Probe cap

Abstract

A cap for mounting to a distal end of a diffuse reflectance spectroscopy, DRS, probe, is provided. The cap is arranged to receive the DRS probe on a first side, and the cap comprises a contact surface for contact with a body surface of a subject on a second side thereof and an aperture extending through said contact surface for allowing light from the DRS probe to be transmitted therethrough. The contact surface has a maximum width at least 10 times greater than a maximum width of the aperture.

Description

[0001] Probe cap

[0002] Technical field

[0003] This invention relates to caps for use with diagnostic probes, for example with diffuse reflectance spectroscopy probes.

[0004] Background

[0005] Those skilled in the art will appreciate that diffuse reflectance spectroscopy (DRS) utilises visible and near-infrared (NIR) light in order to provide information regarding the haemoglobin content and level of oxygenation of the patient’s blood at a microcirculatory level. Typically, the patient’s skin is illuminated with relatively broadband light using optical fibre probes; however, narrowband and single wavelengths may also be used; and the light absorbed / reflected by superficial capillaries in the patient’s skin is used to determine the blood oxygenation and gather information about haemoglobin levels.

[0006] As outlined in the Applicant’s earlier publication WO 2021 / 084134, DRS measurements can be particularly sensitive to the angle of incidence, i.e. the angle between the probe and the patient’s skin. Large angles may result in specular reflections, altered optical lengths and increased scattering effects, affecting the shape of the spectra. In the worst-case scenario, this may affect oxygenation values by providing a large variation in the data due to different angles while recording. The Applicant has previously outlined methods for collecting oxygenation data.

[0007] The Applicant has appreciated that the effect of pressure on spectroscopic measurements on a body surface (e.g. the skin) can also result in various undesirable problems. Firstly, the overall reflectance of the surface may decrease during pressure application and this change in reflectance can lead to unreliable acquisition of spectra. The in-flow and out-flow of oxygenated blood local to the measurement site can affect local circulation. Furthermore, pressure on the body surface at the measurement site can undesirably force blood away from the site, thus, perturbing local blood distribution.

[0008] For DRS measurements, in particular, even a light contact pressure between the probe and the target tissue can significantly affect the tissue optical properties and the light coupling into the fibre optic probe, resulting in a substantial degradation in performance of the quantitative algorithms for spectral analysis.

[0009] The Applicant has determined that the pressure applied to a body surface can easily exceed an acceptable range (e.g. a range above which the reflectance is substantially altered) during DRS measurements and that the pressure is significantly variable depending on the operator’s "style" of using the probe.

[0010] The present invention aims to provide an improved probe cap for diagnostic probes.

[0011] Summary of Invention

[0012] From a first aspect, the invention provides a cap for mounting to a distal end of a diffuse reflectance spectroscopy, DRS, probe, wherein the cap is arranged to receive the DRS probe on a first side thereof; the cap comprising: a contact surface for contact with a body surface of a subject on a second side thereof; and an aperture extending through said contact surface for allowing light from the DRS probe to be transmitted therethrough, wherein the contact surface has a maximum width at least 10 times greater than a maximum width of the aperture.

[0013] Thus, it will be seen that, in accordance with the invention, by providing a contact surface having a maximum width at least 10 times greater than a maximum width of the aperture, the pressure applied by such a cap when interfacing with a body surface can be reduced. Furthermore, having a relatively small aperture reduces the amount of stray light that can enter the cap from external surroundings. Therefore, the cap may also function as a light shield. Optical probe caps, particularly for medical uses, typically have a transparent window such that, in use, the window is positioned between the probe and the body surface, e.g., to reduce the likelihood of contamination of the probe.

[0014] In a set of embodiments, however, the aperture defines a non-contact area in which the body surface has no direct contact with the cap or probe, i.e. the aperture is an open hole. In such embodiments, the area within the aperture may experience substantially less pressure compared to typical measurements where a window or probe surface presses against the same area. The cap preferably maintains a gap between the body surface and the DRS probe, keeping the likelihood of contamination low. Another advantage of the aperture being an open hole is that any interference or scattering due to the window is avoided. For example, the Fabry Perot effect, i.e. multiple beam interference caused by light shining through a cavity bounded by two reflective parallel surfaces, is avoided with the open hole.

[0015] In some embodiments, however, there may be material in the aperture, e.g. the aperture may comprise a window. In such embodiments, the window may be structured for reducing the likelihood of the interference problems, e.g. the Fabry Perot effect, described above. For example, for the structure of the window, highly reflective materials and layers may be avoided.

[0016] Reducing the pressure problem in DRS measurements using a stabilising cap, especially a cap for use in non-contact measurements, has not been widely contemplated in the literature.

[0017] Reducing the influence of localised pressure on DRS spectra has been considered in the prior art. According to the prior art, known approaches include building complicated reflectance probes coupled to pressure-measuring systems, i.e. such as those disclosed in M. Bregar, M. Burmen, U. Aljancic, B. Cugmas, F. Pernus, and B. Likar, “Contact pressure-aided spectroscopy, ” Journal of biomedical optics, vol. 19, no. 2, p. 020501, 2014. Other studies - D. Wang, et al., “Influence of probe pressure on the pulsatile diffuse correlation spectroscopy blood flow signal on the forearm and forehead regions," Neurophotonics, vol. 6, no. 3, p. 035013, 2019 and B. Yu, A. Shah, and V. K. Nagarajan, “Diffuse reflectance spectroscopy of epithelial tissue with controlled probe-to-tissue pressure," in Biomedical Optics. Optica Publishing Group, 2014, pp. BS3A-14, have relied on feedback from sensors (i.e. when pressure exceeds certain thresholds) to control the pressure on tissue with angle-measuring systems used for keeping track of the tilting.

[0018] The influence of probe tilting is an ancillary although impactful problem to the pressure one in the context of operator-induced biases. Given the difficulty to have a real time feedback over the pressure integrated into the probe, the Applicant has determined that a ‘non-contact’ measurement concept is preferable, where the probe tip is not touching the body surface but only staying at a quite close distance. The distance between the body surface and the cap may be no more than the thickness of the cap material. Such a ‘non-contact’ system would still involve contact between the cap and the body surface, surrounding the aperture.

[0019] Too much of distance between the DRS probe and the body surface, on the other hand, may weaken the signal-to-noise-ratio (SNR) required for quality readings and affect the assumptions for the light propagation model that is implemented to calculate clinical parameters. The cap embodying the invention may act as a stabiliser, i.e. to keep the distance constant between the body surface and the probe surface. Embodiments preferably standardise the residual pressure over the areas neighbouring a region of interest of the body surface which is viewed by the DRS probe.

[0020] Non-contact DRS systems, where the DRS probe is not in direct contact with the body surface, are very rare in the state of the art and are typically linked to applications where "non-contact" involves long distances between the probe and the target, e.g. in hyper / multi-spectral imaging systems. Such long-distance DRS set ups come up against a number of issues related to the increased noise from the environment and the modelling of the light beam shape. Therefore, a probe cap having an open hole defined by an aperture which is for interfacing with a body surface can overcome the drawbacks of the long-distance DRS set ups without the problems caused by pressure of the window on the body surface.

[0021] The contact surface may be made from a non-slip material, e.g. matt plastic or silicone. In a set of embodiments, the contact surface is matt, i.e. not smooth. In such embodiments, when the cap is placed on a body surface it is less likely to slip because the matt contact surface provides an increased coefficient of friction between the body surface and the contact surface relative to the coefficient of friction between the body surface and a smooth contact surface. In a set of embodiments, the coefficient of friction between the contact surface and the body surface is comparable to the coefficient of friction of rubber on skin.

[0022] The contact surface may be made from a biocompatible material, e.g. a hypoallergenic material. Therefore, when the cap is being used, allergic reactions or irritations are less likely to occur, which further helps to control the conditions and standardise the DRS measurements.

[0023] The contact surface may be flexible. In embodiments with a flexible contact surface, the pressure on the body surface may be further reduced. However, in a set of embodiments, the contact surface is rigid, e.g. stiff. Having a rigid contact surface may help to keep the angle of the DRS probe relative to the body surface constant and may prevent undesirable tilting of the DRS probe when in use. The entire cap may be rigid. The contact surface of the cap may be shaped to fit the contours of a particular body surface and / or the cap may be part of a DRS system comprising fastening means to hold the cap to the body surface, e.g. the fastening means may comprise an adhesive, a wearable item such as a wristband or glove or a clip. Such embodiments may help to prevent any undesirable movement, e.g. tilting of the DRS probe relative to the body surface, when in use.

[0024] In a set of embodiments, the contact surface comprises a layer of adhesive. The adhesive layer may have a negligible thickness, such that it would not substantively affect the DRS measurement. Such an adhesive may help the cap stick to the body surface, thus helping to prevent undesirable movement of the DRS probe relative to the body surface when in use.

[0025] In some embodiments, the contact surface is optically opaque, i.e. it substantially prevents ambient light from passing through the cap to the DRS probe, e.g. into the optics or an optical fibre of the DRS probe. For example, in some such embodiments, the contact surface is black. Typically, in DRS, light in the surrounding environment adds noise to DRS readings. An optically opaque contact surface helps to improve DRS measurements by reducing the impact of changing brightness or non-uniformity of lighting in the surrounding environment. In this way, DRS measurements may be less affected by changing environments, increasing uniformity in measurement conditions. Preferably, the entire cap is optically opaque, to help ensure that ambient light does not enter the cap, e.g. through the sides of the cap via a wall portion.

[0026] Embodiments wherein the cap comprises a material having one or more of the following properties: stiff, matt, opaque and / or bio-compatible may help to ensure controlled conditions for DRS measurements which are standardised over many uses. For example, bio-compatible materials may help to avoid allergic reactions on the body surface which may alter the measurement and affect the DRS reading.

[0027] In a set of embodiments, the cap has a mass of less than 50 g, e.g. less than 30 g, e.g. less than 20 g. In other words, the cap may be lightweight. This may further reduce the pressure on the body surface compared to a heavier cap. This may be achieved by the cap being made from a plastics material.

[0028] The contact surface, e.g. the diaphragm, may have a larger maximum width than a maximum width of the DRS probe in order to spread the force between the cap and the body surface over a greater surface area.

[0029] In preferred embodiments, the cap comprises a wall portion defining a recess for receiving the DRS probe. The wall portion may extend from the first side of the cap in a direction away from the contact surface, i.e. away from the second side. The wall portion may be spaced from and surrounding the perimeter of the aperture. The wall portion and recess may be formed on the first side of the cap.

[0030] In preferred embodiments, the wall portion has a maximum width less than the maximum width of the contact surface, e.g. the wall portion has a maximum width that is less than two thirds, e.g. less than a half of the maximum width of the contact surface.

[0031] The cap may have any suitable shape. However, in at least some embodiments, the cap is substantially cylindrical. The contact surface may be circular or elliptical. In a set of embodiments, the wall portion may have a circular or elliptical annular cross-section. A (circular or elliptical) annular contact surface means that the contact surface is unlikely to have sharp edges that can cause excess pressure on the body surface affecting the DRS readings.

[0032] The wall portion may comprise at least one, e.g. a plurality of slots arranged to interface with protrusions on the DRS probe. In this way, the DRS probe can be slotted into the cap and optionally be guided into position. The slots may be arranged in a first pair of slots and a second pair of slots each pair being situated on respectively opposing sides of the wall. Having the slots arranged in this way may help to ensure a secure connection to the DRS probe.

[0033] The wall portion of the cap may comprise one or more engagement members (e.g. tab(s)) which are arranged to engage with one or more corresponding engagement members (e.g. groove(s)) on the probe when in use. For example, the engagement members may extend from the inner surface of the wall portion toward the centre of the cap, i.e. in the direction of the aperture.

[0034] The recess for receiving the DRS probe may be arranged so that a surface of the probe is brought into abutment with the cap. In a set of embodiments, the cap is arranged to hold the DRS probe such that a front surface of the probe, i.e. where light is transmitted from, lies flush with a surface of the cap on the first side of the cap, e.g. the first side of the cap being opposite to the second side of the cap (the side comprising the contact surface).

[0035] The recess may help to keep the DRS probe perpendicular to the body surface. The recess may have a size, e.g. depth, suitable for comfortably fitting the probe head inside.

[0036] The contact surface may be a surface of a contact element. The contact element may comprise a diaphragm, i.e. the contact element may be a contact diaphragm. In a set of embodiments, the cap comprises a contact element comprising the contact surface. In a set of example embodiments, the contact element may comprise a circular diaphragm with the aperture in its centre. The size, e.g. maximum width, of the contact surface may be defined by the size, e.g. maximum width, of the contact element. The thickness of the contact element may define the distance that the light travels from the DRS probe before coming into contact with the body surface. In some embodiments, the contact element may have a thickness equal to a predetermined distance between the probe and the body surface. In such embodiments, the front surface of the probe preferably lies flush with the contact element. For example, a thickness of the contact surface (e.g., diaphragm) may be selected to correspond to a predetermined ideal distance between the DRS probe and the body surface.

[0037] This may help to standardise DRS measurements so that the distance between the body surface and the DRS probe remains constant and at a distance that is most suitable for the DRS measurement.

[0038] In a set of embodiments, the contact element has a thickness which is less than 5 mm, e.g. less than 2.5 mm, e.g. less than 1.0 mm. Preferably, the contact element has a thickness from 0.3 mm to 1.0 mm. The Applicant has found that this range is particularly suitable for providing more consistent DRS readings. In some embodiments, the contact element has a thickness from 0.3 mm to 0.8 mm. The Applicant has shown by experiments that thicknesses within this range cause less perturbation of the DRS readings. Thus, the relatively small thickness of the contact element allows less space to be travelled by the light emitted / received by the DRS probe, which helps to reduce disturbances in the DRS readings. The Applicant has determined that the range of 0.3 mm to 0.8 mm offers a good compromise between optical and structural considerations. With this range of dimensions, DRS measurements can be taken very close to the body surface while reducing the risk of fractures in the diaphragm, helping to ensure good durability and usability of the cap.

[0039] The contact element may not have a uniform thickness. In a set of embodiments, the contact element has a first, inner thickness proximal to the aperture and a second, outer thickness distal from the aperture. The first, inner thickness may be smaller than the second, outer thickness. For example, the first, inner thickness may be less than 1 mm, e.g. between 0.3 mm and 0.8 mm and the second, outer thickness may be greater than 1 mm, e.g. between 1 mm and 2 mm. This means an outer portion of the contact element is thicker and thus more robust which could help to prevent breakage or undesirable tilting. A thinner inner portion may help the distance travelled by the light to be reduced without sacrificing the robustness of the overall contact element.

[0040] As described above, the cap can be used to standardise the recording of DRS spectra by relieving residual pressures over a relatively large contact surface compared to the smaller aperture size and by shielding external light. In embodiments where the aperture defines a non-contact area, i.e. the aperture is an open hole, the pressure is further reduced which helps the cap to better avoid the undesirable effect of pressure on DRS measurements.

[0041] The hole may be formed in the contact surface by any suitable means, e.g. drilling. The aperture is preferably sized to allow any emitting and receiving fibres of the DRS probe to be in visual contact with the body surface that is to be measured, e.g. the region of interest.

[0042] The cap may comprise a single aperture. In a set of embodiments, however, the cap comprises a plurality of apertures. Each aperture may extend through the contact surface for allowing light to be transmitted therethrough. Each aperture may be associated with a respective detector. For example, the DRS probe may comprise several detectors and there may be an aperture associated with each detector.

[0043] Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

[0044] Brief description of the drawings

[0045] Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0046] Fig. 1 shows a perspective view of a probe cap embodying the present invention;

[0047] Fig. 2 shows another perspective view of the probe cap of Fig.1 ; Fig. 3 is a schematic drawing of the probe cap of Figs. 1-2 interfacing with a body surface during a diffuse reflectance spectroscopy (DRS) measurement;

[0048] Figs. 4A and 4B are graphs of DRS spectra obtained for a baseline DRS measurement and DRS spectra obtained using a cap embodying the invention for the DRS measurement, respectively;

[0049] Fig. 5 shows a notched boxplot comparing the oxygen saturation readings obtained for the baseline DRS measurement and DRS measurements using a cap embodying the invention;

[0050] Fig. 6 shows a notched boxplot comparing the oxygen saturation readings obtained for the baseline DRS measurement and DRS measurements using caps of different thicknesses;

[0051] Fig. 7 schematically shows a cap embodying the invention, in use, on a human hand;

[0052] Fig. 8 shows a plan view of the cap embodying the invention; Fig. 9 shows a cross-sectional view of the cap of Fig. 8; and Fig. 10 shows a side view of the cap of Figs. 8 and 9.

[0053] Detailed description

[0054] Spectroscopy measurements of body surfaces for determining blood oxygenation are frequently carried out in clinical and hospital settings. However, both systematic and random variations introduced at the interface between the probe and the body surface can result in poor oxygen saturation measurements. Providing control at this interface with a stabilising cap, described below, helps to reduce such variations to improve the reliability of oxygen saturation measurements.

[0055] With reference to Figs. 1 and 2 there is shown a probe cap 1 for mounting to a distal end of a diffuse reflectance spectroscopy (DRS) probe (not shown). The cap 1 has a first side 1a and a second side 1b. The cap 1 is arranged to receive the DRS probe (not shown) on the first side 1a. The cap 1 has a contact surface 2 on the second side 1 b of the cap 1. An aperture 3 extends through the contact surface 2. The aperture 3 defines a non-contact area, meaning that there is no material filling the aperture 3, only an air gap. In other words, the aperture 3 defines a hole. The contact surface 2 comprises a matt plastic material. The contact surface 2 has a maximum width, Dc, at least 10 times greater than a maximum width, Da, of the aperture 3. The contact surface 2 is part of a contact element 6 which has a thickness Tc. The thickness of the contact element 6 is preferably between 0.3 mm and 1 mm.

[0056] The cap 1 comprises a wall portion provided by a wall 4, defining a recess 12 for receiving the DRS probe (not shown), the wall 4 and recess 12 being on the first side 1a of the cap. The wall 4 has a plurality of slots 5a, 5b, 5c, 5d. These slots 5a, 5b, 5c, 5d are arranged to interface with complementary protrusions on the DRS probe so that the DRS probe can be slotted into the cap 1 and be guided into position. The slots are arranged in a first pair 5a, 5b and a second pair 5c, 5d, the pairs being situated on opposing sides of the wall 4.

[0057] The wall 4 of the cap 1 also has two opposing engagement members 7 which are arranged to engage with corresponding engagement member (e.g. a groove) on the probe when in use. In this example, the engagement members 7 are tabs that extend from the inner surface of the wall 4 toward the centre of the cap 1 , i.e. in the direction of the aperture 3.

[0058] The probe cap 1 is made of a polymer-based material - e.g. a bio-compatible plastics material.

[0059] Fig. 3 is a schematic drawing of the probe cap 1 of Figs. 1-2 on a DRS probe 10 interfacing with a body surface 11 during a DRS measurement.

[0060] The cap 1 is ‘clicked’ into place on the DRS probe 10, where the engagement members 7 on the wall 4 of the cap 1 physically engage with grooves on the handle of the DRS probe 10. When in use, the optics of the DRS probe 10 are flush with the side of the contact element 6 opposite the contact surface 2. Therefore, once put in place, the gap between the optics of the DRS probe 10 and the body surface 11 is defined by the thickness of the contact element 6.

[0061] The contact surface 2 of the cap 1 is in contact with the body surface 11 of a subject. In this example, the body surface 11 is the skin of a subject (e.g. a human patient). The aperture 3 allows light from the DRS probe 10 to be transmitted therethrough. The relatively small thickness Tcof the contact element 6 allows less space to be travelled by the light emitted and received by the DRS probe 10, reducing potential disturbances in the DRS measurements.

[0062] As mentioned above, the contact surface 2 has a maximum width, Dc, at least 10 times greater than a maximum width, Da, of the aperture 3. This ratio has been found by the Applicant to advantageously stabilise the probe relative to the body surface 11. This helps to standardise the recording of DRS spectra, as the width of the aperture 3 relative to the width of the contact surface 2 provides relief from the residual pressures over a large surface which is known to affect DRS spectra readings. Furthermore, the design of the cap helps to shield external light which improves the quality of the spectra acquired by the DRS probe.

[0063] The Applicant has conducted experiments to assess the performance of the cap embodying the present invention. The tests aimed to determine whether and to what extent the cap helps with standardising the spectra obtained from DRS measurements and at reducing the effect of external light.

[0064] The results of these experiments are shown in Figs. 4-6 and are discussed in detail below.

[0065] Figs. 4A and 4B are graphs of DRS spectra, showing reflectance (no units) against wavelength (nm). Oxygen saturation measurements (SmV02) can be determined from these spectra and so variability in the spectra results in variability, and therefore, inaccuracy of the oxygen saturation measurements.

[0066] Fig. 4A is obtained for a baseline DRS measurement with no cap. Fig. 4B shows DRS spectra obtained using a cap embodying the invention for the DRS measurement.

[0067] In total, 50 measurements of spectra, were performed at the same local region of the body, i.e. the hand dorsum, on the same subject. For each measurement, the microvascular oxygen saturation (SmVCh) was calculated using a diffusive model for light-tissue interaction.

[0068] The same measurements were performed by adding the cap 1 to the DRS probe tip, maintaining a probe-to-skin distance of 0.3 mm - 0.8 mm.

[0069] A reduction in the integration time for each DRS measurement was found in the measurements using the cap 1 , the ‘integration time’ being the time the spectrometer has to acquire data in order to get a sufficient signal to noise ratio. The integration time is a parameter that is calculated automatically by the instruments. This reduction in integration time was found to apply to all the measurements in a systematic fashion. Furthermore, the experiment showed that the values obtained from each DRS acquisition using the cap 1 were almost constant.

[0070] These results suggested that use of the cap 1 limits the loss of the DRS light and also limits the undesirable collection of stray light from outside the cap 1 , thus, standardising the light conditions.

[0071] The baseline spectra of Fig. 4A obtained from cap-less DRS measurements show visibly significant differences to the spectra of Fig. 4B. The 50 spectra of Fig. 4B are more consistently similar and mostly overlap. The skilled person would appreciate that this shows that the cap 1 embodying the invention has a standardising effect with respect to operator-induced pressure and tilting.

[0072] The distribution of the SmVCh with and without the cap 1 turned out to be statistically different as shown in Fig. 5.

[0073] Fig. 5 shows a notched boxplot comparing the oxygen saturation readings obtained for the baseline DRS measurement and DRS measurements using a cap 1 embodying the invention.

[0074] The skilled person will appreciate that notched boxplots narrow at a median value and that if the notches of the boxes from two different boxplots do not overlap, then there is a statistically significant difference between the medians. As can be seen in Fig. 5, the notches do not overlap and so there is a significant difference between the ‘baseline’ oxygen saturation measurements and the ‘cap’ oxygen saturation measurements. The spread of the ‘baseline’ oxygen saturation measurements is rather pronounced whereas the ‘cap’ oxygen saturation measurements relatively have very little variability.

[0075] The difference in the calculated values of oxygen in the two scenarios was tested via a Kruskal-Wallis test, showing a statistically significant difference in the average value of oxygen saturation between the ‘cap’ and ‘baseline’ test. The variability is also significantly different and was assessed via a Levene’s test. The p-value given for both was p = 0.05.

[0076] The variability reduction induced by the cap 1 was also found when measuring on the same subject, but changing the measuring spot within a local region of interest on the dorsum of the hand of the subject. Such testing mimics operating conditions that are closer to the non-ideal operating conditions found in pre-clinical usage.

[0077] In Fig.6 the values of oxygen saturation determined from DRS measurements using two different caps having contact elements of different thicknesses and thus different distances between the probe and the skin, i.e. 0.7 mm (left) and 0.5 mm (right), are shown next to the baseline oxygen saturation values. There is a spreading effect that must be factored into the comparison which is due to spatial non-homogeneity. For both thicknesses, the variability in oxygen saturation measurements is reduced. For a suitable range of thicknesses, both average and variances are tested to be different in a statistically significant manner (p = 0.05). From the results shown in Fig. 6 it can be concluded that 0.5 mm is a preferred thickness, in terms of DRS performance, compared to 0.7 mm as it shows a smaller spread of the oxygen saturation readings which is more symmetrical about the average.

[0078] The body surface 11 could be any surface of the body suitable for taking DRS measurements. Fig. 7 shows that the body surface 11 may be the skin on a human hand. In use, the cap 1 is placed on the hand so that the contact element 6 rests on the hand as shown in Fig. 7. Figs. 8 to 10 show an example set of dimensions of the cap 1. Fig. 8 shows a plan view of the cap 1 from the first side 1a; Fig. 9 shows a cross-sectional view; and Fig. 10 shows a side view.

[0079] Fig. 8 shows that the aperture 3 has a 3 mm width and the contact element 6 has a 45 mm width, i.e. over 10 times the width of the aperture. Figs. 9 and 10 show that the thickness of the contact element 6 may be non-uniform. Fig. 9 in particular shows that the outer portion has a greater thickness of 1.5 mm and the inner portion has a thickness of 0.6 mm. This means that an outer portion 6b of the contact element 6 is thicker and thus, more robust, than an inner portion 6a of the contact element 6. With this particular arrangement, the thinner inner portion 6a may allow the distance travelled by the light to be reduced to a more optimal distance (i.e. closer to 0.5 mm) without sacrificing the robustness of the overall contact element 6.

[0080] It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

Claims1. A cap for mounting to a distal end of a diffuse reflectance spectroscopy, DRS, probe, wherein the cap is arranged to receive the DRS probe on a first side thereof; the cap comprising: a contact surface for contact with a body surface of a subject on a second side thereof; and an aperture extending through said contact surface for allowing light from the DRS probe to be transmitted therethrough, wherein the contact surface has a maximum width at least 10 times greater than a maximum width of the aperture.

2. The cap of claim 1 , wherein the aperture defines a non-contact area in which the body surface has no direct contact with the cap or probe.

3. The cap of claim 1 or 2, wherein the contact surface is made from a non-slip material.

4. The cap of any preceding claim, wherein the contact surface is rigid.

5. The cap of any preceding claim, wherein the contact surface of the cap is shaped to fit the contours of a particular body surface and / or the cap is part of a DRS system comprising fastening means to hold the cap to the body surface.

6. The cap of any preceding claim, wherein the contact surface comprises a layer of adhesive.

7. The cap of any preceding claim, wherein at least the contact surface and preferably the entire cap is optically opaque.

8. The cap of any preceding claim, having a mass of less than 50 g.

9. The cap of any preceding claim, wherein the contact surface has a larger maximum width than a maximum width of the DRS probe.

10. The cap of any preceding claim, comprising a wall portion defining a recess for receiving the DRS probe.

11. The cap of claim 10, wherein the wall portion extends from the first side of the cap in a direction away from the contact surface; wherein the wall portion is spaced from and surrounding a perimeter of the aperture, the wall portion and recess being formed on the first side of the cap.

12. The cap of claim 10 or 11 , wherein the wall portion has a maximum width less than the maximum width of the contact surface.

13. The cap of any preceding claim, wherein the cap is arranged to hold the DRS probe such that a front surface of the probe lies flush with a surface of the cap on the first side of the cap.

14. The cap of any preceding claim, comprising a contact element comprising the contact surface.

15. The cap of claim 14, wherein the contact element has a thickness equal to a predetermined distance between the probe and the body surface.

16. The cap of claim 14 or 15, wherein the contact element has a thickness which is less than 5 mm.

17. The cap of any one of claims 14 to 16, wherein the contact element has a first inner thickness proximal to the aperture and a second outer thickness distal from the aperture, the first inner thickness being smaller than the second outer thickness.

18. The cap of any preceding claim, wherein the cap comprises a plurality of apertures, each extending through the contact surface for allowing light to be transmitted therethrough.

19. The cap of claim 18, wherein the DRS probe comprises a plurality of detectors and each aperture is associated with a respective detector.

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