Parallax corrected external alignment lights

Claim 2

In a tomography system including a gantry having a generally cylindrical opening to receive a patient, a patient table longitudinally translatable to move the patient from a staged position to a scanning position within the gantry opening, means to tilt at least one of said patient table and said gantry about a lateral tilting axis to provide oblique scans, and external alignment light means mounted on said gantry for indicating on the staged patient the portion of the patient to be scanned, parallax correction means comprising: sensing means to detect the angulation of said gantry with respect to said patient table; and correction means to modify the translation distance of said patient table when advancing to scan plane to compensate for the parallax caused by gantry angulation.

Claim 3

The tomography system of Claim 1, wherein said light means is a laser.

Claim 4

The tomography system of Claim 1, wherein said gantry is tiltable about said tilting axis.

Claim 5

The tomography system of Claim 3, wherein said light means is mounted on said gantry substantially above said tilting axis.

Claim 6

The tomography system of Claim 3, wherein said tilting axis passes through the isocenter of said gantry.

Claim 7

The tomography system of Claim 1, wherein said light means comprises first and second sources of a fan shaped beam of light mounted on said gantry, said first and second light sources being respectively fixed to the right and to the left of the axis of translation of said patient table.

Claim 8

The tomography system of Claim 6, wherein said first and second light sources project a pattern along about 200 degrees of the perimeter of said patient.

Claim 9

The tomography system of Claim 1, wherein said sensing means comprises an angular digital encoder which maintains a digital value corresponding to the angular position of the gantry with respect to its base.

Claim 10

The tomography system of Claim 8, wherein patient table translation is governed by a microprocessor, patient table position is recorded by a linear digital encoder, and said parallax correction means comprises: memory means to store a value for translation distance of said table from said staged position to said scanning position when said gantry is perpendicular to said table; means to read said linear encoder to establish a landmark value for the staged position of said table; means to read said angular digital encoder to establish a gantry angle; means to multiply said stored value for translation by the secant of said gantry angle, whereby to derive an updated value for patient table translation distance; means to add said updated value for translation distance to said landmark value to derive a value for the corrected scanning position of said table; and means to drive said table into said gantry until said linear digital encoder indicates a value for table position equal to said value for corrected scanning position.