Patent Publication Number: US-6035227-A

Title: System and method for identifying malformation of a hip in a human or animal subject

Description:
This is a cip of Ser. No. 08/643,696, filed May 6, 1996 now U.S. Pat. No. 5,704,356. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to imaging of biological tissue and, in particular, it concerns systems and methods for low-dosage imaging in babies or small animals. 
     It is known that a significant proportion of babies, primarily female, are born with a hip-joint defect known as a congenital dysplasia of the hip, abbreviated to &#34;CDH&#34;. Estimates of the incidence of this condition vary between 1 and 15 per 1000 live births. Identification of this defect within the first few days of life is highly advantageous, allowing immediate and straightforward corrective action which typically results in completely recovery towards normal development of the hip. Delay in treatment, on the other hand, allows contracture of parts of the soft tissue surrounding the hip-joint, resulting in displacement or irreversible malformation of the hip, complicating the treatment and causing progressive hip problems later in life. 
     Although clinical neonatal procedures include various manipulations intended to detect CDH, these are not sufficiently reliably. It has been suggested to supplement the physical examination by additionally screening using ultrasound imaging techniques. However, ultrasound imaging can only be performed and the results properly evaluated by specially trained personnel, thereby limiting its usefulness for mass screening. 
     X-ray imaging has been used as a diagnostic tool for many years. Because of the possible harmful effects of high dosages of X-rays, use of X-ray imaging for very young children is conventionally kept to a minimum. Furthermore, conventional X-ray techniques only have diagnostic value after the age of about three months since, until that time, most of the tissue is cartilaginous and is substantially transparent to X-rays. 
     U.S. Pat. No. 4,852,137 to Mackay, U.S. Pat. No. 5,150,394 to Karellas and U.S. Pat. No. 5,187,730 to Fujihara relate to X-ray-based imaging systems in which the X-rays transmitted through the object of interest fall on a layer of phosphorescent material which acts as a secondary source, converting the pattern of incident X-ray radiation into light. The pattern can then be measured by use of a charge-coupled-device (CCD) to generate an image. This arrangement allows a considerable reduction of intensity of the X-ray radiation used compared to that required for conventional film exposures. The above-referenced U.S. patents are hereby incorporated in their entirety by reference as if fully set out herein. 
     There is therefore a need for a simple, reliable and cost effective method for screening newborn babies or small animals to identify malformed hips. It would also be advantageous to have a low-dosage X-ray-based system which may be used as a safe diagnostic tool for a wider range of applications involving newborn babies or small animals. 
     SUMMARY OF THE INVENTION 
     The present invention is of methods and systems for low-dosage diagnostic imaging in a human or animal subject. 
     According to the teachings of the present invention there is provided, a method for low-dose imaging of a part of a human or animal subject, the method comprising the steps of: (a) determining a weight of the subject; (b) calculating from the weight a minimum necessary dosage of radiation for forming an image of the part of the subject; and (c) directing radiation corresponding substantially to the minimum necessary dosage towards the part of the subject for forming an image. 
     According to a further feature of the present invention, the weight is determined by positioning the subject on a weighing platform, and wherein the radiation is directed towards the part of the subject while the subject is positioned on the weighing platform. 
     According to a further feature of the present invention, the radiation is X-ray radiation, and wherein the radiation is directed towards a stimulated emission light source, the method further comprising sensing light generated by the stimulated emission light source. 
     According to a further feature of the present invention, the light generated by the stimulated emission light source is sensed by use of a charge-coupled-device having a full-frame shutter. 
     According to a further feature of the present invention, the X-ray radiation is transmitted for less than about 5 milliseconds. 
     According to a further feature of the present invention, the X-ray radiation is transmitted for up to about one millisecond. 
     According to a further feature of the present invention, the X-ray radiation includes a primary pulse of radiation having a first frequency for generating a primary image of soft tissue surrounding the cavity and a secondary pulse of radiation having a second frequency for generating a secondary image of hard tissue surrounding the cavity, and the step of forming an image includes processing the primary and secondary images to form a composite image. 
     According to a further feature of the present invention, the part of the subject is at least part of the chest. 
     According to a further feature of the present invention, the part of the subject is a hip, the method further comprising applying tensile force to the leg of the subject so as to generate a temporary cavity within the hip in accordance with the vacuum phenomenon. 
     According to a further feature of the present invention, the step of applying tensile force exerts a corresponding force on a force sensor, the step of directing radiation being initiated in response to an output of the force sensor. 
     There is also provided according to the teachings of the present invention, a system for low-dose imaging of a part of a human or animal subject, the system comprising: (a) a weighing platform for receiving the subject, the weighing platform producing an output indicative of a weight of the subject; (b) a processor electrically connected to the weighing platform, the processor deriving from the output a minimum necessary dosage of radiation for forming an image of the part of the subject; and (c) a source of radiation positioned relative to the weighing platform for directing radiation towards the part of the subject while the subject is on the weighing platform, the source of radiation being electrically responsive to the processor to direct the minimum necessary dosage towards the part of the subject. 
     According to a further feature of the present invention, the source of radiation is an X-ray generator, the system further comprising: (a) a phosphorescent screen positioned so as to receive at least part of the X-ray radiation transmitted through the part of the subject, the phosphorescent screen generating light in response to incident X-ray radiation; and (b) a charge-coupled-device for generating an image of the light generated by the phosphorescent screen. 
     According to a further feature of the present invention, the part of the subject is a hip, the system further comprising a sensor producing an output indicative of a tensile force applied to the leg of the subject, the source of radiation being responsive to the sensor output to direct the minimum necessary dose of radiation towards the hip of the subject. 
     According to a further feature of the present invention, the source of radiation is activated in response to an output from the sensor indicative of application of a tensile force greater than a threshold value to the leg of the subject. 
     According to a further feature of the present invention, the processor is further responsive to the output from the weighing platform to derive the threshold value. 
     There is also provided according to the teachings of the present invention, a method for identifying malformation of a hip in a human or animal subject, the method comprising the steps of: (a) applying tensile force to the leg of the subject so as to generate a temporary cavity within the hip in accordance with the vacuum phenomenon; (b) forming an image of soft tissue surrounding said cavity; and (c) identifying the hip as one of a normal hip and a malformed hip in accordance with the shape of said cavity. 
     There is also provided according to the teachings of the present invention, a system for use in identification of malformed hips in a human or animal subject, the system comprising: (a) a sensor producing an output indicative of a tensile force applied to the leg of the subject; (b) an X-ray generator responsive to said output to generate at least one pulse of X-ray radiation directed towards the hip of the subject; (c) a phosphorescent screen positioned so as to receive at least part of said X-ray radiation transmitted through the hip, said phosphorescent screen generating light in response to incident X-ray radiation; and (d) a charge-coupled-device for generating an image of said light generated by said phosphorescent screen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1A is a schematic cross-sectional view through the hip joint of a newborn baby; 
     FIG. 1B is a schematic cross-sectional view through the hip joint of FIG. 1A when a longitudinal force is applied to the leg of the baby; 
     FIG. 1C is a schematic cross-sectional view through a dislocated hip joint of a new baby when a longitudinal force is applied to the leg of the baby; 
     FIG. 2 is a schematic side view of a system, constructed and operative according to the teachings of the present invention, for identifying malformed hips in newborn babies or small animals; 
     FIG. 3A is a schematic representation of an X-ray image of a hip joint taken without inducing the vacuum phenomenon; 
     FIG. 3B is a view similar to FIG. 3A showing the shape of a cavity formed by the vacuum phenomenon in a normal hip; 
     FIG. 3C is a view similar to FIG. 3A showing the shape of a cavity formed by the vacuum phenomenon in a hip exhibiting congenital dysplasia of the hip (CDH); 
     FIG. 3D is a view similar to FIG. 3A showing the shape of a cavity formed by the vacuum phenomenon in a dislocated hip; and 
     FIG. 4 is a schematic side view of a further system, constructed and operative according to the teachings of the present invention, for low-dosage imaging in newborn babies or small animals. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a method and system for low-dosage imaging in a human or animal subject. 
     The principles and operation of systems and methods according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Both the method and system may be applied to a wide range of applications in which minimum exposure to radiation is important. As an initial and primary example, the invention will be described with reference to FIGS. 1-3 in an embodiment for diagnosis of malformed hips in babies or small animals. Then, with reference to FIG. 4, a further embodiment useful in thoracic X-rays will be described. 
     By way of introduction to the first embodiment of the invention, and before addressing the systems and methods of the present invention in detail, a physiological effect known as the &#34;vacuum phenomenon&#34; will be described. It is a particular feature of both the methods and the systems of the present invention as applied to diagnosis of hip malformations that use is made of the vacuum phenomenon to facilitate identification of CDH and other malformations of the hip. 
     Referring now to the drawings, FIGS. 1A, 1B and 1C show three different states of a hip joint, generally designated 10, of a newborn baby. For convenience of reference, hip joint 10 will be referred to herein, in the description and claims, as hip 10. 
     FIG. 1A shows hip 10 as it would appear in a resting position of a normal hip. In this position, the head 12 of the femur 14 is snugly engaged in the socket 16 of the pelvis (acetabulum) 18. Inmmediately after birth, the portions of both femur 14 and pelvis 18 adjacent to the hip joint are made of cartilage 20 and only the lower part of femur 14 and the inner part of pelvis 18 have hardened to bone 22. Although this fact allows easy correction of a dislocation which is discovered at this stage, the nature of the tissue involved complicates clear imaging of the joint structure by usual X-ray techniques. 
     FIG. 1B illustrates the effect of applying tensile force to the femur, i.e., the leg, of the healthy hip of FIG. 1. When sufficient force is exerted, head 12 of femur 14 is temporarily displaced slightly from socket 16, thereby generating a generally crescent-shaped cavity 24 between them. This effect is known as the &#34;Vacuum Phenomenon&#34;. The cavity has the same radio-translucency as air and appears as a black shape on usual X-ray images. 
     FIG. 1C illustrates the effect of applying a tensile force similar to that applied in FIG. 1B to a dislocated hip. In this case, head 12 of femur 14 is not firmly bound to socket 16, so that any gap formed between femur 14 and pelvis 18 is irregular in shape. 
     Turning now to FIG. 2, there is shown a system, generally designated 30, constructed and operative according to the teachings of the present invention, for identifying malformed hips in newborn babies or small animals. The system may also be used for analysis of the condition of a malformed hip during or after treatment. Although system 30 may be used effectively on a range of non-human subjects of various types, it will be illustrated herein, by way of example only, in relation to human babies. 
     Generally speaking, system 30 includes an X-ray generator 32 for generating a pulse of X-ray radiation directed towards the hip of the baby, a phosphorescent screen 34 positioned so as to receive at least part of the X-ray radiation transmitted through the hip, for generating light in response to the incident X-ray radiation, and a charge-coupled-device (CCD) 36 for generating an image of the light generated by phosphorescent screen 34. System 30 is then employed to form an image of soft tissue of the hip while a sufficient tensile force is applied to the leg of the baby to generate a cavity within the hip in accordance with the vacuum phenomenon. The hip is then identified as either normal or malformed on the basis of the shape of the cavity observed. 
     It will be appreciated from the following description that system 30 represents a low-cost and simple-to-use system requiring only a brief training for the operator and suited for widespread use in a program of mass screening for CDH. 
     Turning now to the features of system 30 in more detail, it is a particular feature of the present invention that X-ray generator 32 delivers one or more short pulses of very low intensity radiation. Typically, X-ray generator 32 is designed to transmit a pulse of less than about 5 milliseconds, and preferably, up to about one millisecond, at an energy of less than about 75 kV. Preferably, a dual frequency X-ray source is used to provide a primary pulse for generating an image of soft tissue so as to differentiate between fat, muscle and cartilage in the region of the hip surrounding the cavity, and a secondary pulse at a second frequency for generating a secondary image of hard tissue such as bone. Typically, the primary pulse employs X-rays in the energy range of about 25-30 kV, and the secondary pulse employs X-rays of about 70 kV. Although labelled &#34;primary&#34; and &#34;secondary&#34;, the two X-ray pulses may be performed in either order. The time delay between them is generally a small fraction of a second, but must be significantly longer than the memory of phosphorescent screen 34. 
     Phosphorescent screen 34 is preferably made using rare-earth phosphor which is highly sensitive and has a &#34;memory&#34; of a few thousandths of a second. Alternatively, any other stimulated emission light source which generates visible or invisible light may be used. It should be understood in this context that the term &#34;phosphorescent&#34; is used to refer to the properties of the material as a stimulated emission light source, and is not intended to imply specifically a phosphor-containing material. 
     CCD 36 is preferably a full-frame shutter device. By avoiding any scanning process during generation of the image, it is possible to form an image from a single short pulse of X-ray radiation. This allows the dosage of X-ray pulses employed by the present invention to be very much smaller than that of conventional X-ray imaging techniques, thereby making system 30 acceptable for use with babies. For example, a typical conventional system for use in dental applications or the like employs a 20 millisecond pulse of 100 mA corresponding to a dosage of 2 mA s. In contrast, the device of the present invention delivers two pulses each of up to about 1 millisecond, typically of about 50 mA. This corresponds to a dosage of less than 0.1 mA s, or less than 5% of the conventional dosage. 
     System 30 also features a processor 38. Processor 38 serves to synchronize the operation of X-ray generator 32 and CCD 36, and to process the output of CCD 36 to generate the desired image. The image is then typically displayed on a display screen 40 to enable immediate diagnosis. Alternatively, or additionally, the image may be transferred to a local computer or remote network, such as for automated analysis or for storage within a database. 
     In the case that a single pulse X-ray exposure is employed, processor 38 performs only trivial image processing. In a preferred embodiment, two exposures of different frequencies are employed to provide information about both soft tissues (fat, muscle and cartilage) and hard tissue in the region of the baby&#39;s hip. In this case, processor 38 performs algorithms for combining the two electronic images to form a single composite image. Such algorithms are generally known in other areas of medical imaging, as well as in the fields of aerial imaging and astronomical research, and will therefore not be described here in detail. 
     It is a further feature of a preferred embodiment of the present invention that system 30 is activated only when sufficient tensile force is applied to the leg of the baby to generate the Vacuum Phenomenon. In order to ensure that the necessary force is being applied, a force sensor 42 is provided which produces an output indicative of a tensile force applied to the leg of the baby. Force sensor 42 typically includes a lever 44 pivotally attached at one end and extending between the legs of the baby, and a pneumatic pressure gauge 46 acted upon by lever 44. An electrical output from pneumatic pressure gauge 46 is supplied to processor 38. Processor 38 is programmed to disable activation of system 30 unless pneumatic pressure gauge 46 indicates that the tensile force currently being applied to the leg of the baby is above some threshold value considered sufficient to generate a cavity in accordance with the Vacuum Phenomenon. System 30 may be programmed to be activated either directly in response to above-threshold tensile force, or manually contingent on the presence of above-threshold tensile force. It should be noted that, while the present invention has been described in an embodiment in which the force applied the leg of the baby is longitudinal, comparable results may be achieved by applying tensile force in other directions. In a case in which a different direction of force is to be used, the structure of force sensor 42 may be varied accordingly. 
     In a preferred feature of the present invention, an X-ray shield made from lead or other X-ray absorbent material is provided to protect the genital region of the baby from potentially damaging X-ray radiation. This shield may conveniently be formed as a butterfly-shaped device 45 attached to the upper end of lever 44. 
     For convenience of use, the components of system 30 are, in general, mounted in relation to a traction table 48 on which the baby is placed during the imaging process. In this case, lever 44 extends through a slot (not shown) through traction table 48. Traction table 48 is either made from a substantially X-ray-transparent material, or has an opening corresponding to the operative imaging area of the table. 
     It has been found that the force necessary to generate a cavity according to the Vacuum Phenomenon is directly correlated to the weight of the baby. In order to estimate correctly the appropriate force threshold to be used for each baby, it is therefore preferable to supply processor 38 with a measurement of the weight of the baby. For this purpose, the traction table 48 on which the baby is placed is preferably mounted on a load cell 50 or other weighing mechanism thereby forming a weighing platform which supplies a weight measurement to processor 38. Alternatively, a conventional free-standing electronic scale may be used as a weighing platform to weigh the baby prior to imaging. The weight measurement is then supplied electronically or manually to processor 38. 
     It is a further particular feature of certain preferred embodiments of the present invention that the weight measurement from load cell 50 or other weighing mechanism is employed to derive a minimum required dosage of radiation for forming an image of the part of the subject in question. For a heavier baby, slightly higher quantities of radiation are required, whereas for a lighter baby, a lower quantity will suffice. This derivation is typically performed by processor 38 and may be in addition to, or as an alternative to, the calculation of a threshold force value for the vacuum phenomenon. 
     The appropriate quantity of radiation, referred to herein as the &#34;minimum necessary dosage&#34;, is preferably derived by accessing look-up tables of exposure against weight for given parts of the body stored in a memory device associated with processor 38. The look-up tables are typically established by clinical trials which start with radiation levels conventionally considered acceptable and reduce them in steps until a minimum necessary dosage is established for each weight bracket. For higher precision, interpolation may be used to estimate the optimal minimum necessary dosage for any weight of subject. Processor 38 may also be programmed to update the tables based on feedback from the system operator, for example when a given dosage is found to be insufficient. In an alternative implementation, processor 38 employs an algebraic function based on a numerical model and/or clinical tests to define a minimum necessary dosage as a function of subject weight. Where two pulse dual-frequency exposure is used, separate look-up tables are provided for each frequency. 
     In use, the baby is first placed on the weighing platform and its weight input to processor 38. Processor 38 then calculates the appropriate force threshold value necessary to generate the Vacuum Phenomenon for this baby and derives the minimum necessary dosages of radiation required. The baby is then held in position with lever 44 between its legs with one hip aligned between X-ray generator 32 and phosphorescent screen 34. Traction is then applied to the baby&#39;s leg, typically by hand, and, when processor 38 determines that sufficient tensile force is being applied, X-ray generator 32 is activated to deliver the minimum required dosage of X-rays. The simplicity of the system and its operation enables it to be operated by semi-skilled personnel with relatively brief training. 
     X-ray generator 32 transmits a primary and a secondary pulse, as detailed above, typically about a twentieth of a second apart. Each pulse generates an image on phosphorescent screen 34 which is sensed by CCD 36 and input into processor 38. Processor 38 then processes the images to form a single compound image which is then displayed on display screen 40, and may also be stored electronically or printed for inclusion in medical records. 
     FIGS. 3A to 3D illustrate the X-ray images which would be expected under various different conditions. FIG. 3A illustrates the difficulty of accurate diagnosis in the absence of a vacuum phenomenon cavity. Since all the soft tissue is usually indistinguishable, the structure of the hip joint is unclear. 
     In contrast, FIG. 3B shows the image of a normal hip subjected to sufficient force to generate a cavity in accordance with the vacuum phenomenon. In this case, a distinct crescent-shaped cavity allows rapid identification of the hip as normal. 
     FIG. 3C shows an image comparable to that of FIG. 3B, but with an irregularly shaped cavity. This irregularity is indicative of a malformation such as CDH. 
     Finally, FIG. 3D shows the more extended irregular shape of the cavity in a case of hip displacement. 
     Turning now to FIG. 4, this shows a further system, generally designated 60, constructed and operative according to the teachings of the present invention, for low-dosage imaging of a part of a baby or small animal subject, in this case, a thoracic X-ray image. System 60 is generally similar to system 30 described above, equivalent elements being labeled similarly. Since no tensile force need generally be exerted during imaging of parts of the subject other than hips, the various force sensing elements are omitted. 
     In a manner similar to that described above, a load cell 62 or other weighing mechanism in combination with table 48 constitute a weighing platform for providing processor 38 with a signal indicating the weight of the subject. On the basis of this signal, processor 38 derives a minimum necessary dosage or dosages for forming an image of the relevant part of the subject, as described above, to be produced by X-ray generator 32. 
     In all other respects, the structure and operation of system 60, and the corresponding method, will be readily understood by analogy to system 30 described above. 
     It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the spirit and the scope of the present invention.