The present invention relates generally to patient positioning systems for multimodality imaging systems, and particularly to maintaining linear and angular registration between a plurality of imaging systems of a multimodality imaging system. 
Imaging systems acquire images of a patient, such as images of a suspected tumor, for diagnosis and subsequent treatment or therapy.  Commonly used medical imaging systems include fluoroscopy, computerized tomography (CT), magnetic resonance (MRI) or position emission tomography (PET), for example. 
Many imaging systems employ a patient table or couch (the terms being used interchangeably herein throughout the disclosure and claims) upon which a patient is supported throughout the imaging process.  In general, the patient lies on the table, which may move along a first axis (generally the azimuth axis).  The theoretical isocenter is defined as the symmetry axis of the gantry.  The imaging system identifies the spatial coordinates of the suspected tumor, with reference to the isocenter, or for guiding the planning of surgery or other treatments.  Accurate measurement of these coordinates is crucial for subsequent treatment of the suspected tumor, because the position of the tumor, as defined by the imaging system, is then used as the target for irradiation, such as by a stereotactic radiotherapy system A typical stereotactic radiotherapy system uses a linear accelerator (LINAC) gantry, which rotates about the longitudinal axis of the table.  It is essential that the isocenter of the LINAC gantry be as close possible to the isocenter of the imaging system. 
Multimodality imaging systems employ a plurality of imaging systems, such as CT and PET imaging heads or gantries aligned along a common longitudinal axis.  The patient registration should be the same for both imaging systems.  Specifically, the isocenter of the first imaging system should be aligned as accurately as possible with the isocenter of the second imaging system.  This is not a trivial task because no mechanical assembly is perfect, due, inter alia, to tolerances and the act that mechanical parts are not infinitely stiff. 
Some systems have attempted to solve this problem by means of a fixed alignment between the two imaging systems during the manufacturing process of the multimodality system.  Basically this approach places high restrictions on tolerances and mechanical accuracies during production and assembly of the system.  This method has the drawback of being quite expensive.  Another approach is the use of a fixed calibration during assembly of the system.  An accurate jigging fixture is used to align the two imaging systems with each other.  This approach is less expensive than the first approach, but still has a disadvantage of being time-consuming.  Both methods have a further disadvantage, in that if it is required to service one or more of the imaging systems, the systems must be re-aligned, which can be a cumbersome, time-consuming and tedious task. 
The present invention seeks to provide improved apparatus that substantially maintains linear and angular registration between al plurality of imaging systems of a multimodality imaging system. 
In the present invention, at least one of the imaging systems is slidingly mounted on a set of one or more rails.  The rails may be adjusted linearly along a vertical axis perpendicular to the longitudinal axis of the rails, and adjusted rotationally about a third axis perpendicular to the two aforementioned axes.  This provides several advantages.  First, mounting one or more of the imaging systems on rails enables separating the imaging systems easily and quickly for servicing.  Secondly, after servicing there is generally no need for time-consuming recalibration after re-positioning the displaced imaging system to its original position.  Third, since the rails are capable of linear and rotational adjustment, the invention enables quick and straightforward initial calibration and alignment of the imaging systems with each other. 
There is thus provided in accordance with a preferred embodiment of the present invention a multimodality imaging system including a plurality of imaging systems, and at least one rail upon which at least one of the imaging systems is slidingly mounted. 
In accordance with a preferred embodiment of the present invention the at least one rail is adjustable linearly along a vertical axis generally perpendicular to a longitudinal axis of the at least one rail. 
Further in accordance with a preferred embodiment of the present invention the at least one rail is adjustable rotationally about another axis generally perpendicular to the vertical axis and the longitudinal axis. 
Additionally in accordance with a preferred embodiment of the present invention a leveling device is provided which is operative to adjust a height of at least one of the imaging systems generally along a vertical axis generally perpendicular to a longitudinal axis of the at least one rail. 
In accordance with a preferred embodiment of the present invention an isocenter of one of the imaging systems is substantially collinear with an isocenter of another of the imaging systems. 
Further in accordance with a preferred embodiment of the present invention one of the imaging systems includes a computerized tomography (CT) imaging system, and another of the imaging systems includes a positron emission tomography (PET) imaging system
There is also provided in accordance with a preferred embodiment of the present invention a multimodality imaging system including a plurality, of imaging systems, and a set of wheels attached to it least one of the imaging systems by means of an adjustable suspension assembly mounted to the at least one of the imaging systems. 
In accordance with a preferred embodiment of the present invention the suspension assembly includes a linkage member pivotally mounted to the at least one of the imaging systems. 
Further in accordance with a preferred embodiment of the present invention a leveling device is connected to the linkage member. 
Still further in accordance with a preferred embodiment of the present invention an arresting device is adapted to fix at least one of the imaging systems at a predetermined location. 
There is also provided in accordance with a preferred embodiment of the present invention a method for installing a multimodality imaging system, including providing at least one rail upon which at least one imaging system is slidingly mountable, aligning a first imaging system, such that the first imaging system is substantially aligned and leveled with a longitudinal axis of the at least one rail, and slidingly mounting a second imaging system on the at least one rail. 
In accordance with a preferred embodiment of the present invention the first imaging system is also mounted on the at least one rail. 
Further in accordance with a preferred embodiment of the present invention the second imaging system is aligned with the first imaging system such that an isocenter of the first imaging system is substantially collinear with an isocenter of the second imaging system. 
Still further in accordance with a preferred embodiment of the present invention the imaging systems are separated from each other by sliding the second imaging system along the at least one rail. 
Additionally in accordance with a preferred embodiment of the present invention at least one of the imaging systems is serviced after separating them from each other.  The imaging systems may be slid back towards each other after servicing.