed beam 16 impinges on arm 21 and vein 31 and photons are scattered 14 in many directions and some will be detected by detectors 5 and 7. Photons whose (multiple) scattering path takes them generally along the line of the vein 31 before reaching the detectors 5, 7 will have an increased probability of absorption by the blood within the vein 31. The scattered light detected in the configuration shown in figure 3c will therefore be at a minimum compared to when the head is positioned over the surrounding tissue as shown in figure 3b, and not directly over the vein 31.

Similarly, the scattered light detected will be a minimum when the detectors 5, 7 and the source 31 are perfectly aligned along a vein 31. In this way, the alignment of the head giving the minimum detected signal will also indicate the direction in which the vein 31 runs.

As mentioned briefly in relation to figure 1, the optimum spacing for the detectors 5,7 from the source 3 is roughly equal to the penetration depth of the radiation into the skin.

Using wider separations between the detectors 5, 7 and the source 3 result in better vein contrast. However, a larger separation leads to a bulkier head and lower detected signal levels. Using narrower separations between the detectors 5, 7 and the source 3 can result in lower contrast between regions of tissue containing and not containing veins because radiation may be detected after just one or two scattering events at shallow depths above the veins, i.e. before the radiation has had a chance to be absorbed by the blood in the veins. However, narrower separations between the source 3 and detectors 5 and 7 result in a more compact head.

Figure 4a illustrates some results from the device of Figure 1. The device was positioned as shown in Figure 2 and then moved laterally across the arm 21 perpendicular to the general direction of the veins. The source emitted radiation at a wavelength of 760 nm. The photo current from the Si detectors is plotted along the y axis and time/distance is plotted along the x axis. The head was moved at a speed of approximately 1cm per second and sharp dips 41, 43, 45, 47and 49 are observed corresponding to 5 veins. The device travels approximately 47 mm from the first vein 41 to the fourth vein 47. The device is then held stationary over the fifth vein, 49.

Obviously, once the device is held stationary, the x axis only corresponds to time and not distance.

Thus, the position of the veins can be easily determined from the dips in the signal.

Further, since the result is displayed in terms of a relative measurement, i.e. the result from one part of the subject's arm is displayed next to result from a different part of the subject, thus making it easy for an operator to determine where a vein is most likely to be regardless of variations in the thickness or scattering efficiency of surrounding tissue.

For example, there is a significant difference in the scattering efficiency between the skin of the upper arm and that of the lower arm, or indeed, between the skin of the inner and outer forearm.

When the head is placed over veins which are close to the surface, little radiation is scattered and hence a deep and narrow dip is observed. More radiation will be scattered when the head is placed over deeper veins and hence a shallower wide dip is observed.

Hence, by estimating the full width at half maximum (or minimum) it is possible to obtain an indication of the depth of the vein below the surface.

Once a vein has been located and the head is positioned stationary above the vein, the vein may be injected either directly using a needle mounted in sleeve 13 or the position of the vein may be marked using a marker provided in sleeve 13 for later injection.

Figure 4b schematically illustrates some further results taken by the device of figure 1.

The device was positioned as shown in Figure 2 and then moved laterally across the arm 21 perpendicular to the general direction of the veins. The source emitted radiation at a wavelength of 760 imi. The photo current from the Si detectors is plotted along the y axis and time in arbitrary units is plotted along the x axis.

The device is moved across one vein which is identified by dip 51, the device is then reversed over the vein and second dip 53 is noted. The device is then moved forwards and backwards a further three times over the same vein demonstrating that the device produces reliable and repeatable results.

Figure 5 shows a variation on the head of Figure 1. To avoid uimecessary repetition, like reference numerals will be used to denote like features.

The detector head of Figure 5 is substantially similar to that of Figure 1, except for the provision of narrow pass filter 51. Narrow pass filter 51 is provided on edge 15 such that the radiation detected by detectors 5 and 7 is filtered to ensure that these detectors primarily detect radiation scattered from source 3.

Figure 6 shows a yet further variation on the head of Figure 1. In this specific example, sleeve 13 is provided at the side of elongate head 1. Again, the sleeve is mounted so that marking or injection can take place at the point where the skin is illuminated by source 3.

Figure 7 shows a yet further variation on the design of detector head. In this specific detector head, a single annular photodetector 61 is provided in a ring around source 3.

This annularly symmetric design means that there is no need to orientate the head so that the detectors and source are aligned along the direction of the veins as there will always be a part of the detector in line with both the vein and the source. The annular detector may also take the form of a ring of smaller, individual, detector elements.

In the previous figures, the detector head has been free from the subject. Figure 8 shows a variation on the detector head where the device is mounted to the subject.

The source 71 is mounted in a line between two detectors 75 and 76. The source 71 and two detectors 75 and 76 are mounted on a slide 87. The slide is itself slidably mounted on a cuff (or strap) 81 which may be placed around an arm or leg etc. of a subject. The slide 87 slides around the cuff 81 and perpendicular to the line along which the detectors 77, 76 and source 71 are mounted such that the slide slides perpendicular to the direction of the veins.

Holder 79 is mounted such that its position is fixed with respect to the source 71 to allow the point on the subject's skin which is illuminated by the source 71 to be marked.

The slide 87 may be moved manually or scanned automatically. The position of the veins could be marked on a second pass of the slide 87 or a return scan.

Figure 9 shows a preferred embodiment of the present invention. As for the embodiment as described with reference to figure 1, the device comprises a source 103 located between two detectors 105, 107. However, in this particular embodiment, the source 103 is set back from the detectors 105, 107. The detectors and source are mounted in casing 109. Casing 109 comprises a body 110 and first and second posts and 101. The first andsecond posts 100, 101 are generally parallel to one another and extend from the same end of the body 110. Detectors 105 and 107 are provided at the distal end of each post 100, 101. A shield material 190 is provided on each detector 105, 107. The shield material 190 is opaque to NIR radiation and has a window 191 positioned such that only photons which are emitted directly under the detectors 5, 7, when the device is positioned on the skin, reach the detector. Thus the shields 190 prevent stray photons from reaching the detectors 105, 107.

The source 103 is provided on the body 110 of the casing 109 between the two posts 100, 101. The source 103 is provided behind focussing optics 118 and is positioned so that it emits a beam of radiation 116 which will enter the subject, at a point 117 in-line and at the midpoint of the two detectors, when the device is in position on a subject.

In addition to the source 103 and the focussing optics 118, the body of the casing comprises a signal display 192. The signal display 192 may be an LCD screen, or an array of LEDs or similar. In order to match the fullscale deflection of the display to the change in signal when a device passes over a vein, the device comprises gain control 196 and suppression control 194. The size of the detected signal will differ from patient to patient. Therefore, the gain and suppression means allow the measured maximum signal to be fitted to the full scale deflection of the display 192.

The suppression control 194 is configured to operate in a calibration mode where it processes a signal which is measured from a point on the subject which should be away from a vein and display this signal at a set percentage of the maximum deflection of the display. If the measurement is performed away from a vein, the measurement should be close to the maximum, so the value can be displayed at a high level on the display, for

example around 80%.

Thus for one subject "Person A", the measured signal strength away from a vein might be 160 jiA, whereas for a different subject "Person B" it might be only 8OpA. Thus, the suppression means can automatically determine the most appropriate value for the maximum deflection shown on the display. For person A the maximum deflection would be 200 jiA, whereas for person B it would be 1 OOjiA.

The gain control 196 then determines the contrast in signal which can be displayed.

The gain control may have an automatic setting which sets the maximum change in the signal which may be displayed to be a value, for example 60%. Thus, taking the above values, the display would be set to read between 8OjiA and 200 jtA for person A, and between 40pA and I OOjiA for person B. However, in some cases it may not be suitable to use a pre-set value for the gain. Therefore, the gain control may also be manually controlled, for example, if person B has veins with poor contrast, the gain control 196 could then manually be adjusted to read between (say) 801.tA and 100 jiA - in this way a vein giving only 20% real contrast could be displayed as giving 100% full scale deflection.

The gain control 196, suppression control 194, power supply 129 and gainlsuppression electronics 198 are all provided within the body 110 of casing 109.

Sleeve 113 is provided mounted to the casing 109 to hold a pen or needle etc to allow marking or injecting at the correct site. The sleeve 113 is aligned so that the point of a pen or needle when passed through the sleeve touches the point at which the radiation enters the subject.

Claims (1)

1. CLAIMS: 1. A device for locating a vein in a subject, the device

   comprising: a source of collimated or focussed radiation for irradiating a point on said subject, a detector for detecting radiation from the source which has been scattered from said point and its surrounding area and indicating means for indicating the size of the detected radiation signal relative to the size of the detected radiation collected from other points on the subject.

   2. A device according to claim 1, wherein said indicating means indicates at the same time the size of the scattered signal from at least two points on the subject.

   3. A device according to any preceding claim, wherein said source is monochromatic.

   4. A device according to any preceding claim, wherein the distance from the point on said subject to the centre point of the detector is 1 to 30mm.

   5. A device according to any preceding claim, wherein the detector is positioned so that there is no reflective interface overlying the subject in the space between the detectors and the point on said subject.

   6. A device according to claim 5, wherein two detectors are provided in a line with the point on said subject and the source is set-back from the line.

   7. A device according to any preceding claim, further comprising a shield which is opaque to radiation emitted from the source, the shield being provided over the detectors and having a window such that radiation from the source which has been scattered by the subject reaches the detector.

   8. A device according to any preceding claim wherein said source emits at least one wavelength in the range from 400 to 450 nm and/or 535 to 550 nm and/or 575 to 585 nm and/or 700nm upwards.

   14. A device according to any preceding claim, further comprising marking means for marking the position of a vein derived from the detected radiation signal.

   15. A device according to any preceding claim, further comprising means to vary the focus of the radiation from the source on the skin.

   16. A device according to any preceding claim, wherein the indicating means comprises a display configured to show the magnitude of the measured signal and the device further comprises suppression means configured to process a calibration measurement and display the calibration measurement at a predetermined value on the display.

   17. A device according to claim 16, further comprising gain means configured to amplify the measured signal so that the full scale deflection of the display represents a predetermined percentage change in the signal.

   18. A device according to any preceding claim, further comprising mounting means to mount the detector and source to the subject.

   19. A method for locating the position of veins or the like in a subject, the method comprising: irradiating a point on the subject with a source of collimated or focussed radiation; detecting radiation scattered from said point and its surrounding area with a detector; and indicating the size of the detected radiation signal from the point relative to the size of the detected radiation from other points on the subject.

   20. A method according to claim 19, wherein said detector and source are aligned along the expected direction of veins in the subject.