Abstract:
A method for determining a time for acquiring contrast-enhanced images from a CT scanner of a subject into whom a contrast medium has been injected, the method including: 
     identifying a region of interest in an initial CT image of the body of a subject; 
     defining an attenuation data segment corresponding to the region of interest by reprojecting the region of interest in the initial image; 
     receiving attenuation data within the segment from a subsequent scan of the patient; and 
     independently processing the attenuation data received within the segment to estimate an optimal time for performing a diagnostic scan of the body, without reconstructing all or a portion of the CT image.

Description:
RELATED APPLICATION 
     The present application is a US national stage application of PCT/IL97/00036, filed Jan. 29, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to CT imaging, and specifically to methods for contrast enhancement in CT imaging. 
     BACKGROUND OF THE INVENTION 
     Contrast enhancement in CT imaging, by injection of contrast media into the bloodstream of a subject is well known in the art. A bolus of a suitable contrast medium, for example, comprising iodine, is injected intravenously into the subject&#39;s body and is carried by the blood flow to an organ of interest, such as the liver. Features of the organ, such as tumors, which may ordinarily be difficult or impossible to discern, preferentially take up the contrast medium from the blood. After a certain delay, to allow the medium to reach the organ of interest in sufficient concentration, a CT scan of the subject is performed. If the scan is performed at the appropriate time, the contrast-enhanced features of the organ are clearly seen in the resultant CT images. 
     Generally, in performing such contrast-enhanced CT scans, a standard delay is allowed between the beginning of injection and the beginning of diagnostic scanning, for example 60-70 seconds in liver scans. The time required for the contrast medium to reach sufficient concentration in the organ, however, varies substantially from patient to patient. These variations are the result of differences in cardiac output and other circulatory parameters, and are very difficult to predict. Furthermore, after the concentration has reached its desired value, giving a high contrast for imaging, the medium begins to wash out of the organ, and contrast drops off. Therefore, when a standard scanning delay is used for all patients, a relatively large dose of contrast medium or a relatively long CT scanning period, with consequently increased radiation dosage, must be used in order to ensure that an image of sufficient contrast is captured. 
     General Electric Medical Systems (Milwaukee, Wis.) has introduced an optional modification to CT scanners of its manufacture known as “SmartPrep,” which is intended to provide a patient-dependent variable delay between bolus injection and the beginning of CT scanning. This modification is described in an article by Silverman, et al., entitled “Optimal Contrast Enhancement of the Liver Using Helical (Spiral) CT: Value of SmartPrep,” in the American Journal of Radiology, vol. 164 (1995), pages 1169-1171, which is incorporated herein by reference. 
     In SmartPrep, an initial scan is performed to produce a CT image of a slice through an organ or area of interest the patient&#39;s body. A user, generally a physician, marks up to three regions of interest in the image, for example, the aorta, portal vein and hepatic parenchyma, if the liver is the object of the procedure. Injection of the contrast medium is begun, and starting a short time thereafter, a sequence of CT scans are performed of the same slice, preferably at a reduced level of irradiation. Images from these scans are reconstructed, and the CT values, in Hounsfield units (H), in the images for each of the regions of interest are compared with baseline values from the initial scan. When the CT value in one of the regions, preferably the aorta, reaches a desired threshold over the corresponding baseline, the sequence of scans is terminated. After a delay of approximately 10 sec. a diagnostic helical scan over the organ or area is initiated. 
     SmartPrep thus allows the beginning of the diagnostic scan to be cued according to the time it takes the contrast medium to reach an area in the image slice in sufficient concentration to produce strong image contrast. According to the above-mentioned article, SmartPrep is effective in optimizing the delay from the bolus injection to the beginning of diagnostic scanning, such that, for example, in a sample of 75 patients studied by Silverman and co-workers, delay times ranged from 57 to 86 sec, as against the standard 60-70 sec delay. These delays are typical for contrast-enhanced imaging of the hepatic parenchyma, which normally receives the contrast medium through the portal vein with a lag of a number of seconds relative to the aorta. 
     SmartPrep may not be effective, however, for imaging features and areas of organs that receive the contrast medium with only a short lag behind the appearance of the medium in the aorta, before there has been a substantial amount of perfusion through the body. Such features include arterial lesions in the liver and pulmonary areas. The delays inherent in reconstructing the images from the sequence of CT scans, and then waiting until the threshold is reached before terminating this sequence, are typically greater than the short time that it takes the contrast medium to reach the organ. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method for contrast-enhanced CT scanning, by predicting when a bolus of contrast medium will reach a desired concentration level in an area of suspected pathology in the body. 
     In preferred embodiments of the present invention, a patient is positioned in a CT scanner, and X-ray attenuation data are acquired with respect to a slice through an area of suspected pathology in the patient&#39;s body. The data are preprocessed, filtered and back-projected, as is known in the art, to reconstruct a reference CT image of the slice. One or more regions of interest (ROIs) are identified within this slice, wherein preferably, one of the ROIs includes the aorta or other major artery feeding a site of the suspected pathology, and another ROI includes the site itself. These ROIs are reprojected onto the preprocessed attenuation data, so as to define segments within the data corresponding respectively to each of the ROIs. 
     A bolus of a contrast medium, for example, comprising iodine, is injected into the patient&#39;s bloodstream, and a CT scan of the slice, preferably a continuous dynamic scan, is initiated. During this scan, attenuation data from the ROI-related segments are acquired and preprocessed continuously. Preferably, the preprocessed attenuation data are tracked so as to generate one or more functional curves describing the increase in attenuation over time, due to influx of the contrast medium into the ROIs. The shapes of the initial portions of these curves are used to predict when the attenuation at the site of the suspected pathology will reach a predetermined value or peak. Based on this prediction, the continuous dynamic scan is terminated, and a helical scan is initiated, preferably automatically, to acquire a diagnostic, contrast-enhanced image of the area. 
     Preferably, the segments defined within the preprocessed attenuation data comprise segments of data acquired with respect to each ROI from a plurality of different angular views. As the CT scanner scans through these different views in succession, the preprocessed attenuation data acquired in each view are used in turn in generating the functional curves. Preferably, the data acquired in different views are combined, for example, by correlation, to increase the sensitivity of detection of increases in attenuation. 
     Alternatively or additionally, the attenuation data from the ROI-related segments may be back-projected to reconstruct images of one or more of the ROIs, as described in a PCT patent application filed on even date with the present application and entitled “REAL TIME DYNAMIC IMAGE CONSTRUCTION” and which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference. The CT values in the ROIs may be used to generate the functional curves, which are used in controlling the scanner as described herein. 
     Preferably, the one or more functional curves are displayed by the scanner, for example, on a video screen, as they are generated. This display allows a user to view, substantially in real time, the progress of the contrast medium entering the subject&#39;s body. If the user observes that the curves do not show a normal increase in attenuation, he or she may intervene, for example, to initiate the diagnostic scan independent of the prediction or to terminate the scan, as appropriate. 
     CT images of the slice may be reconstructed intermittently while the attenuation tracking is going on. However, unlike methods of bolus tracking known in the art, such as the above-mentioned SmartPrep method, the present invention enables the progress of the bolus to be predicted without reconstructing the full slice image. The progress of the bolus may be predicted using only the preprocessed attenuation data, independent of and without the need for any image reconstruction, or by reconstructing only the ROI portion of the CT image. Therefore, the present invention allows much more rapid and precise prediction of the time at which the attenuation at the site of the suspected pathology will reach a desired value. Typically, in preferred embodiments of the present invention, the prediction is completed within a few seconds, for example seconds, so that the diagnostic scan can begin when the bolus reaches the region of interest, for example about 10 seconds after the attenuation at the aorta begins to rise. Therefore, unlike methods known in the art, the present invention may also be used in diagnostic scanning of arterial phase lesions. The rapid prediction afforded by the present invention may also be useful in reducing the dosages of contrast medium and radiation that are administered to the subject. 
     Although preferred embodiments are described herein with reference to certain combinations of axial and helical scans by the CT scanner, it will be appreciated that the principles of the present invention, whereby the attenuation data themselves are used directly and rapidly in predicting changes in contrast within an area of suspected pathology, may be applied using other scanning combinations and procedures. 
    
    
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a CT scanner, operative in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a schematic illustration showing an image reconstructed by the CT scanner of FIG. 1, on which regions of interest have been marked, in accordance with a preferred embodiment of the present invention; and 
     FIG. 3 is a graph that schematically illustrates an aspect of the operation of the scanner of FIG. 1, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1, which shows a CT scanner  20 , operative in accordance with a preferred embodiment of the present invention. Scanner  20  comprises a bed  24 , supported by a base  26 , on which bed a subject  22  lies while his body is being imaged by the scanner. Scanner  20  further comprises an X-ray tube  28 , which irradiates subject  22 , and a detector array  30 , which receives X-rays from tube  28  and generates signals responsive to the attenuation of the X-rays in passing through the subject&#39;s body. 
     Tube  28  and array  30  are mounted on an annular gantry  32 , so as to revolve about subject  22 . Bed  24  is advanced through gantry  32  along an axis  34 , which is generally parallel to the long axis of the subject&#39;s body. As will be described below, in preferred embodiments of the present invention, scanner  20  operates alternately in an axial mode, wherein bed  24  is held stationary while tube  28  and array  30  revolve thereabout, or in a helical mode, wherein tube  28  and array  30  revolve simultaneously with the advance of bed  24  through gantry  32 . 
     Scanner  20  as pictured in FIG. 1 is of a type known in the art as a third-generation CT-scanner, characterized in that both tube  28  and detector array  30  revolve about subject  22 . It will be appreciated, however, that the principles of the present invention and the methods of image reconstruction to be described below are equally applicable to other types of CT scanners, in particular fourth-generation CT scanners, in which the detectors form a substantially stationary ring around subject  22 . 
     At each of a plurality of selected locations of tube  28  along its scan path, data acquisition circuitry  36  acquires a “view,” i.e., the circuitry receives signals from each element  23  of array  30  responsive to X-ray attenuation. For each view, circuitry  36  performs signal normalization and logarithm operations, as are known in the art, to derive X-ray attenuation data corresponding to each of elements  23 . Image reconstruction circuitry  40  received these data and interpolates, filters and back-projects the data, using methods known in the art, to produce one or more planar image slices through the body of subject  22 . A plurality of these planar image slices may be used to reconstruct a three-dimensional CT image set of all or a portion of the body of subject  22 . Preferably, these image slices are stored in image memory  42  and displayed by display unit  44 , and they may be otherwise printed and/or processed as is known in the art. Preferably a system controller  46  controls and coordinates the operation of the various elements of scanner  20 . 
     An infusion pump  50 , for example, a power syringe pump, as is known in the art, is charged with a predetermined quantity of a contrast medium, for example, iodine, and is connected intravenously to subject  22 . Preferably, pump  50  is coupled to controller  46  so that scanning operation of scanner  20  can be triggered by the activation of the pump, as will be described below. 
     FIG. 2 is a schematic representation of a sectional image  60 , acquired by scanner  20 , of a “slice” through the body of subject  22 , in accordance with a preferred embodiment of the present invention. Image  60  is acquired, preferably by means of a single axial scan of scanner  20  at a selected position of bed  24 , before initiating operation of pump  50 . The position of the bed is chosen so that the slice intersects an area of suspected pathology, in this case liver  64 . 
     A user of scanner  20  observes image  60  on display  44 , and indicates regions of interest (ROIs) in the image: preferably, a first ROI  66  at the location of aorta  62 , and a second ROI  68  at a site of suspected lesion  70 . Additional ROIs may be indicated at other locations, for example, along a blood vessel leading to site  70 . Although ROIs  66  and  68  are round, other shapes, such as rectangular ROIs, may similarly be used. The locations of the ROIs are input to scanner  20  and are preferably displayed on image  60 , for example, using dashed lines as shown in FIG.  2 . 
     The locations and extents of ROIs  66  and  68  are fed back to image reconstruction circuitry  40 , which reprojects these image areas back onto the preprocessed attenuation data received from data acquisition circuitry  36 . The object of this reprojection is to define segments within the attenuation data that correspond, respectively, to each of the ROIs. Each such segment comprises attenuation data acquired along rays from tube  28  to array  30  that pass through the corresponding ROI. Preferably, the segments that are defined with respect to each ROI include attenuation data acquired from multiple views at different angles of revolution of gantry  32  with respect to subject  22 . 
     FIG. 3 is a graph that schematically illustrates the operation of scanner  20 , after initial image  60  has been acquired and ROIs  66  and  68  have been chosen, in accordance with a preferred embodiment of the present invention. The upper portion of the figure includes curves  80  and  82 , respectively representing measured attenuation in the segments corresponding to ROIs  66  and  68 , as a function of time. The attenuation is measured directly on the reprojected segments of preprocessed attenuation data, without back-projecting the data to find CT values. The lower portion of the figure is a timing diagram, on the same time scale as the upper portion. 
     At a beginning time T 0 , pump  50  is activated to begin injecting the contrast medium into subject  22 . Shortly thereafter, controller  46  initiates a continuous dynamic axial scan of scanner  20  at the position of bed  24  at which image  60  was acquired. In this continuous dynamic scan, tube  28  revolves continuously around subject  22 , and attenuation data are acquired and preprocessed. Image reconstruction circuitry  40  optionally back-projects the data to reconstruct and intermittently update an image of the slice, like image  60 . However, curves  80  and  82  are generated without reference to such an image. Preferably, during the continuous dynamic scan, tube  28  is controlled to operate at a low level of irradiation of subject  22 , in order to reduce the radiation dosage that the subject receives. 
     As shown by curves  80  and  82  in FIG. 3, the attenuation data in the data segments corresponding respectively to ROIs  66  and  58  are tracked by controller  46 , or by another suitable computing device, independently of any image reconstruction going on. Preferably, as gantry  32  scans through its multiple angular views, the preprocessed attenuation data acquired within the data segments corresponding to the ROIs in the different views are used in turn in generating the curves. Preferably, the data acquired in different views are combined, for example, by correlation, to increase the sensitivity of detection of increases in attenuation. 
     Alternatively or additionally, the attenuation data from the ROI-related segments may be back-projected to reconstruct images of one or more of ROIs  66  and  68 , as described in the above-mentioned PCT patent application. The CT values in the ROIs may then be used to generate curves  80  and  82 , which are used in controlling the scanner as described herein. 
     However, the curves are generated, they are, preferably, also plotted graphically on display  44 . 
     Beginning at a time T 1 , when a curve  80  rises by an increment Δ over its baseline, T 0  value, the attenuation data in curve  80  are fitted to a model function, for example, a second order polynomial. This attenuation tracking and fitting continue as long as required to obtain an accurate curve fit, terminating at a time T 2 . The fit is used to predict when curve  80  will reach its peak, i.e., when the contrast medium in aorta  62  will reach its maximum concentration. Preferably, curve  82  is monitored simultaneously with curve  80 , to predict how much longer the medium will take to reach its maximum concentration at site  70  in liver  64 . 
     Once the fitting and prediction computations have been completed, at time T 2 , the continuous dynamic scan of scanner  20  is automatically brought to an immediate “soft stop.” The scanner is then positioned and prepared to begin a diagnostic helix scan. The helix scan begins at a time T 3 , when curve  82  is predicted to be nearing its maximum. Shortly after time T 3 , the injection of the contrast medium by pump  50  is terminated, and the scan continues until a time T 4 , when the curve has begun to drop off. 
     Time T 3  may be as little as 10-15 seconds from the initial of injection at time T 0 , particularly in the case of lesions that receive the contrast medium in the arterial phase of blood flow in liver  64 , i.e., receiving the contrast medium directly from the arteries. The rapid acquisition and fitting of the attenuation data, without the need for image reconstruction, in accordance with the principles of the present invention, enable this prompt operation. Alternatively, time T 3  may be in the range of 50-90 seconds after T 0  for portal venous-phase lesions, as in other bolus tracking methods, as are known in the art, such as the above-mentioned SmartPrep method. In either case, the early, accurate determination of times T 3  and T 4  in accordance with the method of the present invention allows optimal diagnostic images to be acquired by scanner  20 , with minimal dosages of contrast medium and radiation to subject  22 . 
     The user of scanner  20 , observing curves  80  and  82  on display  44 , may intervene in the event that the curves do not follow a normal, expected pattern. For example, if subject  22  has poor blood circulation, the curves may rise abnormally slowly. In this case, the user may preferably override the automatic operation of the scanner to begin the helical scan at a default or estimated delay time. Alternatively, a malfunction in the operation or intravenous connection of pump  50  will also be observed to affect the rise of the curves, in which case the scan is preferably terminated while the problem is corrected. 
     Although the above preferred embodiment has been described with reference to lesions of liver  64 , it will be appreciated that the principles of the present invention may equally be applied to contrast-enhanced scanning of other organs of the body, as is known in the art. Furthermore, although in the above preferred embodiment, a helical-mode diagnostic scan in used to acquire contrast-enhanced images, the method described here may be used, mutatis mutandis, together with axial-mode contrast-enhanced CT scanning techniques. 
     It will also be appreciated that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.