Imaging apparatus

Abstract: In an imaging method and apparatus an x-ray source (12) is mounted to a rotatable gantry (16). The x-ray source irradiates an examination region (14) with penetrating radiation as the x-ray source rotates around the examination region and a subject/patient support (30) moves a patient axially through the examination region. Detectors (24) positioned on an opposite side of the examination region convert radiation which has traversed the examination region along spiral paths (80) into data. Data from the detectors are collected and stored (44) in spiral data sets. In the prior art spiral data on either side of a thin slice central plane was interpolated into a thin slice data set, each thin slice data set was reconstructed into a thin slice image, and several thin slice images were combined to make a thick slice image with significant partial volume artifacts. In the method and apparatus of the invention an integrating interpolator (46) weights (88) data values in several spiral sets in accordance with weighting values W₁, W₂, etc. and the weighted sets are combined (90) to form a thick slice data set. The thick slice data set is reconstructed (50) into a thick slice image having reduced partial volume artifacts.

Description: This invention relates to an imaging apparatus. It finds particular application in connection with spiral volume imaging using computed tomography (CT) scanners for medical diagnostic imaging of patients and will be described with particular reference thereto. It should be appreciated, however, that the present invention also finds application in connection with other types of volume imaging e.g. volume imaging of manufactured articles to detect internal structures or flaws and the like. In conventional spiral or helical scanning, an x-ray source or tube emits a fan beam of radiation as it rotates continuously around an examination region as a subject support table moves therethrough at a constant, linear velocity. Detectors positioned across the examination region from the x-ray tube convert x-rays which traverse the subject, e.g. a patient, into corresponding electronic data. The collected data effectively represents a helical path of constant pitch through the subject or patient. The helical data is reconstructed into a volumetric image representation, typically a series of consecutive thin slice images of the patient. One technique for reconstructing image slices from the stored spiral data involves interpolating collected spiral data into planar data. For example, for each plane transverse to a central axis, corresponding rays in the two closest spirals to either side of the plane are interpolated to generate one of the rays of the planar data set. This same interpolating procedure is repeated for each ray of a complete data set, e.g. a data set extending over 180°. The data from the transverse plane is reconstructed into a planar or thin slice image representation of the volume image representation using conventional reconstruction techniques. This technique is for example disclosed in EP-A-471 455 which describes an imaging apparatus according to the preamble of claim 1. Heretofore, partial volume artifacts have degraded CT scanner image quality. Prior art techniques have attempted to reduce the adverse effects caused by partial volume artifacts by combining several of the reconstructed thin slice images. Two or more adjacent thin slice images are combined or summed to form a single thick slice image with reduced partial volume artifact effects. Combining thin slice images has the benefit of retaining low contrast resolution, but suffers from possible motion artifacts due to the long acquisition time for each summed image as well as the additional reconstruction time to generate the several thin slice images prior to combination. When six thin slice images are combined to reduce partial volume artifacts in the resulting slice representation, six reconstruction operations are first performed. The length of time to perform six reconstructions and combine the resultant six thin slice images into a partial volume artifact reduced thicker slice image causes a significant delay. The present invention provides an apparatus which overcomes the above problem. In accordance with the invention there is provided an imaging apparatus according to claim 1. In one particular embodiment the apparatus includes a filter memory which stores at least a selected filter function that indicates corresponding weighting factors for each of the spiral data sets; and the integrating interpolator includes a multiplier for combining the weighting factors with the spiral data set to generate weighted data; and an adder for combining the weighted data. In one such arrangement the integrating interpolator includes: a weighting means for weighting data from the spiral data sets within a preselected number of spiral slice thicknesses of a central plane of a thick slice with a first weighting value selected in accordance with the preselected number and for weighting data from the spiral data sets beyond the preselected number of spiral slice thicknesses of the thick slice central plane with a roll-off filter that diminishes from the first weighting value to zero with distance from the thick slice central plane; and means for combining the weighted and roll-off filtered spiral slice data into a thick slice data set. The apparatus suitably further includes a video processor for converting the slice image representation into a thick slice cross sectional image for display on a monitor. The apparatus according to the invention is in particular adapted to put into practice a method of imaging wherein partial image artifacts from cross sectional images are reduced, said method comprising: irradiating a subject along a spiral path in an examination region; detecting radiation which has traversed the examination region; and converting the detected radiation into electronic data; and characterized by the steps of: storing the electronic data in spiral data sets, each data set consisting of electronic data collected along a spiral arc segment of less than one rotation; generating weighting values in accordance with a selected slice thickness; weighting and combining the electronic data of a plurality of the spiral data sets in accordance with corresponding weighting values; and reconstructing the combined weighted electronic data into a single thick slice image representation. In a preferred method to be put into practice, the weighting values for electronic data of the spiral data sets adjacent a central transverse plane of the thick slice are inversely proportional to the number of sets selected for combination. One advantage of the present invention is that integration of the electronic data occurs prior to reconstruction of the resulting image. Reducing the number of reconstructions reduces processing time. Another advantage of the present invention is that partial voluming is selectively reducible. Another advantage of the present invention is that only selected cross-sectional images of the patient are reconstructed for viewing eliminating unnecessary storage and processing. Another advantage is that thin slice 3D images can be reconstructed from the same data set. Yet another advantage resides in the ability to select slice profiles including rectangular profiles. A method and apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic illustration of a spiral CT scanner in accordance with the present invention; Figure 2 illustrates an interpolation function used in the apparatus; and; Figure 3 is a diagram illustrating the geometric relationships of spiral data, transverse slices, and the interpolation function. Referring to Figure 1, the apparatus is a spiral CT scanner 10 which includes a radiation source 12, such as an x-ray tube, for projecting a fan beam of radiation through an examination region or scan circle 14. The x-ray source 12 is mounted on a rotatable gantry 16 to rotate the fan beam of radiation around the examination region 14. A collimator and shutter assembly 18 collimates the radiation to a one or more narrow planar beams and selectively gates the beams on and off. The beams may also be gated on and off electronically at the x-ray source. A motor 20 provides motive power for rotating the gantry 16 continuously around the examination region 14. A rotational position encoder 22 is connected with the motor and the gantry to measure the rotational position of the gantry. In the illustrated fourth generation CT scanner, a ring of radiation detectors 24 are mounted peripherally around the examination region. In a source fan geometry, an arc of detectors which span the radiation emanating from the source are sampled concurrently at short time intervals as the radiation source 12 rotates around the examination region to generate a source fan view. In a detector fan geometry, each detector is sampled a multiplicity of times as the radiation source rotates behind the examination region to generate a detector fan view. The path between the x-ray source and each of the detectors is denoted as a ray. Contiguous fan views which span 180° plus the angle of the fan constitute a complete set of data for reconstruction into a slice image. A patient support 30 supports a patient subject in a reclined position. A motor 32 advances the patient support through the examination region at a selected, preferably constant velocity. An encoder 34 is connected with the motor 32, the movable patient support 30, and the drive mechanism therebetween for monitoring an axial or longitudinal position of the patient support as it moves the patient through the examination region 14. A sampling processor 40 samples the fan views corresponding to each angular position around the examination region 14 for each of a multiplicity of spirals. The apexes of the fan views are disposed along a helix and are defined by an angular orientation  around the imaged volume and as longitudinal position along the imaged volume. Data within each fan is further identified by the angle α within the fan. A view processor 42 converts the spiral sampled views into planar data sets. The view processor includes memory 44 which stores the spiral sampled views. An integrating interpolator 46 integrates corresponding rays of a preselected number of contiguous sets of spiral data with a filter or interpolation function retrieved from a memory 48. The memory 48 stores a plurality of filter or interpolation functions for integrating different numbers of spiral fan views. The more views that are combined, the thicker the corresponding slice. Thereafter, the combined set of data is conveyed to a reconstruction processor 50. The reconstruction processor of the preferred embodiment includes a convolver 52 which convolves the thick slice interpolated data sets and a backprojector 54. The backprojector 54 back projects the convolved data sets into a reconstructed image memory 56. A video processor 60 retrieves slice, projection, 3D rendering, and other image information from the image memory 56 and formats the image data into appropriate formats for display on video monitor 62 or the like. With reference to Figure 2, an interpolation function 70 is a linear weighting function whose values are dependent upon the number of sets or views selected for combination to form a thick slice image of the subject patient. The interpolation function is defined especially over two or more revolutions of the x-ray source 12. As shown in Figure 2, the interpolator function has a flat plateau and tapers at each end over a distance comparable to a rotation angle of ½ of a revolution angle. The interpolator function is normalized such that the area under the curve is substantially unity. In other words, the plateaus value of the interpolator is inversely proportioned to the width of the thick slice. The interpolator function of Figure 2 has a maximum weighting value 72 of .33 which is inversely proportional to a selected three rotations sought to be combined, i.e. the integration factor. Using this interpolation function, the reconstructed images have an effective slice thickness to the distance traveled in 3 revolutions of the x-ray source. With reference to Figures 2 and 3, a center point 74 of the filter function is fixed to match a center 76 of a thick slice 78. It will be appreciated that the data follows a helical path while the center of the filter is axially fixed. Thus, when the next corresponding rays of each of the adjoining data sets are integrated, their intersection points with the filter shifts and the filter factors normally change. Depending on the pitch, one data set may spiral out of the filter as another spirals in. The data fans along spiral paths 80 are arranged into 180° plus fan angle data sets with the "plus fan angle" fans being duplicative with adjacent data sets. Accordingly, the data sets are each nominally one-half the distance traveled by the patient in the time that the x-ray source rotates one revolution. Of course, the data sets can span 360°, can have non-duplicative "plus fan angle" fan views in which case the data sets are nominally spaced apart by the distance that the patient travels in the time the x-ray source rotates one revolution. Changing the pitch of the helix changes the physical spacing of the data sets. An integration controller 82 converts the physical dimension of the selected thick slice into data space, e.g. average number of spanned spiral data sets. Each data set value that is encountered within the plateau at the top of the filter curve, between about 3.75mm and -3.75mm of the center of the thick slice, is multiplied by 0.33 in the example of Figure 2. Data set values in data space regions between +3.75 and +5.5 and between -3.75 and -5.5 are multiplied by the corresponding smaller fraction. The integration controller 82 controls a filter or weighting factor calculator 84 which calculates the 0 to 0.33 or other appropriate weighting values W 1 , W 2 , ... n W for each retrieved data value. The filter factors are loaded in filter factor buffers 86a, 86b, .... A multiplier 88 multiplies each of the received data set values by the corresponding filter function. A summing amplifier 90 sums all weighted data values corresponding to rays spaced 360° apart. The resulting 260° weighted and summed data set is then convolved and backprojected by the processor 50. In the illustrated embodiment, to obtain equivalent thick slice thickness of about 10mm with a 3mm spiral scan width an integration factor of 3 would be used with a scan pitch of 1.0 to 1.25. Larger pitch factors will result in unnecessarily broad slice profiles. Using this interpolator, the measured slice thickness corresponds to the following formula: Measured Slice Thickness (FWHM) =(Pitch Factor)x(Integration Factor)x(Slice Thickness).

Claims: An imaging apparatus which includes an x-ray source (12) for irradiating an examination region (14) with penetrating radiation, the x-ray source (12) being mounted for rotation around the examination region, (14), a subject support (30) which moves a subject through the examination region (14) concurrently with rotation of the radiation source (12), radiation detectors (24) disposed across the examination region (14) from the x-ray source (12) to receive X-ray radiation which has traversed the examination region (14) and convert the received radiation into electronic data, and a data set memory (44) which stores the electronic data in spiral data sets where each spiral data set contains data relating to less than 360° around the subject, the apparatus being characterized by : an integrating interpolator (46) for integrating the electronic data from a predetermined plural number of the spiral data sets into a planar data set in accordance with a filter function that has a Full-Width Half-Maximum value that is at least equal to the distance travelled in two revolutions of the x-ray source and a reconstruction processor (50) for reconstructing the planar data set into a single thick slice (78) image representation, whereby the measured slice thickness of the thick slice (78) is substantially equal to the Full-Width Half-Maximum value. An apparatus as set forth in claim 1 including a filter function memory (48) which stores at least a selected filter function that indicates corresponding weighting factors for each of the spiral data sets; and wherein the integrating interpolator (46) includes a multiplier (88) for combining the weighting factors with the spiral data set to generate weighted data; and an adder (90) for combining the weighted data. An apparatus as set forth in Claim 1 wherein the integrating interpolator (46) includes: a weighting means (82, 84, 86) for weighting data from the spiral slice data sets within a preselected number of spiral thicknesses of a central plane of a thick slice with a first weighting value selected in accordance with the preselected number and for weighting data from the spiral data sets beyond the preselected number of spiral slice thicknesses of the thick slice central plane with a roll-off filter that diminishes from the first weighting value to zero with distance from the thick slice central plane; and means (88, 90) for combining the weighted and roll-off filtered spiral slice data into a thick slice data set. An apparatus as set forth in claim 3 wherein the integrating interpolator (46) includes means (84) for generating the weighting values as a function of position along a longitudinal axis of the patient. An apparatus as set forth in any preceding claim including a video processor (60) for converting the slice image representation into a thick slice cross sectional image for display on a monitor (62).

Inventors: HEUSCHER DOMINIC J, HEUSCHER, DOMINIC J.
Applicants: PHILIPS MED SYST INC, PHILIPS MEDICAL SYSTEMS (CLEVELAND), INC.
Application year: 1995
Main classification: A61B6
IPC codes: G06T11, A61B6