WO2011022769A1

WO2011022769A1 - Shuttter and method of use

Info

Publication number: WO2011022769A1
Application number: PCT/AU2010/001093
Authority: WO
Inventors: Charlene Shia-Ying Chua; Simon Phillip Adam Higgins; Andreas Fouras
Original assignee: Monash University
Priority date: 2009-08-25
Filing date: 2010-08-25
Publication date: 2011-03-03

Abstract

A shutter for a beam of electromagnetic radiation, such as that provided by a synchrotron, said shutter comprising at least one voice coil having a radiation attenuating head, wherein actuation of the voice coil moves the head on a linear trajectory between a first position wherein the beam of electromagnetic radiation is unattenuated and a second position wherein the beam of electromagnetic radiation is attenuated.

Description

SHUTTt=R AND METHOD OF USE

FIELD OF INVENTION

The present invention relates to the field of shutters, particularly for specialist X-ray scanning systems such as those used for medical diagnostic use.

In one form, the invention relates to shutters, particularly high speed shutters suitable for controlling the emission of X-rays in systems such as computer axial tomography (CAT) scanners.

In another form, the invention relates to a mechanical shutter with synchronous or asynchronous control and sub-millisecond operation.

' The present invention is adapted for controlling the emission of X-rays however it will be readily appreciated that the shutter could also be used for controlling the emission of radiation from any part of the. electromagnetic spectrum.

It will be convenient to hereinafter describe the invention in relation to CAT scanners and synchrotron imaging, however it should be appreciated that the present invention is not limited to that use only and has a wider range of application to advanced x-ray equipment. Furthermore the present invention is not limited to medical applications and has a wider range of application to advanced x-ray equipment for other applications such as industrial and research applications.

BACKGROUND ART

It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein. Shuttering generally

Medical scanners typically comprise a source of radiation that rotates around a patient, projecting radiation that passes through the patient and is received by a detector.

Medical scanners typically include a shutter to selectively block or allow passing of a radiation beam. Shutters are also used extensively in other optical equipment and laser systems. Shuttering is required to optimise image quality by reducing motion smear and for dosage control to the locus.

Shutter performance is typically measured in terms of 'response time¹ or 'time to open', 'time to close¹ or 'rise time' or 'fall time'. A full cycle of 'open-close- open' or 'close-open-close' is called 'total window time'. Shutter response time is the time between receiving a signal and the first noticeable change in the status of the shutter, that is, from input of a signal to close and the first part of the closing motion. Computer x-ray tomography scanning systems often use shutters that operate with a very short response time in order to minimize wasted x-ray dosage to a patient. Useful data cannot be collected until the shutter is fully opened, therefore during the time the shutter is moving from fully closed to fully open, the patient or target tissue receives an extraneous, non-utilized dose of x- rays. Exposure to excessive or continuous radiation can cause premature deterioration or damage of tissue. Provision of the appropriate amount of x- radiation is also important for optimal data-capture by a camera.

In essence, shutters are used to selectively block (attenuate) or allow the transmission of a beam of electromagnetic radiation. Attenuation of the beam may be provided by two mechanisms; absorption and scattering, due to the interaction of the beam with attenuating material.

Attenuating x-rays through shuttering is more difficult than for other wavelengths of the electromagnetic spectrum, such as visible light. This is due to the requirement of the flag, the portion of attenuating material used to block the beam, to have high x-ray attenuation qualities. Materials of high density will greatly reduce the maximum speed at which the flags can travel.

X-ray equipment for applications such as x-ray tomography, the shutter also needs to be capable of operation at very high speed (sub-millisecond or close to sub-millisecond operation). This allows, for example, biological flow rates in patients, model arteries or ex-vivo (mouse and rat) arteries to be synchronised with image capture in terms of the camera's frame rate. When utilising synchrotron x-ray sources for biomedical research, accurate shuttering is required to improve image quality and is also essential for dosage control to the target, as may be required for ethical, preservation and safety measures.

Shutter optimisation

Shutters fall into distinct categories based on their operation mechanism.

^• Focal plane shutters have either single or multiple curtains placed directly in front of a focal plane ahead of photographic film or an image sensor. However they suffer from shudder due to their weight, noise, premature mechanical failure of moving parts and distortion of images of fast moving subjects. Leaf shutters include both diaphragm and irises shutters and comprise a mechanism with one or more pivoting metal leaves. They provide high-speed operation and good flash-synchronisation with limited image distortion. Rotary disc shutters are semicircular mirrors that rotate in front of a film gate and are used extensively in the motion picture industry to restrict motion blur between frames.

Through necessity, curtain, iris leaf and rotary discs have all been adapted for x-ray imaging and offer different performance outputs based on compromises of functional needs. Although these mechanisms have been adequate solutions for visible light photography, construction of the curtains and leaves from x-ray attenuating materials generally has significant drawbacks. In particular, the curtains and leaves are too heavy, slow, and prone to friction.

There have been many attempts to optimise shutters for attenuation of x- ray beams such as, for example, using voice coils. A voice coil includes a coil of wire around a bobbin and electrical current flowing through the coil creates a magnetic field that causes a proportionate movement of a head. For example, International patent: WO 2008/060600 relates to a shutter that uses a single voice-coil to drive shutter movement in both the open and dosed directions, the shutter comprising blades that work in a rotational fashion like an iris. US patent 4,839,679 uses two rotary voice-coils to drive a shutter, each coil being operating off completely separate cores and isolated by a stainless steel mounting of low magnetism. The blades of the shutter close from opposite directions on a radial trajectory. US patent 4,592,083 relates to a device used in x-ray scanning machines used for CAT scans. The device uses solenoids that are similar to voice-coils to actuate shutter blades that work in a rotational fashion.

US patent 2,846,588 relates to a device that uses a DC motor to drive a single shutter blade which has a linear motion. However, this shutter is comparatively slow, which is a problem intended to be addressed by the present invention.

Published papers by Maguire et al and Scholten (Maguire, L. P., Szilagyi, S. & Scholten, R.E. (2004) High performance laser shutter using a hard drive voice-coil actuator, Review of Scientific Instruments, 75(9), 3077-3079; Scholten, R.E. (2007) Enhanced laser shutter using a hard disk drive rotary voice-coil actuator, Review of Scientific Instruments, 78, 026101 ) describe a drive system for a rotary voice coil actuated visible light shutter system. This design used a plastic flag to block visible light.

As mentioned above, shutter attenuation of x-rays is more difficult than attenuation of other wavelengths of energy such as visible light, because the shutter must be composed of material having radiation blocking properties, typically dense metals such as tungsten or molybdenum. Accordingly, the aforementioned visible light shutter system of Maguire et al and Scholten could not be used to attenuate x-rays. The shutter would operate too slowly if the plastic flag was replaced with a suitably thick high density material, such as a tungsten flag.

Vincent Associate's Uniblitz™ XRS series of shutters are specifically designed for x-ray applications and use a translational blade mechanism. Although the XRS series shutters offer a cost effective off-the-shelf solution suitable for imaging static samples or for general safety purposes, they do not operate at sufficiently high frequencies and do not provide sufficiently fast opening times for PIV imaging sequences that typically involve asynchronous, instantaneous frequencies over 300 Hz (to capture an image pair) combined with much lower cycle frequencies.

Attempts have also been made to shutter beams using 'choppers' - spinning mechanisms that 'chop' a light source at a known rate. These systems include rotating diffracting crystals or mirrors which scatter a beam, and absorption choppers that comprise a metal disk attached to a motor and spun at a known rate. The perimeter of the disk has multiple apertures or 'slots' that allow the transmission of the beam as each aperture crosses the beam path. Shutters utilising rotary discs are aligned with their axis of rotation either parallel, or perpendicular to the x-ray beam.

Gembicky et al (Gembicky M, Oss D₁ Fuchs R & Coppens P (2005) Journal of Synchrotron Radiation 12, 665-669) have disclosed an accurate slotted rotary wheel chopper having low jitter for synchrotron experiments. However slotted rotary wheel chopper designs suffer from numerous drawbacks including very small fields of view, no asynchronous control with the detector, and the necessary manufacture of different slotted rotary wheels if different fields of view or different time lapses between frames are needed. Moreover, Gembicky and Coppens (Gembicky M & Coppens P (2007) Journal of Synchrotron Radiation 14, 133-127) suggest that a perpendicular orientation is necessary to achieve short opening times, whereas a parallel geometry is needed for a high repetition rate requirement. Both of these features are necessary if dosage control and high frame rate capture are required.

So, although slotted rotary wheel, rotary voice coil and commercially available iris-leaf shutter designs have been successfully demonstrated for x-ray imaging, they do not provide adequate aperture size, the option of asynchronous control, or the speed required to perform imaging techniques such as x-ray particle image velocimetry for examination of physiological flows in biological models.

There is therefore a need for a high-speed, high attenuation shutter that is capable of operation as a slave to a data acquisition unit.

SUMMARY OF INVENTION

An object of the present invention is to provide a high speed shutter mechanism for controlling the emission of electromagnetic radiation, particularly x-rays.

A further object of the present invention is to alleviate at least one disadvantage associated with the related art. It is an object of the embodiments described herein to overcome or alleviate at least one of the above noted drawbacks of related art systems or to at least provide a useful alternative to related art systems.

In a first aspect of embodiments described herein there is provided a shutter for a beam of electromagnetic radiation, the shutter comprising at least one voice coil having a radiation attenuating head,

wherein actuation of the voice coil moves the head on a linear trajectory between a first position wherein the beam of radiation is unattenuated and a second position wherein the beam of radiation is attenuated.

Where used herein the term 'voice coil' is the colloquial term for a fast and reliable actuator motor, with the force of a magnetic coil causing a proportionate movement of the head. Linear (and rotary) voice coil actuators consist of a coil of wire wound around a bobbin that encases a permanent magnet. When electrical current flows through the coil and electromagnetic field is induced, the voice coil is affected by this field, causing a proportionate movement of the bobbin and coil (head) either towards or away from the fixed permanent magnet, on a linear (or rotary) trajectory. By reversing the current through the voice coil, movement is driven in the reverse direction. Voice coil actuators are extensively used in speakers - a current (audio) waveform acting as input to the voice coil to generate movement of the speaker's diaphragm to produce desired sound. The present invention exploits the electromagnetism of the coils to use voice coils effectively in a shutter.

It will be appreciated that the shutter can provide various degrees of attenuation depending on the thickness and structure of the head and the nature of the one or more materials of construction and.the nature of the radiation being attenuated. By controlling these features, and the number and timing/synchronisation of the movement of the shutter heads it is possible to control the degree of attenuation or the intensity (particle density) of the radiation beam. This includes accurate control of the time of exposure for concomitant accurate control of the dose of electromagnetic radiation delivered.

The head may be integral with the magnetic coil or a bobbin of the voice coil and is comprised of material that attenuates the electromagnetic beam. Typically, when the shutter is in the second position the attenuation is preferably at least 98%, more preferably at least 99%. The exact amount of attenuation will depend on the thickness of the head or heads and the nature of the one or more materials of construction. The attenuation is also dependant on the flux of the electromagnetic radiation source, but for a known specified source at a known flux, the attenuation is changed by the properties of the head.

Preferably the shutter has a frequency of operation of at greater than

500Hz, that is, the time taken for a full cycle of operation that returns the shutter to its original position, or 'closed-open-closed'. Shutters according to the present invention have been shown to operate a full cycle (close-open-close) in 2.7ms, which corresponds to approximately 350 to 360 Hz.

The direction of movement of the head depends on the direction of current flow through the voice coil. For example, when the voice coils are operated, a flow of current moves the head in one direction and a change in polarity reverses the direction of movement.

In a second aspect of embodiments described herein there is provided a shutter for a beam of electromagnetic radiation, the shutter comprising;

a first voice coil having a first core and a radiation attenuating head, a second voice coil having a second core and a radiation attenuating head wherein actuation of the voice coils moves each head on opposing linear trajectories between a first position wherein the beam of electromagnetic radiation is unattenuated and a second position wherein the beam of electromagnetic radiation is attenuated.

In another aspect of embodiments described herein there is provided a shutter mechanism for a scanner, the scanner having a source or electromagnetic radiation that projects the radiation at a target, radiation passing through the target being received by a detector, the shutter mechanism comprising:

a voice coil having a radiation attenuating head, wherein actuation of the voice coil moves the head on a linear trajectory between a first position wherein the radiation beam is unattenuated and a second position wherein the radiation beam is attenuated, and

a stop means for halting movement when the head reaches the second position. This shutter mechanism may further comprise a second voice coil having a second radiation attenuating head, wherein the -second head moves on a linear trajectory in the opposite direction to the linear trajectory of the first head.

In yet a further aspect of embodiments described herein there is provided a method of attenuating a beam of electromagnetic radiation, the method comprising the steps of

initiating a flow of current through a voice coil having a radiation attenuating head to move the head on a linear trajectory from a first position wherein the radiation beam is unattenuated to a second position wherein the radiation beam is at least 99% attenuated, and

reversing the current flow to move the head on a linear trajectory from the second position to the first position.

In a yet further aspect of embodiments described herein there is provided a method of controlling the intensity of a beam of electromagnetic radiation, the method comprising the steps of:

initiating a flow of current through a voice coil having a radiation attenuating head to move the head on a linear trajectory from a first position to a second position, and

reversing the current flow to move the head on a linear trajectory from the second position to the first position

wherein the radiation intensity is attenuated by the head between the first and second positions.

Other aspects and preferred forms are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention.

In essence, embodiments of the present invention stem from the realization that simple, linear voice coils will allow high shutter speed from a small displacement, the force-dispiacement relationship of the actuators being exploited to provide optimal acceleration and combined. In contrast, due to their comparatively large rotary displacements, shutters of the prior art such as iris- leaf, rotary wheel choppers and rotary voice coil activated shutters do not yield asynchronous control, adequate aperture size and adequate performance for techniques such as x-ray imaging. This contrast with shutters of the prior art that use an iris shutter mechanism. g

Advantages provided by the present invention over some or all of the shutters of the prior art comprise the following:

• simpler operation and fewer moving parts;

• ability to operate at high repetition rate;

• short minimum time-to-open;

• scalable field of view with no adverse effects on performance;

• well suited to use in a vacuum, particularly when frictionless bearings are used;

• higher speed x-ray attenuation with greater repeatability and longer continuous use capability;

• spacers can be positioned within the voice coil to limit its motion to a range that provides the greatest response;

• complete asynchronous control can be used, with any timing configuration required;

• signals received can be synchronised with other instruments, the shutter thus operating as a slave, and not the master;

• scalable to accommodate various fields of view by introducing different coil conformations;

• good aperture format for synchrotron light sources, allowing for the use with wide beams;

• can be driven by a timing device or a detector;

• easier maintenance; and

• can be easily transported to different locations and readily installed, for example, by being bolted straight onto an optical table, rail stage or adaptor plate.

Furthermore, the present invention is not only suitable for specialist x-ray systems in the field Qf medical diagnostics and radiation therapy, but can be used to control the emission of radiation across the entire electromagnetic spectrum rendering it suitable for a wide range of uses and applications. These include: • medical diagnostics and therapy including radiotherapy;

• medical or industrial imaging, particularly biomedical imaging using techniques such as 3-dimensional particle image velocimetry; • applications in which the dose of electromagnetic radiation delivered must be very accurately controlled (eg applications in which a target dose is administered to kill or injure cancer cells);

• synchrotron applications, particularly those utilising a narrow/small diameter, high flux x-ray beam;

• • synchrotron applications utilising very high aspect ratio (eg. 6mm x

300mm or 1 :50);

• experiments using electromagnetic radiation such as laboratory scale (table-top) experiments involving x-rays or visible light from lasers, lamps, etcetera; and

• any experiments or other applications which require a high speed .camera with high rate data acquisition, for example to measure high flow rates of working fluids.

In the case of high aspect ratio beams (used typically with synchrotron x- rays), the linear design of the shutter provides efficient scaling over the prior art that includes an iris or rotary mechanism. In the case of shutters using an iris or rotary mechanism, the diameter is increased to accommodate the width of the beam. This inherently decreases the speed of the closing/opening time. The present invention can be scaled for increases in beam width without any losses in the speed of the closing/opening time.

Further scope of applicability of embodiments of the present invention will become apparent from the detailed description, given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which: • Figure 1 illustrates the main steps involved in operating the shutter;

• Figure 2 depicts the output of the electronics box operating the shutter;

• Figure 3 illustrates one embodiment of a shutter according to the present invention, the shutter being in the open position;

• Figure 4 illustrates the shutter of Figure 3 in the closed position;

• Figure 5 illustrates a further embodiment of a shutter having two heads according to the present invention, the shutter being in the open position;

• Figure 6 illustrates the shutter of Figure 5 in the closed position;

• Figure 7 illustrates a cross sectional view of the shutter of Figures 5 and 6; and

• Figure 8 illustrates the scalable nature of the invention for wide beam applications.

• Figure 9 is a plot of typical linear voice coil Force v. Stroke, showing a bell shaped profile and illustrating that there- is an Optimal operation zone¹ where the central stroke position outputs a larger force than in the lower and upper stroke regions;

• Figure 10 is a shutter time trace in which beam intensity is plotted against frame number when three different exposure signals are received.

• Figure 11 is a simplified schematic of a shutter according to the present invention in use at a synchrotron.

• Figure 12 is an illustration of x-ray PIV results showing recirculating blood flow in a glass capillary using a system incorporating a shutter according to the present invention.

DETAILED DESCRIPTION

The invention is powered by linear voice coil actuators, which offer simpler operation and fewer moving parts than shutters of the prior art, which typically operate using either:

• rotary voice coil motors that require greater shutter travel distance due to the rotary motion and are hence slower to close; or • solenoid shutters that are comparatively slow; or

• iris operation for a small beam window which suffers from the limitations imposed by leaf travel distance, has many moving parts and there is attenuation of the eye of the beam (highest flux) at the very end of the leaf travel.

When the x-ray shutter includes two linear voice coils having heads that approach each other from opposite directions, in the closed position both heads block the x-ray beam, thus providing double x-ray attenuation. Furthermore, the use of two heads provides a higher closure speed than a shutter with one head.

Figure 1 illustrates the operating sequence of the high speed linear voice coil shutter of the present invention. Specifically, when the initial voltage is applied, drive capacitors are charged under the influence of an electronic controller such as an LMD 18200 3A, 55V H-Bridge chip. The timing device sends a signal to the electronic controller to dump current through the coils. An electromagnetic field is generated due to the current in the coil loops that cause the voice coil head to be pushed away from the fixed magnet configuration. The electronic controller holds the coil in the closed position by applying a small 'holding' voltage. To open the shutter, the timing device sends another signal which is reversed through the circuitry, now sending current in the opposite direction of the voice coil's loops, pulling the voice coil heads back towards the rare each magnet core. The cycle repeats, triggered by the incoming TTL signal.

Figure 2 illustrates the output of a suitable custom built electronics box. The response time (that is, the time between sending the signal and the first noticeable movement of the shutter is approximately 8ms. In practice, shutters according to the present invention are capable of much faster response times. The power supply may need to be custom built to provide sufficient driving power for the electronics if commercially available power supplies do not provide enough power. A timing device coordinates an activation signal that dumps current over the voice coils. The current generates a magnetic field around a bobbin, such as a solid core magnet, to provide a driving force that moves the coil at very high velocity. A head on the voice coil is thus moved forward in a linear trajectory across a beam of radiation. A stop means such as a foam stopper halts the head at a position where it fully attenuates the radiation beam. At this point the shutter is fully closed and remains closed as long as the current is held. This current is not the same as the current used to drive the voice coil in the first instance. It is much, much lower, possibly in the vicinity of 5% of the peak current utilised, and it is specifically chosen for the circuit. Accordingly it is held at a lower "holding current".

Reversing the current to the voice coil provides a driving force in the opposite linear direction, moving the head back across the radiation beam to the fully open position. This motion is dampened at its extreme not by a foam stopper (as is movement in the opposite direction) but by a rubber spacer that has the dual function of increasing the initial displacement of the voice coil head (as discussed above) to optimise the force and hence acceleration, and also to provide a dampener for the opening motion of the head.

Voice coils provide high acceleration, as their force curve is not linear with reference to displacement position, but peaks through the mid-range of travel that arises from the inductance of the magnet surrounded by the coils of wire. Accordingly, the initial offset position of a voice coil is within this high force region of operation. This occurs because the rubber spacer mentioned above shifts the initial displacement into the "high force region". Other motors do not have this type of force relationship.

Using capacitor-coupled linear voice coil actuators in an arrangement of the type described above, it has been possible to achieve an extremely fast shutter movement of at least 700 microseconds with negligible jitter for travel of 6+ mm, with recorded performance readings for a slot that is 4mm in height. .

Figure 3 depicts a shutter according to one embodiment of the present invention in the open position. In this embodiment the shutter is encased within a housing comprising aluminium outer plates (1a, 1 b) and high density_, foam mounting pads (2) for dampening purposes and to hold the steel inner housing tightly within the outer housing. The shutter may be fixed to a translational stage. Alternatively it can be directly attached onto a standard optical table using a mounting plate. Both options eliminate the need to tediously align the shutter with the beam and target in the x and y planes and completely isolate any remaining movement to the foam isolation mounts.

The front and rear aluminium outer plates have been removed to show the internal components. The single, linear voice coil (3) having a radiation attenuating head (4) is fixed to the steel inner housing by a screw (5). In a preferred embodiment each voice coil consists of twin bobbin copper coils attached to a moving aluminium head which sits around a 12 mm diameter, cylindrical solid-core, rare earth magnet configuration.

The voice coil operates at up to 100G calculated from moving mass (including coil, head and leads). The inner housing chassis is typically a stainless steel construction, featuring slot apertures in the front and back faces. In Figure 3, a beam of radiation (6) is shown passing into the housing through an aperture, through a sample on the outside (far side) of the shutter. The x-rays reach the sample on the outside when the shutter is open and a detector (camera) captures the x-rays passing through the sample.

A high density foam pad (8) of a material such as neoprene acts as a damping end-stop to halt the head at one extreme of its movement. The inner housing sits on softer, low density foam isolation mounts (10) located in the bottom of the outer housing. These mounts (10) hold the inner housing in place, dampen vibration and reduce any residual motion of the inner housing caused by the rapid halting of the voice coil heads at their ends of travel. More low density foam is located adjacent the top of the outer plates of the housing, but is not shown in this view for clarity.

The overall rigidity of the device comes from the solid metal inner and outer housings, which have a mass ratio to the two moving voice coil heads of, for example >20:1 , or more preferably >25:1 or >27:1. The heavy mass of the housings reduced movement and provides long-term reliability. This construction has supported shutter runs of as much as 100,000 cycles without failure or need for replacement of parts. The simplicity of construction allows easy access for service and replacement.

When the head (4) moves to the fully closed position abutting the foam pad

(8) it blocks the beam of radiation (5) so that the beam no longer emerges from the aperture in the housing. Thus attenuation is provided by the head (4) of the voice coil's moving mass, which in this embodiment provides 25.4 mm of aluminium at the centreline (densest X-ray beam locality) of the window. The upper portion of the aluminium head provides radiation attenuation, which is particularly effective for attenuating soft x-rays. It is supplemented with the addition of a heavy metal sheet (such as tungsten, molybdenum or lead; nominally 1mm thick - or as required) placed perpendicular to the beam, directly in the centre of the coil's moving head and covers the entire width of the window adequately attenuates high X-ray flux by 99%. The combination of two different density metals (aluminium to block soft x-rays plus molybdenum or tungsten to block hard x-rays) at their corresponding thicknesses will effectively block a spectrum of x-rays.

Movement of the shutter can be optimised by inclusion of features such as DryLin™ low friction rails for improved stability, reliability and longer service intervals while maintaining lubrication free status required for operation of the shutter within a vacuum.

The shutter can also be configured with dual heads (20a, 20b), each on its respective voice coil (22a,22b) as illustrated in Figure 5 (in the open position) and Figure 6 (in the closed position). Figure 6 is a cross sectional view of the shutter of Figures 5 and 6. In a preferred embodiment the shutter signal inputs can be connected to a generic timing device and/or computer to activate the shutter at either regular intervals or asynchronously. Shutter signal outputs can be connected to camera or other detectors to synchronize image capture with shutter operation. For example, infrared sensors could be used with transistor-transistor logic (TTL) feedback. For this embodiment, the following are typical operation steps:

1. Within the controller, capacitors charge ready to dump current across the voice-coil(s) and the voice-coils are at the closed shutter position (voice coil heads extended against the stopper)¹. The coil heads having moved from opposing directions to close from both top and bottom simultaneously. 2. When a signal is sent to the shutter to open (input voltage is set to high) (a) A reverse signal is given from the timing device to the electronic device (to open), and the capacitors dump their current through the circuit to the voice-coils

(b) The high current through the voice-coils loops of wire generates a strong electromagnetic force (EMF).

(c) The generated EMF interacts with the fixed rare-earth magnet of the voice-coils, and drives the heads towards the magnet at high acceleration (d) The head of the voice coils moves away from the foam stoppers, opening the aperture of the shutter allowing an opening for the X- ray beam path.

(e) The holding current of the circuit (which has been set to a specified value based on the voice coils specifications) then holds the voice- coil in the open position.

3. Shutter remains in the open position until the signal is sent to the shutter to close (input voltage is set to low)

(a) The electronic controller commences dumping of the opposite charge current into the voice coils loops of wire

(b) Through the generated EMF in the opposite direction, the voice coil head(s) are driven away from the magnet(s)

(c) The voice coil heads move into the X-ray beam's path, attenuating the X-rays with the mass of aluminium voice coil head and optional insert to increase attenuation.

Having two heads move in opposite directions means:

• complete closure is achieved once voice coils pass each other across the midline of the aperture, reducing the closure time;

• once both voice coils are fully closed, - additional X-ray attenuation is • achieved by virtue of the mass of the two heads and inserts made of appropriate x-ray attenuating material, such as molybdenum; and

• a certain degree of redundancy is achieved because the shutter can still work with just one operational voice coil.

It will be appreciated that the material(s) of construction of the head and its structure could be configured such that when the head is in the path of the radiation beam the particle density (radiation intensity) can be controlled. Having two heads moving in opposite directions provides^' potentially greater control of the profile of radiation intensity over time.

In the case of high aspect ratio beams (used typically with synchrotron x- rays), the linear design of the shutter provides efficient scaling over the prior art that includes an iris or rotary mechanism. In the case of shutters using an iris or rotary mechanism, the diameter is increased to accommodate the width of the beam. This inherently decreases the speed of the closing/opening time. The present invention can be scaled for increases in beam width without any losses in the speed of the closing/opening time. This is simply illustrated in Figure 8 that demonstrates that multiple voice coils can be added to make up the width of any beam.

Linear voice coils have a distinctive bell shaped force versus stroke relationship as represented in Figure 9. The curved nature of the relationship arises from the inductance of the magnet surrounded by the coils of wire. By exploiting the curve's peak through the mid-stroke region, greater overall acceleration can be achieved by limiting the voice coil's total stroke length to this Optimal operation zone¹. This is achieved by choosing a voice coil with a longer rated stroke length than is necessary for the device. The initial displacement of the voice coil head is then moved to the lower boundary of the Optimal operation zone', for instance by non-magnetic spacer, and the total stroke travel is limited by the design of the housing. The force across the full .stroke travel is then maximised, at or near the peak force, to maximise acceleration overall. The high speed opening motion is hence dampened at its extreme, not by^' high density foam as is the case at the closing end of travel, but by a rubber spacer that has the dual function of increasing the initial displacement of the voice coil head to an optimal force location and acting as damper.

In the case of high aspect ratio beams (used typically with synchrotron x- rays) the linear trajectory of the shutter of the present invention provides efficient scaling by adding more voice coils to increase the width with no effect on performance. This is beneficial as compared to an iris or rotary mechanism shutter where the diameter must be increased to accommodate the width of the beam that is, both height and width must be increased where only an increase in one is required. Moreover, increasing the diameter inherently increases the time- to-open or time-to close.

The shutter may also provide degrees of attenuation resulting from alterations to the thickness and structure of the voice coil head as may, be required for use as a purely safety device. In particular the thickness and nature of any insert material place within it can be customised and optimised according to the wavelength nature of the radiation being attenuated. By controlling these features along with the shutter's timing and synchronisation, it is possible to control the degree of attenuation or the intensity of the radiation beam allowed to pass through the device. Overall the device is easily adaptable to suit various requirements.

Examples

The present invention will be further described with reference to the following non-limiting examples.

Example 1

In a specific example the shutter depicted in Figure 3 was used in the following manner. Drive capacitors were charged under the influence of an electronic controller (an LMD 18200 3A, 55V H-bridge chip). A timing device coordinated an activation TTL (transistor-transistor logic) signal that instigated the capacitors to dump the stored current across the two linear VCAs in a short time frame. The current generated a magnetic field around the bobbins, which were repelled by the magnetic field developed by the solid-core rare-earth magnet configuration, to provide a high driving force that moved the linear voice oil heads away from the magnet configuration at very high velocity, with an average acceleration of approximately 415g's and an average blade peed of 4.0m. s^"1 in a linear trajectory across the radiation beam. The moving heads were halted by the device's steel inner housing, placing the voice coil heads directly in the beam path where they fully attenuated the beam. At this point the shutter was fully closed and remained closed with a 'holding' current of approximately 5% of the peak current (specifically chosen to suit the component ratings of the circuit). In the event of circuit failure, redundancy was achieved through the vertical orientation of the voice coils, because one of the voice coil heads sat in the closed position due to gravity and the absence of an electromagnetic field.

Reversing the current to the coils through the H-bridge provided a driving force in the opposite linear direction, moving the voice coil heads back to the fully open position, exposing the beam. At this point the timing device had accurately synchronised the detector to begin acquiring the knages. Example 2 - Testing of the shutter

During the time-to-open (rise time) and the time-to-close (fall time) the target receives extraneous closes of x-rays that- do not contribute to the image.

- Excessive or continuous radiation may cause permanent deterioration of the sample, hence minimising unnecessary does by shortening the rise time and fall time is critical.

High speed photography at 115,609 fps was used to test the linear voice coil shutter's open, closed and jitter parameters. The device was placed between a high intensity light and a high speed IDT MotionPro Y6 CMOS camera. The beam intensity as seen by the detector (camera) was plotted against frame number (represented as time lapse in ms), when three different exposure time signals were sent by the timing device to the shutter. Figure 10 illustrates the outputs representing the subsequent rise time, fully-exposed duration (window time) and fall time of the shutter for each different exposure signal sent. The total pixel intensity depicts the number of pixels which have captured a grey level by the CMOS detector in that particular frame, that is, when the shutter is fully closed, all pixels are black and zero grey levels are detected; when the shutter is fully open, all pixels are illuminated by some grey level and count towards the total pixel intensity. In this instance the maximum total pixel intensity is approximately 350,000, as governed by the CMOS detector's pixel size and by matching the field to the aperture.

The plots show smooth operation across all exposures and minimal bounce at the ends of travel. The 15ms plot represents signal cycles where the capacitor is fully charged at the commencement of open and close. The 3ms plot represents a shorter exposure signal sent where the maximum pixel intensity is still witnessed from the captured image, that is, the aperture is fully open when the image is acquired. It should be noted that although the image pixel intensity is unaffected, the time-to-close performance has been reduced as seen by the gentler gradient of the fall-time. This is caused by the capacitors inability to fully recharge in the time between dumping the current to open the shutter, and receiving the signal to dump the current to close the shutter. As the input signal gets smaller the fall time performance diminishes. The 1.5ms plot represents the shutter performance when the duration between open and close signals is much shorter than both the minimum rise time and shorter than the capacitors ability to fully recharge. When the signal is received to close the shutter, the shutter is only partially open represented by the reduced peak of total pixel intensity. The shutter then reverses direction to close ant the total pixel intensity drops back to zero. Although the peak total pixel intensity is greatly reduced for the 1.5ms signal, the shutter operates as if drawing back two curtains, and the central horizontal portion of the image will capture pixels at full quality and intensity.

The minimum time-to-open, not including electronic lag from the controller, is represented in the plots as the corresponding rise-times of 700 microsecs to maximum total pixel intensity. This -sub-millisecond rise-time represents an advantage over high-speed x-ray shutters of the prior art.

Very little jitter or bounce was experienced, due to the frictionless operation and overlapping mechanism of the voice coils.

The shutter provides asynchronous, pulsed operation with a minimum window time (fully-closed to fully-open to fully-closed) of 2.8 ms and is fully continuous up to 50 Hz. These values are not limited by .the maximum speed of the voice coil heads but by the residual EMF generated by the coil at high frequency. The EMF has a minimum discharge time, preventing fully continuous operation up to the theoretical maximum repetition rate of 360Hz.

Example 3 - Comparison

Table 1 sets out a performance comparison of high-speed x-ray shutters of the prior art.

Table 1:

*Width can be increase with no adverse effect on minimum time-to-open

The Uniblitz™ XRS shutters yield reasonable minimum times-to-open for their corresponding aperture widths. The circular nature of their apertures limits the scalability of these shutters, as can be seen by the manufacture of three different sizes and by the increased minimum time-to-open for the larger aperture versions.

The shutters of Lee et al (Lee SJ, Kim GB₁ Yim DH & Jung SY (2009) Review of Scientific Instruments 80, 033706) and Gembicky et al (Gembicky M, Oss D, Fuchs R & Coppens P (2005) Journal of Synchrotron Radiation 12, 665- 669) comprise of a spinning disk type (slotted rotary wheel chopper) whereby the imaging system must be synchronised accurately to the shutter. This makes for very complex experimental set ups that are inflexible with respect to the exposure times and inter-frame ratios. Gembicky et al (2005) has a very small aperture which restricts field of view, but hence is able to achieve high repetition rates with microsecond minimum time-to-open. However, very high flux is required from the synchrotron source for practical application at this maximum speed. The shutter of Lee et al (2009) has a significantly larger field of view and provides a good minimum time-to-open but subsequently can only operate at low repetition rate.

The Cammarata et al (Cammarata M, Eybert L, Ewald F, Reichenbach W₁

Wulff M, Anfinrud P, Schotte F₁ Plech A, Kong Q₁ Lorenc M₁ Lindenau B₁ Rabiger J & Polachowski S (2009) Review of Scientific Instruments 80, 015101) system is a sophisticated device custom-built to isolate single bunches for pump-probe experiments with ultrafast lasers. The asynchronous millisecond shutter component serves to lower the default frequency offered by the other two rotary chopper components. The shutter has a fixed 0.4mm x 0.07mm aperture to match the 1D09B beam line and due to its specific functionality offers a small range of exposures based on the device's tunnel geometry.

Example 4 - In situ testing

A shutter according to the present invention has been tested and use for experiments at both the Spring-8 Synchrotron (Hyogo, Japan) and the Australian Synchrotron (Melbourne, Australia). The shutter design was optimised for use on the BL20XU Beamline at the Biomedical Imaging Centre at the Spring-8 synchrotron, leading to its 12mm x 4mm aperture.

The system arrangement is shown schematically in Figure 11. The shutter (30) according to the present invention was placed in a beam that had passed from the synchrotron storage ring (32), through an undulator (33), then a monochromator (34). It was placed adjacent the monochromators (34) and directly ahead of a sample (36), aligned so that the front and back apertures matched. This was achieved through the mounting holes located at the bottom of the shutter's outer casing. Alignment in the z plane was performed with a z stage, and snapshot photography of the sample until the aperture matched with the desired field of view of the sample (36). A detector (38) picked up the beam after it had passed through the sample (36).

Figure 12 illustrates an example of x-ray PIV measurements of blood flow achieved with the use of the linear voice coil shutter of the present invention. The schematic of the capillary is overlaid with the velocity vector field and streamlines. The recirculation is caused by rapid injection of a bolus of contrast agent immediately prior to imaging. The experiment was only possible with reduced dose to the sample to prevent cell damage and clotting. The analysed images were captured at 3ms exposures, at an instantaneous frame rate of 300 fps for each image pair.

The raw phase-contrast images contained clarity with little smear, features that allowed the acquisition of data quality needed for PIV analysis. Use of the shutter also provided accurate radiation dose control to the sample as the shutter was only triggered to open with the acquisition of an image. If this were not the case, the fluid would heat up, promoting natural convection and would alter the fluid dynamics exhibited. In the cases where blood and tissue are used, dose reduction is paramount to minimising cell damage and clotting.

While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.

Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the Invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures. For example, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface to secure wooden parts together, in the environment of fastening wooden parts, a nail and a screw are equivalent structures.

It should be noted that where the terms "server", "secure server" or similar terms are used herein, a communication device is described that may be used in a communication system, unless the context otherwise requires, and should not be construed to limit the present invention to any particular communication device type. Thus, a communication device may include, without limitation, a bridge, router, bridge-router (router), switch, node, or other communication device, which may or may not be secure.

It should also be noted that where a flowchart is used herein to demonstrate various aspects of the invention, it should not be construed to limit the present invention to any particular logic flow or logic implementation. The described logic may be partitioned into different logic blocks (e.g., programs, modules, functions, or subroutines) without changing the overall results or otherwise departing from the true scope of the invention. Often, logic elements may be added, modified, omitted, performed in a different order, or implemented using different logic constructs (e.g., logic gates, looping primitives, conditional logic, and other logic constructs) without changing the overall results or otherwise departing from the true scope of the invention.

Various embodiments of the invention may be embodied in many different forms, including computer program logic for use with a processor (e.g., a microprocessor, microcontroller, digital signal processor, or general purpose computer), programmable logic for use with a programmable logic device (e.g., a Field Programmable Gate Array (FPGA) or other PLD), discrete components, ^' integrated circuitry (e.g., an Application Specific Integrated Circuit (ASIC)), or any other means including any combination thereof. In an exemplary embodiment of the present invention, predominantly all of the communication between users and the server is implemented as a set of computer program instructions that is converted into a computer executable form, stored as such in a computer readable medium, and executed by a microprocessor under the control of an operating system.

Computer program logic implementing all or part of the functionality where described herein may be embodied in various forms, including a source code form, a computer executable form, and various intermediate forms (e.g., forms generated by an assembler, compiler, linker, or locator). Source code may include a series of computer program instructions implemented in any of various programming languages (e.g., an object code, an assembly language, or a high- level language such as Fortran, C, C++, JAVA, or HTML) for use with various operating systems or operating environments. The source code may define and use various data structures and communication messages. The source code may be in a computer executable form (e.g., via an interpreter), or the source code may be converted (e.g., via a translator,. assembler, or compiler) into a computer executable form. ^{• •}

The computer program may be fixed in any form (e.g., source code form, computer executable form, or an intermediate form) either permanently or transitorily in a tangible storage medium, such as a semiconductor memory device (eg, a RAM, ROM, PROM, EEPROM, or Flash-Programmable RAM), a magnetic memory device (e.g., a diskette or fixed disk), an optical memory device (e.g., a CD-ROM or DVD-ROM), a PC card (e.g.,^" PCMCIA card), or other memory device. The computer program may be fixed in any form in a signal that is transmittable to a computer using any of various communication technologies, including, but in no way limited to, analog technologies, digital technologies, optical technologies, wireless technologies (e.g., Bluetooth), networking technologies, and inter-networking technologies. The computer program may be distributed in any form as a removable storage medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the communication system (e.g., the Internet or World Wide Web).

Hardware logic (including programmable logic for use with a programmable . logic device) implementing all or part of the functionality where described herein may be designed using traditional manual methods, or may be designed, captured, simulated, or documented electronically using various tools, such as Computer Aided Design (CAD), a hardware description language (e.g., VHDL or AHDL), or a PLD programming language (e.g., PALASM, ABEL, or CUPL).

Programmable logic may be fixed either^' permanently or transitorily in a tangible storage medium, such as a semiconductor memory device (e.g., a RAM, ROM, PROM₁ EEPROM, or Flash-Programmable RAM), a magnetic memory device (e.g., a diskette or fixed disk), an optical memory device (e.g., a CD-ROM or DVD-ROM), or other memory device. The programmable logic may be fixed in a signal that is transmittable to a computer using any of various communication technologies, including, but in no way limited to, analog technologies, digital technologies, optical technologies, wireless technologies (e.g., Bluetooth), networking technologies, and internetworking technologies. The programmable logic may be distributed as a removable storage medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over the communication system (e.g., the Internet or World Wide Web).

"Comprises/comprising" and "includes/including" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, Integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', 'includes', 'including' and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A shutter for a beam-of electromagnetic radiation, the shutter comprising at least one voice coil having a radiation attenuating head,

wherein actuation of the voice coil moves the head on a linear trajectory between a first position wherein the beam of electromagnetic radiation is unattenuated and a second position wherein the beam of electromagnetic radiation is attenuated.

2. A shutter for a beam of electromagnetic radiation according to claim 1 , the shutter comprising two voice coils, each having a radiation attenuating head, wherein actuation of the voice coils moves each head on opposing linear trajectories between a first, position wherein the beam of electromagnetic radiation is unattenuated and a second position wherein the beam of electromagnetic radiation is fully attenuated.

3. A shutter according to claim 1 or claim 2 which can provide attenuation of the electromagnetic beam of >99%. 4. A shutter according to claim 1 or claim, 2 wherein the electromagnetic beam is generated by a synchrotron, and the shutter provides attenuation of >99%.

4. A shutter for a beam of electromagnetic radiation according to claim 1 which further comprises;

a non-magnetic inner housing for thB at least one voice coil, an end stopper for halting the voice coil head at the second position, an outer housing,

isolation mounts intermediate the inner housing and the outer housing for damping the effects of the voice coil movement.

5. A shutter according to claim 4 wherein the ratio of the total mass the inner and outer housings to the mass of the head of the voice coil is >20:1.

6. A shutter according to any one of the preceding claims wherein the minimum window time is <2.8 ms.

7. A shutter according to any one of the preceding claims which operates at better than 500 Hz, preferably better than 300 Hz₁.

8. A method of attenuating a beam of electromagnetic radiation using the shutter of claim 1 , the method comprising the steps of:

initiating a flow of current in a circuit to drive the voice coil head from the second position to the first position on a linear trajectory at high acceleration,

applying a holding current to the circuit to maintain the voice coil head in the first position, and

reversing the current flow in the circuit to drive the voice coil head from the first position to the second position on a linear trajectory at high acceleration,

wherein the initiation, application and reversing of current is asynchronous and under electronic control by a timing device or detector.

9. A method according to claim 8 when used for an application chosen from the group comprising imaging, diagnostics, therapy, measurement or combinations thereof.

10. A method according to claim 8 when used for attenuating a beam of electromagnetic radiation having a high aspect ratio.

11. A system for imaging a sample comprising;

an electromagnetic beam from a synchrotron,

an undulator for producing a beam of selected energies for transmission to a monochromator,

a monochromator for selecting a range of band lengths to provide a narrowed beam, a shutter according to claim 1 or claim 2 through which the narrowed beam passes,

the sample located in the path of the narrowed beam,

a detector for detecting the narrowed beam after it passes through the sample and creating a data set that is capable of translation into an image.