GB2245365A - Sample mount for nmr and optical microscopy - Google Patents

Abstract

A sample mount for NMR microscopy and NMR spectroscopy which enables a specimen to be accurately placed within an NMR apparatus and if required in a preferred embodiment for the specimen to be examined under an optical microscope to enable the images to be accurately compared.

Description

SAMPLE MOUNT FOR NMR MICROSCOPY The present invention relates to sample mounts for NMR microscopy and more particularly to a sample mount which can be used for both optical microscopy and NMR microscopy by which is meant the production of two dimensional images of the object and/or the production of NMR spectra.

In optical microscopy tissue specimens and cell cultures are prepared for examination by staining in order to increase contrast so as to highlight particular structures.

An object of the present invention is to devise a specimen mounting that would enable examination by both optical microscopy and NMR microscopy. This is particularly important in establishing correspondence between specimen features.

A sample mount for NMR microscopy and spectroscopy including a substrate layer, an electrical coil mounted adjacent to the substrate layer, a cover slip mounted adjacent to the coil on the opposite side to the substrate layer, the cover slip and substrate layer being sealed together to environmentally seal the coil in which the RF coil forms part of a tuned circuit and in which additional electrical components are mounted integrally onto the substrate slide and in which the RF coil is formed to be able to be externally coupled to enable a signal generated within the RF coil to be received and processed.

Preferably, the coupling means is capacitive, inductive or radiative.

Preferably, the sample mount further comprises a further glass cover slip removably mounted on to the cover slip on the opposite side of the cover slip to the substrate, the further cover slip being spaced apart from the cover slip, a specimen to be examined being positioned between the two cover slips and adjacent to the RF coil.

Embodiments of the present invention will now be described, by way of example with reference to the accompanying drawings, in which: Figure 1 shows a side view of a sample mount according to the present invention illustrating a slide coil assembly and in which the position of a typical specimen is also indicated; Figure 2 shows a plan view of the slide coil assembly of Figure 1 showing the coil an tuning arrangement, the coil being sealed between the slide substrate and cover slip; Figure 3 shows an equivalent circuit of the slide mounted tuned coil arrangement (circuit 2).Also shown is the circuit drive and coupling inductor L1, alternative capacitive drive arrangement being provided when points A and B are joined together and L1 replaced by a capacitor C; Figure 4 shows the effective circuit of the slide arrangement shown in Figure 2; Figure 5(a) shows an optical micrograph of one layer of onion cells, and Figure 5(b) shows an NMR image of one layer of onion cells, the image comprises 256 x 256 pixels with 4.7 fim in-plane resolution; Figure 6 shows an alternative slide circuit arrangement which uses a separate inductive coil; Figure 7(a) shows an alternative slide coil assembly in which the B1 field is produced by a flat conductive strip or set of parallel wires. This coil arrangement is fixed to a glass substrate and protected by a glass cover slip.The specimen is brought close to the plate and the effective magnetic field B is enhanced and made uniform by the presence of a highly conductive metallic sheet, Figure 7(b) shows a side view of the coil arrangement in Figure 7(a), Figure 8 shows a plastic holder forming part of an NMR probe illustrating accurate positioning of the slide within an NMR apparatus and Figure 9 shows schematically the slide and slide mount located within an NMR apparatus.

In the NMR specimen mount according to the present invention, the sample 100 is placed on a slide 200 which includes an integrated tuned RF coil 202. The RF coil is sealed between a standard glass microscope slide substrate 204 and a cover slip 206 so the whole slide assembly can be washed for re-use. The use of a glass substrate allows for the sample to be viewed under a normal optical microscope but if this is not required then the substrate could be made from a suitable plastic or other material. The specimen 100 is placed between cover slip 206 and a further removable cover slip 208.

The slide can be examined using conventional optical microscopy and then placed in the NMR magnet and imaged, or spectroscopic information obtained using the optical picture as a guide to particular areas of interest. There is no physical link to the slide, the signal picked up in the coil 202 being coupled to a loosely coupled secondary coil (see for example figures 8) which is preferably matched to 5052.

The slide assembly mounts on the Rf stage of an NMR apparatus in a holder (figure 8) which positions the sensitive region in the centre of the gradient coil (not shown). The microscope slide system would be particularly suitable for use with flat gradient coils.

NMR apparatus and gradient coils for use therein are well known and these will not therefore be further described. The operation is as follows: The sample 100 is placed close to the flat RF coil 202 comprising N turns, Fig. 1. Let the coil radius be a and the axial distance from the centre of coil be x. The field Bx at distance x from coil is

T jeld relative to that obteined at the @@il centre is

ir . is given by x = ka Over a 100 m slice using a 900 m diameter coil 202 and 170 ijm cover slips 206, 208. the average Brel approaches 0.85 so that this arrangement, though not perfect loses only around 15% of the available signal.

A plan view of the coil arrangement is shown in Fig.

2. The coil forms part of resonant circuit tuned with a chip capacitor (210). Fine tuning is achieved with a sliding copper foil trimmer 212. The slide coil 202 is inductively coupled to the transmitter/receiver system of the NMR microscopy (not shown). The circuit arrangement is represented schematically in Fig. 3.

For the il loop we have

where VO is the output voltage across points P and Q, i1 is the primary current in loop 1, L2 is the secondary current in loop 2, rl, cl and L1 are resistance series capacitance and inductance of loop 1. M is the mutual inductance between loops 1 and 2. r2, c2 and L2 are the resistance capacitance and inductance respectively of loop 2.

For the i2 loop we have j#Mil - L2i2 - 2:2 - i2/j#c2=o which givesives i2 = I jwM / (jcJt2 r2 .l/j;C) The imput i.pedance is gives by

At resonance j#L1 = 1/j#C1 and j#L2 = -1/j#C2 which gives

and Q is the quality factor.

xperiinentally the coefficient of coupling k is approximately 0.01.

The voltage transfer function Vo = -i@@@@ . For the '-1 loop where Z1 is the load impendance and Vt is the load voltage.

Vo - i1r1 - j#Lli1 + j#Mi2 - i1 j#C1 =0 and for the i2 loop -v# # j#M1 - j#L2i2 - i2r2 - i2/jwC2 = 0 Coooining these results we obtain

At resonance the above result reduces to

If 11 < < ZL and Zin = ZL then

This result is identical to that obtained when the tuned sample coil 202 is directly coupled transmitter/receiver system, i.e. when A is joined to 5 and L1 is replaced by a capacitor C. In this case Zin = r1Q2(cl/ci)2 for Ci 3 C1. Setting k = (C1/C21)2 gives the same result as that for inductive coupling. The electrical equialence of the two coupling arrangements means that inductive coupling may be used without degrading the signal reception, thus obviating solder or other connections.

Figures 1 and 2 show the construction of the slide coil. The effective circuit on the slide is shown in Fig. 4. The coil assembly is sealed against liquid ingress using glass cover 206. The copper foil tuning tab 212 on the reverse side of the slide is at earth potential thus electrically balancing the coil 202. The chip capacitor 210 is preferably placed about 10 mm from the coil thereby minimising its adverse effects on an image due to the ceramic and metal parts it contains.

The coil 202 preferably comprises 4 - 6 turns of polyurethane insulated 34 SWG copper wire. The microscope slide arrangement 200 is placed in a holder (not shown) which contains the pickup coil (not shown) that couples to the slide circuit and also contains components for further tuning and matching o-ffl the i=.

Images of cells at less than 5rn resolution have been obtained where the average thickness of the cells is around 70cm.

The quality of the results is dependent on sample preparation which must exclude air bubbles which cause image artefacts due to local gradients. The right liquid medium is important to achieve susceptibility matching. Water with various dopants seems best but we have tried organic solvents like Cm14.

Using gadolinium or manganese doped water good susceptibility matching is achieved an good outline of the cell structure produced with an inversion recovery NMR sequence to null out the extra cellular water.

Figure 5 shows a comparison between an optical micrograph of a layer of onion cells 5a and the corresponding NMR image 5b.

In this example Gd-DPTA/ water was used to match susceptibility across the extra cellular spaces. The NMR resolution was 4.7put There are three categories of non-contact coupling: capacitive, inductive and radiative. Capacitive coupling can be used to the slide mounted circuit and requires an arrangement of plates to couple, tune and match. The examples of inductive methods described above require only loose coupling which imposes fewer mechanical constraints. A further approach which would allow more space around the sample area and also obviate coupling to the pickup coil is to use a second coil 202' on the slide as in Fig 6. An optimum circuit geometry which takes account of the inductive loop and resistance of the leads to the capacitor may be approached experimentally.In a further modification the receiver coil is set into the cover plate 206 or a depression is made in the glass plate 206 to bring the sample closer to the receiver coil, thus increasing the filling factor. In yet further modifications an RF preamlifier (not shown) is included on the slide. In this arrangement coupling to the NMR microscope receiver system would be by either an inductive or capacitive link. In this case there would have to be a separate decoupled transmitter coil.

In a third coupling mode, the NMR signal could be transmitted via an antennae from a micro-transmitter housed on the slide mount.

An advantage of the flat circular coil 202 is its restricted field of view. However, this results in a lower filling factor and a spatially inhomogenous RF field. A larger filling factor and more uniform field would be obtained using two coils in a Helmholtz arrangement.

Further flat coil arrangements are possible in which a wire coil array or a single flat conducting strip 220, with current return path 221, Fig 7, is brought close to a conducting sheet 222 with the specimen sandwiched between. The presence of the conducting sheet increases the RF field in the sample, improving both signal sensitivity and RF inhomogeneity. The conductive sheet may be a metallic glass substrate. For this arrangement coupling to the strip coil 220 may be by direct connection or by a capacitive, inductive or radiative link, as described above.

With reference now to figures 8, 9, the slide 200 is shown mounted in a suitable plastic holder 230 which forms part of the NMR probe 310 and is mounted within an NMR apparatus 300 which in a known manner comprises a permanent magnet 331 gradient and field coil structures 332, 334. (Further description of the operation of the NMR apparatus is considered unnecessary since the techniques for such operation though both complex and varied are well known to those skilled in the art).

The slide 200 is mounted onto a plastic holder 230 which forms part of an NMR probe. The plastic holder 230 will be accurately positioned in a defined position within an NMR apparatus and the slide will be therefore accurately positioned within the holder 230 and hence within the NMR apparatus.

The slide 200 is held in holder 230 by, for example, -a slotted arrangement 232. The RF pickup coil 250 for the NMR apparatus is mounted into the holder 230 and the coil 202 in the sample mount 200 is thus aligned accurately with this coil 250 and also with the gradient coils 332. Coil 250 is connected to a printed circuit board 251 for support and a timing capacitor 253 iS, also positioned on this board.

The NMR RF Feed/output is taken via cable 238 to suitable transmitter/amplifier apparatus 340 (Fig 9). A suitable NMR spectrometer 342, computer 344 and display apparatus 346 is shown and operates in known manner.

Alternatively as described hereinbefore a microtransmitter/antennae arrangement 242 could be incorporated into the slide mount to directly transmit the NMR signal to a suitable receiver 244 (Fig 9).

Claims (11)

1. A sample mount for NMR microscopy and spectroscopy including a substrate layer, an electrical coil mounted adjacent to the substrate layer, a cover slip mounted adjacent to the coil on the opposite side to the substrate layer, the cover slip and substrate layer being sealed together to environmentally seal the coil in which the RF coil forms part of a tuned circuit and in which additional electrical components are mounted integrally onto the substrate slide and in which the RF coil is formed to be able to be externally coupled to enable a signal generated within the RF coil to be received and processed.

2. A sample mount as claimed in claim 1 in which the substrate and cover slip are made from clear glass whereby a sample may be positioned on the cover slip to be thereby viewable under an optical microscope prior to or subsequent to NMR Examination.

3. A sample mount as claimed in claim 1 in which the coupling means is capacitive, inductive or radiative.

4. A sample mount as claimed in claim 1 or 2 in which the sample mount further comprises a further cover slip removably mounted on to the cover slip on the opposite side of the cover slip to the substrate, the further cover slip being spaced apart from the cover slip, a specimen to be examined being positioned between the two cover slips and adjacent to the RF coil.

5. A sample mount as claimed in any one of claims 1 to 4 in which the RF coil is inductively coupled to a transmitter coil in the main NMR probe such that the RF coil on the slide mount behaves as both RF transmitter to and receiver of signals from the specimen under study.

6. A1 sample mount as claimed in any one of claims 1 to 4 in which the RF coil is provided with an amplifier, transmitter and energy source integral with the microscope slide allowing the signals picked up from the sample to be transmitted to a remote receiver.

7. A sample mount as claimed in any one of claims 1 to 4 in which RF pulses are transmitted to the sample and signals received from the sample by the RF coil which is capacitively coupled to the main NMR probe.

8. A sample mount as claimed in any one of claims 1 to 4 in which the RF coil is formed by a flat conducting strip placed in proximity to a metallic conducting sheet the specimen being placed between insulating glass sheets which are effectively sandwiched between the coil and the metallic conducting sheet.

9. A sample mount as claimed in any one of claims 1 to 4 in which the RF coil is formed from a set of parallel wires placed in proximity to a conducting sheet the specimen being placed between insulating glass sheets which are effectively sandwiched between the RF coil and the conducting sheet.

10. A sample mount as claimed in claims 8 and 9 in which the RF coils form part of a tuned tank circuit mounted on the glass substrate which is directly energized via a screened twin feeder cable.

11. A sample mount as claimed in claims 8 and 9 in which the RF coils are inductively coupled via a separate tank circuit mounted on the glass substrate and which is itself directly coupled to the RF coil.