Abstract:
A method of imaging motion of an organ that changes volume in a patient including the steps of monitoring change in volume of the organ, and recording multiple in vivo images of the organ, wherein the change of organ volume between the images is constant or of some other predetermined value.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to the field of imaging, particularly imaging of organs such as the lung. 
         [0002]    In one form, the invention relates to the field of imaging organs, such as the lung for diagnostic or research purposes. 
         [0003]    In one particular aspect the present invention is suitable for use as a technique for assessing organ motion. 
         [0004]    It will be convenient to hereinafter describe the invention in relation to human medical applications however, it should be appreciated that the present invention is not limited to that use only and could, for example, be used for veterinary applications. 
         [0005]    Furthermore, it will be also convenient to hereinafter describe the invention in relation to CTXV, however it should be appreciated that the present invention is not limited to that imaging technique and could, for example be used with other forms of imaging including X-ray computer tomography (CT), particularly 4D-CT, fluoroscopy, MRI, ultrasound or any other imaging method that acquires multiple images over time. 
       BACKGROUND ART 
       [0006]    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&#39;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. 
         [0007]    In the past, X-ray CT has been extensively used in radiography to investigate the anatomy and function of the lungs. CT uses computer processed X-rays to produce tomographic images (virtual ‘slices’) of specific areas of a lung. Digital geometry processing is used to generate a three-dimensional image of the inside of the lung from a series of two-dimensional radiographic images taken around a single axis of rotation. 
         [0008]    X-ray slice data is generated using an X-ray source that rotates around the object in a circular shroud. X-ray sensors are positioned on the opposite side of the circle from the X-ray source. Early versions of the imaging machines operated by rotating the X-ray source and detectors around a stationary patient. Following each complete rotation, the patient would be moved axially and the next rotation carried out. Newer machines have been designed to allow continuous rotation of the X-ray source and detectors while the patient is slowly and smoothly slid through the X-ray circular shroud. These are called helical or spiral CT machines. 
         [0009]    CT can be used for imaging most body structures. It is particularly useful for detecting both acute and chronic changes of structure inside the lungs. These changes may not be detectable using conventional two-dimensional X-ray imaging. A variety of techniques are used. For example, evaluation of chronic interstitial processes such as emphysema or fibrosis is carried out by taking thin slices of data and applying high spatial frequency reconstructions. 
         [0010]    One of the principal limitations of these approaches is the need to image the lung while it is stationary in order to minimise blurring. In particular, CT has poor temporal resolution limiting its use for a dynamic lung test. CT scanning is typically performed at two different time intervals—upon breath hold at inspiration or expiration—usually minutes apart. 
         [0011]    This has obvious drawbacks and limits the ability of CT to be used for dynamic lung function testing. Specifically, it produces a sampling of the lung and an interpolation is required to deduce lung motion between the two steady state conditions, but such methods assume that the motion follows a linear or defined path. They cannot produce continuous real-time images that would provide information regarding lung motion and be used to detect diseases that cause subtle changes in lung structure or function. 
         [0012]    With the advent of fast CT scanning, it has been possible to develop methods called 4D-CT that capture images over time to depict movement of body organs, such as inflation and deflation of a lung. A typical 4D-CT scan involves 10 to 50 rotations around the patient, each coordinated with the table moving through the circular shroud. Fast scanning can be carried out at a rate of about 4 rotations/sec, with the detector taking about 1000 ‘snapshots’/rotation. Each ‘snapshot’ is carried out at one position (angle or ‘projection’) of the X-rays source. Typically between 40,000 to 200,000 images are collected in a 4D-CT. 
         [0013]    For example, 4D-CT has been used for measurement of lung function, including expansion using traditional absorption based imaging, but has the drawback of delivering significant levels of radiation dose to the patient. Visualising, controlling and tracking patient specific respiratory motion is key to more precisely targeted irradiation treatment for disorders such as chest and abdominal cancers that move with the diaphragm. Motion of cancers causes problems with irradiation treatment because moving targets may appear with distorted shapes and in wrong locations on CT images. In order to compensate for this, larger irradiation fields and concomitant large radiation doses are used to ensure that the tumour is not missed. 
         [0014]    Accordingly, respiratory gating may be combined with the 4D-CT for treating ‘moving’ tumours such as lung tumours. One approach is through the use of a small “box” placed on the patient&#39;s chest/upper abdomen and specialised cameras are used to monitor the motion of this box during respiration. This information is used to correlate the position of the lung tumour with specific phases of the respiratory cycle. At treatment, motion of the box allows the treatment beam to be turned on and off (gating) during specific phases of the breathing cycle. (Jiang S B et al,  Int J Radiat Oncol Biol Phys  2008; 71:S103-7 ; Phys Med Biol  2008; 53:N315-27). 
         [0015]    Use of CT has increased dramatically over the last two decades. An important issue within radiology today is how to reduce the radiation dose during CT scanning without compromising the image quality. In general, higher radiation doses result in higher-resolution images, while lower doses lead to increased image noise and blurred images. However, higher radiation doses increase adverse side effects, including the risk of radiation induced cancer. 
         [0016]    In the past, attempts have been made to reduce exposure to ionizing radiation during a CT scan including: 
         [0000]    1. new software technology to more effectively utilise the data recorded at the detector,
 
2. individualising the scanning and adjusting the radiation dose to the body type and body organ examined, and
 
3. evaluating the appropriateness of avoiding CT scanning in favour of another type of examination.
 
         [0017]    Notwithstanding these efforts, the relatively high radiation dose imposed on a patient by a CT scan (especially by a 4D-CT scan), is a key problem acting against the increased use of CT. In addition to CT, there are many other forms of imaging capable of dynamic imaging of dynamic biological processes, such as breathing. 
         [0018]    The fluoroscope is ideally suited to dynamic imaging of the thorax and is used in a wide range of procedures such as barium swallow examinations, cardiac catheterization, arthrography, lumbar puncture, placement of intravenous (IV) catheters, intravenous pyelogram, hysterosalpingogram, and biopsies including bronchoscopy. Fluoroscopy may be used alone as a diagnostic procedure, or may be used in conjunction with other diagnostic or therapeutic media or procedures. 
         [0019]    One specific application of the fluoroscope, and related medical imaging configurations, is in the acquisition of dynamic sequences for CTXV analysis. CTXV analysis is possible from acquisition sequences including (but not limited to): multiple images acquired from multiple perspectives (projections); multiple images acquired from a number of perspectives (projections) simultaneously; and, continuous acquisition with a moving perspective. 
         [0020]    The work of Keall et al (US20140192952) attempts to reduce the clustering of projections that occurs when gating to a respiratory signal and using a constant rotation speed in 4DCBCT. To avoid this clustering of projections and achieve an even distribution of projections around the subject, Keall changes two variables: (1) the rotation speed of the gantry and (2) the time interval between projections. 
         [0021]    With the advent of advanced imaging systems such as those used in CTXV and advances with 4D-CT technologies, fast imaging rates are achievable. These higher rates of image acquisition allow for more complex timing of when images are acquired throughout a dynamic event. This increased speed of acquisition allows for enhanced temporal resolution of rapidly occurring dynamic processes or events. 
         [0022]    Accordingly there is an ongoing need to improve the quality of information derived from scanning without increasing, or preferably decreasing patient radiation exposure. 
         [0023]    In the field of imaging based motion analysis, a key parameter that affects the image quality and dynamic range of the measurement is the magnitude of the sample&#39;s displacement that occurs between sequential images. This will be determined by the speed of the sample and the rate of acquisition. If the sample displacement is too large or too small, the measurement quality will be degraded. Imaging of samples that exhibit a large range of speeds during the measurement sequence using a constant acquisition rate will therefore inevitably cause degraded measurement quality for some of the frames. 
         [0024]    In the field of imaging based motion analysis there are several techniques that are commonly employed to improve the quality of the motion measurement or the dynamic range of the measurements. One such example is the use of a ‘frame skip’ in a PIV (particle image velocimetry) analysis. This is where the image analysis compares images that are more than 1 image apart. As such the optimum signal to noise can be obtained for slower particles that have a very small displacement between sequential images. 
         [0025]    When using X-ray images as the input, techniques such as these result in an increase in radiation dose imparted to the subject as extra (redundant) images are acquired. In X-ray imaging, extra images equate to extra dose delivered to the subject. Therefore for motion measurement from X-ray images, such as in a CTXV analysis, other techniques must be employed to overcome this. 
       SUMMARY OF INVENTION 
       [0026]    An object of the present invention is to provide improved techniques for assessing lung function and diagnosing lung conditions. 
         [0027]    Another object of the present invention is to provide improved technology for obtaining data of lung motion. 
         [0028]    A further object of the present invention is to alleviate at least one disadvantage associated with the related art. 
         [0029]    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. 
         [0030]    In a first aspect of embodiments described herein there is provided a method of imaging motion of an organ in a patient including the steps of: 
         [0031]    monitoring a parameter corresponding to motion of the organ, and 
         [0032]    acquiring multiple in vivo images of the organ, 
         [0000]    wherein the timing of image acquisition is correlated with a predetermined change in parameter value. 
         [0033]    Where used herein the term ‘organ’ is intended to refer to any collection of tissues joined in a structural unit to serve a common function, or part thereof, and includes the heart, lungs and blood vessels. In a particularly preferred embodiment the organ comprises one or both lungs or lobes thereof. 
         [0034]    The aforesaid correlation may be direct, or indirect correlation. 
         [0035]    In a more specific aspect of the aforesaid embodiment there is provided a method of imaging motion of an organ that changes volume in a patient including the steps of: 
         [0036]    monitoring change in volume of the organ, and 
         [0037]    acquiring multiple in vivo images of the organ, 
         [0000]    wherein the timing of image acquisition is correlated with change of organ volume between the images. 
         [0038]    Preferably the change in parameter value between images is a predetermined value, which may be constant or varied over time. More preferably the predetermined value is a constant value. The parameter value relates to change in any one or any combination of shape, volume, flow, pressure, shear, displacement, temperature, colour, orientation or location of the organ. In a particularly preferred embodiment the parameter value is volume. 
         [0039]    Further to this, it will be appreciated by a person skilled in the art, that flow may be integrated to provide volume information and volume may be differentiated to provide flow information. As such either flow or volume signals can be considered to contain the same information. 
         [0040]    Pressure is a related parameter to flow and volume, and pressure has the advantage of being a simpler and more common measure that is clinically made. In one embodiment of the present invention, pressure alone is used as a direct physiological measure and to this trigger the imaging system. 
         [0041]    When the organ being imaged is the lung, lung tissue motion or displacement may be measured from many means including imaging methods such as those discussed throughout this disclosure. Measurements of lung tissue motion allow for accurate optimization of the desired signal timing when used for a motion analysis of the lungs, thus allowing a real-time feedback system. 
         [0042]    Measurements of chest wall displacement are also related to the displacement of lung tissue and in particular a change in lung volume. Chest wall displacement may be measured from a number of techniques including laser-based grids projected onto the subject, markers placed on the subject and stereo video techniques. Measuring the chest wall displacement can often have the advantage of not requiring any equipment to contact the patient. It will be appreciated by a person skilled in the art that many other physiological measurement techniques and many other physiological parameters can be used to infer information of the displacement or volume change of lung tissue. 
         [0043]    In the past, images of organs that change volume, such as the heart, blood vessels or lungs have been recorded at a constant imaging rate—typically the fastest possible or at a rate sufficient to capture the most rapidly occurring dynamic event. For systems where events are not occurring at a constant rate, the desired temporal resolution (in a relative sense) is not constant, potentially leading to an excess of captured images. An excess of images may correlate to an increase of cost for analysis, review and storage of those images. In the case of X-ray imaging systems, an excess of images results directly results in an excess of X-ray dose. In the case of the lungs, this leads to a large proportion of images being captured at the end of inspiration and the end of expiration where very little change in the lung occurs, with only a small proportion of images being captured at the most transient moments, being during peak inspiratory flow or peak expiratory flow. 
         [0044]    Specifically in the case of CTXV imaging the accuracy of the airflow measurements is a function of the change in volume of the system over the imaging period. Small changes in volume over a measurement period may result in large errors relative to the signal. Large changes in volume over a measurement period may result in large errors due to inadequate temporal resolution of non-linear phenomena. Similarly, large errors may result from sub-optimal cross-correlations that result from a large volume change over the measurement period. 
         [0045]    By contrast, the method of the present invention, is directed to optimising the quantification possible from dynamic imaging sequences. It is expected that this approach will minimise the number of images captured outside of times of most rapid change, concomitantly minimising the dosage of radiation to the subject. In the case of measuring airflow within the lungs, the method of the present invention allows the optimisation of the imaging sequence, both minimizing airflow measurement errors and X-ray dose. 
         [0046]    The change of organ volume may be monitored by any convenient means, including measurement of another directly related physiological parameter, such as airflow or blood pressure. These include, in the case of lungs, airflow at the mouth, spirometry, chest wall measurements using a laser generated grid or image, or a band around the thorax, or fitting to standard respiration curves. In the case of the heart or blood vessels they can include measurements of blood pressure or blood flow. It will be readily apparent to the person skilled in the art that the change of organ volume is not limited to these methods and many other monitoring means could be used. For instance, when imaging the heart, using an ECG to infer the temporal variations of blood volume may be used. 
         [0047]    In a second aspect of embodiments described herein there is provided a method of imaging motion of an organ in a patient including the steps of: 
         [0048]    monitoring change in a parameter corresponding to motion of the organ, such as change of volume of the organ to create monitoring data that is fed in real time to an imaging control system, and 
         [0049]    acquiring multiple in vivo images of the organ in response to an actuation signal from the control system, 
         [0000]    wherein the timing of image acquisition is correlated with predetermined change in parameter value. 
         [0050]    Typically the predetermined change in parameter value is varied over time, or alternatively is constant. The collection of images corresponding to a constant or other predetermined parameter change of an organ may be achieved by any convenient means. 
         [0051]    In one preferred embodiment, images may be collected when the organ changes volume by a predetermined amount by use of HV switching of the kV generator, or shuttering the X-ray source/detector, or by altering the manner in which a radiation beam impinges on the target organ, or other triggering means. 
         [0052]    In another aspect of embodiments described herein there is provided an application to collect images according to the method of the present invention, said application comprising a predetermined instruction set for initiating recordal of images that correspond to a predetermined change of organ volume. 
         [0053]    In a further aspect, the method of the present invention may include the step of measuring a data field of organ motion between two or more of the acquired images. 
         [0054]    The acquisition of images according to the present invention may be carried out from multiple perspectives. In a particularly preferred embodiment the multiple in vivo images are recorded from two or more perspectives, each image comprising encoded data, and a spatial velocity data field of organ motion is reconstructed from the encoded data. 
         [0055]    In another embodiment of the method of the present invention, the method further includes monitoring change in the parameter to create monitoring data that is fed in real time to an imaging control system, and 
         [0056]    acquiring multiple in vivo images of the organ comprising encoded data from a first perspective in response to an actuation signal from the control system, 
         [0057]    acquiring multiple in vivo images of the organ comprising further encoded data from at least one further perspective in response to an actuation signal from the control system, 
         [0058]    and subsequently, reconstructing a spatial velocity data field of organ motion from the encoded data. 
         [0059]    In yet a further aspect of embodiments described herein there is provided, in an apparatus comprising:
   (i) one or more energy source; and   (ii) one or more detectors for acquiring images created by energy from the one or more energy sources passing through a subject intermediate the energy source and detector;
 
a signal processing unit comprising a sensor to monitor the change in volume of the organ, the output of the sensor being analysed to provide an input to trigger the timing of image acquisition in accordance with the present invention.
   
 
         [0062]    In another aspect of embodiments described herein there is provided a computer readable storage medium for storing in non-transient form an application to enable monitoring, acquiring images and timing according to the method of the present invention. 
         [0063]    In a yet further aspect of embodiments described herein there is provided an application stored on non-transitory medium adapted to enable imaging, said application comprising a predetermined instruction set adapted to enable a method according to method of the present invention. 
         [0064]    The person skilled in the art will appreciate that the method of the present invention can be used in conjunction with any well known imaging technique. Preferably the imaging of motion according to the present invention is carried out using an imaging method chosen from the group comprising computer tomographic X-ray velocimetry, computer tomography, cone-beam computer tomography, fluoroscopy, magnetic resonance imaging or ultrasonography. 
         [0065]    The patient may be human or another animal and the present invention is thus useful for a range of human and veterinary applications in the fields of diagnostics and research. 
         [0066]    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. 
         [0067]    In essence, embodiments of the present invention stem from the realization that the existing time based paradigm of imaging is not suitable for organs that move, especially when in a non-linear fashion, when an analysis of the motion of the organ is carried out. In the case of the lung, it maximises collection of images during periods of greatest diagnostic interest in terms of motion and minimises collection of images during periods of least diagnostic interest in terms of motion. 
         [0068]    By dispensing with the existing paradigm of imaging based on time, significant advantages are provided by the present invention including the following:
       collection of images is maximised at times when the most useful information can be obtained and minimised at other times, improving efficiency of data collection and quality of motion analysis,   improvement in functional motion data available for clinical diagnosis and treatment,   data collection and storage is reduced compared to conventional imaging, improving computing efficiency,   patient radiation dosage is minimised,   lower operating power can be used for scanners, reducing required cooling times and increasing patient throughput and overall economy.       
 
         [0074]    One embodiment of this technique aims to cluster the images into each projection, by taking multiple images from each view. This then allows for a calculation of motion from each aforesaid view and therefore a reconstruction of the full 4D motion field of the lungs. Techniques for image reconstruction such as CT, 4DCT and 4DCBCT aim to distribute the projections evenly around the patient from a large number of views. 
         [0075]    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 
         [0076]    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: 
           [0077]      FIG. 1  illustrates lung inspiration and expiration by plots of lung volume against time (sec) ( FIG. 1 a   ) and lung air pressure (cm/H 2 O) versus time (sec) ( FIG. 1 b   ); 
           [0078]      FIG. 2  is a plot of lung volume against time as shown in  FIG. 1 a   , marked up to indicate a scanning regime according to the present invention during inspiration and expiration ( FIG. 2 a   ) and during expiration alone ( FIG. 2 b   ); 
           [0079]      FIG. 3  is a plot of lung volume against time as shown in  FIG. 1 a   , marked up to indicate a scanning regime according to the prior art during inspiration and expiration ( FIG. 3 a   ) and during expiration alone ( FIG. 3 b   ); 
           [0080]      FIG. 4  illustrates a typical arrangement of components in a CTXV scanner. 
           [0081]      FIG. 5  illustrates the output from the application as it detects when to trigger the imaging system based on a constant and preset change of volume ( 5   a ) and the non-linear time base is depicted on a subset of the same data ( 5   b ). 
           [0082]      FIG. 6  is a flow chart summarizing the function of one embodiment of the application of a system according to the present invention. 
           [0083]      FIG. 7  is a graph illustrating radiation dose savings according to the present invention for preset volume change value for a system that is ordinarily run at 30 fps. This data can also be seen in Table 1. 
           [0084]      FIG. 8  illustrates the system and apparatus used for the method of this invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0085]      FIG. 1 a    is a plot of lung volume (litres) versus time (sec) during inspiration and expiration of a human lung. The plot illustrates inspiratory reserve volume (IRV)  1 , VT  2 , expiratory reserve volume (ERV)  3 , residual volume (RV)  4 , functional reserve capacity (FRC)  5 , inspiratory capacity (IC)  6 , vital capacity (VC)  7 , total lung capacity (TLC)  8   
         [0086]      FIG. 1 b    is a plot of pressure (cm/H 2 O) versus time (ms) for inspiration and expiration of a human lung. The flow of air into and out of the lung can be correlated with the change in volume of the lung as shown in  FIG. 1   a.    
         [0087]    In the past, images of organs that change volume, such as the heart, blood vessels or lungs have been recorded at the fastest imaging rate possible with a constant time between images. 
         [0088]    With reference to lungs, this was an attempt to capture the fast motion that occurs during the beginning of the inspiratory cycle and the beginning of the expiratory cycle.  FIG. 3  is a plot of lung volume (litres) against time (sec) that illustrates sequential scanning according to the prior art technique during inspiration and expiration ( FIG. 3 a   ) and during expiration only ( FIG. 3 b   ) with a constant time period between each image. 
         [0089]    However, as illustrated in  FIG. 3 a   , this will result in many images being captured during the end of inspiration and expiration where the volume (and pressure) curves have begun to asymptote towards their final values and little lung motion is actually happening. 
         [0090]    By contrast,  FIG. 2  illustrates volume based gating for scanning according to the present invention with a constant lung volume change between each image.  FIG. 2  is a plot of lung volume against time during inspiration and expiration ( FIG. 2 a   ) and during expiration alone ( FIG. 2 b   ). Thus, for a predetermined unit of air flow or change in lung volume, a predetermined number of images can be taken. This optimises the proportion of images taken during the course of inspiration and expiration when the lung is working hardest and the most information regarding its health can be obtained. It also minimises the number of images taken at other times, such as the end of inspiration and expiration when the lung is not working as hard, concomitantly minimising the dosage of radiation. 
         [0091]    The method of the present invention does not require images to be taken as fast as possible at over-power levels. Instead, using the method of the present invention the CTXV scanner can be run at a lower power level without the need to cool-down, or for a shorter cool-down period. This allows more efficient throughput of patients, lower power usage, and thus overall better economy of usage of a CTXV scanner. 
         [0092]    The current invention is particularly valuable when used in CTXV imaging and CTXV scanners of the prior art as depicted in  FIG. 4 .  FIG. 4  is a schematic diagram outlining the basic design of a CTXV system according to the present invention. The diagram shows three polychromatic X-ray beams transmitted through a sample and converted to visible light by scintillators. High-speed detector systems then produce a set of images. Multiple projection data are gathered simultaneously without rotating the sample. Coordinates (typically Cartesian co-ordinates (x,y,z)) are fixed to the sample and rotated at an angle θ from the beam axis p. With reference to  FIG. 4 , each imaging line would typically consist of the following key components: 
         [0093]    a. video speed or double shutter X-ray camera ( 21 ); 
         [0094]    b. cone beam X-ray source ( 22 ); 
         [0095]    c. source modulation system ( 23 ); 
         [0096]    d. basic source alignment hardware ( 24   a ); 
         [0097]    e. high-resolution camera alignment hardware ( 24   b ); 
         [0098]    f image capture and analysis hardware ( 25 ); and 
         [0099]    g. user interface ( 26 ). 
         [0100]      FIG. 5  shows an actual volume vs time graph captured from a subject. The points chosen to trigger for image acquisition correlates to an equal volume spacing between images. This can be seen more clearly in  FIG. 5   b.    
         [0101]      FIG. 6  shows a flowchart of one particular embodiment of the application relating to this technology. This lists the processing steps that can occur for this technique to be used effectively. 
         [0102]    Table 1 displays an example of the dose savings that can be achieved using this technique. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Examples of Dose Saving for a Given Volume interval 
               
             
          
           
               
                   
                   
                 Number of frame 
                   
               
             
          
           
               
                   
                 Volume 
                 Standard 30 
                   
                 Dose reduction 
               
               
                   
                 ΔL 
                 fps 
                 Triggered 
                 Saving 
               
               
                   
                   
               
             
          
           
               
                   
                 0.1 
                 90 
                 34 
                 62% 
               
               
                   
                 0.2 
                 90 
                 19 
                 79% 
               
               
                   
                 0.3 
                 90 
                 13 
                 86% 
               
               
                   
                 0.4 
                 90 
                 10 
                 89% 
               
               
                   
                 0.5 
                 90 
                 9 
                 90% 
               
               
                   
                 0.6 
                 90 
                 7 
                 92% 
               
               
                   
                 0.7 
                 90 
                 6 
                 93% 
               
               
                   
                 0.8 
                 90 
                 6 
                 93% 
               
               
                   
                 0.9 
                 90 
                 5 
                 94% 
               
               
                   
                 1 
                 90 
                 4 
                 96% 
               
               
                   
                 1.1 
                 90 
                 4 
                 96% 
               
               
                   
                   
               
             
          
         
       
     
         [0103]      FIG. 7  is a plot showing the dose reduction that is possible using the method of the present invention on a 30 frame per second imaging system as compared with imaging at a constant rate. The plot shows how the dose reduction changes ( 30 ) as the predetermined volume change of the lungs is set to different levels (triggered  32 ). 
         [0104]      FIG. 8  shows the system and apparatus of the present invention. The physiological input ( 39 ) is sent from the patient located between the source ( 42 ) and the detector ( 44 ) to the sensor ( 48 ). The signal processing system ( 38 ) in  FIG. 8 a    shows how a physiological parameter is used for controlling the timing of image acquisition of  FIG. 8 b   . The sensor turns this physiological information into a parameter signal ( 50 )I that is then sent to the signal processing unit ( 52 ). The signal processing unit converts the signal to an appropriate trigger ( 40 ) that feeds into the imaging system controller ( 46 ). The signal processing may include some other input information from the user such as preset volume change value or an equation for a series of volume change values or another set of rules by which the input physiological information is to be converted to an appropriate triggering signal for the imaging system. 
         [0105]    Typically, a CTXV scanner of this type comprises multiple energy sources and multiple detectors used simultaneously or in close temporal sequence. This current invention, when utilised in CTXV imaging (irrespective of the number of energy/detector pairs) includes application to imaging of animals and also application to imaging of humans. Due to the nature of CTXV technology often being utilised for direct measures of lung volume change, dramatic reductions in dose can be achieved for little or no loss of information gathered during a scan by using a volume based gating system. 
         [0106]    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. 
         [0107]    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. 
         [0108]    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. 
         [0109]    “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”.