This invention relates generally to imaging systems and methods and more particularly to systems and methods for retrospective internal gating.
As a patient undergoes an imaging procedure, it is useful that the patient remain still. If a patient moves during the imaging procedure, the image may be blurred and thus lack clarity. For example, when an image is taken of a patient's chest or diaphragm area with a helical scan procedure as the patient breathes, the image lacks clarity. As another example, due to the respiratory motion of the patient, images of tumors or other areas of concern disposed within the patient's chest or abdomen tend to be blurred and lack clarity such that the tumor or area tends to appear larger or smaller than its actual size thus may render an inaccurate estimate of a size of a tumor.
Sometimes radiation therapy is used to treat a tumor. In radiation therapy, the area in which the tumor is located is exposed to a dose of radiation so as to irradiate the tumor. In order to lessen the chance of radiating normal tissue surrounding the tumor, it is useful to accurately locate the position of the tumor. This may be accomplished by imaging the tumor and the area surrounding the tumor using an imaging device such as a computed tomography (CT) imaging system, a Flouroscope, a magnetic resonance imaging (MRI) system and/or a positron emission tomography (PET) imaging system.
Although a helical CT scan can cover a typical scanning distance, for example, 20-30 cm, during a normal breathe hold, and thus completely scan the tumor during this breathe hold, the radiation therapy is a relatively long process and takes around 15 minutes. Therefore, it is not possible for the patient to hold his or her breathe during the therapy procedure. When the patient breathes, the internal organs move by as much as several centimeters, causing the tumors to move in and out of a radiation treatment field.
CT perfusion is another examination that requires an accurate image registration to compensate for the respiratory motion during the study. For example, during a CT liver perfusion procedure two types of image scans are typically performed, an arterial phase and a venous phase. The arterial phase of the imaging procedure normally produces images once a second for the first thirty seconds of breath hold time during which a contrast injection is administered to the patient. The venous phase of the imaging procedure is then performed and is normally measured in an interval of five to ten second intervals and may have a total image acquisition time of two to three minutes or longer. Because the arterial phase of the imaging procedure only takes approximately thirty seconds, holding the patient's breath can reduce or eliminate any image artifacts due to respiratory motion. However, because the venous phase may take a couple of minutes or longer, some patients may not be able to hold their breath for the entire venous phase and image artifacts may be in images obtained.
One method to address respiratory motion of the patient during imaging and application of a radiation dose is respiratory gating. In respiratory gating, movement during imaging is tracked, and even more tightly shaped conformal dose distribution is used. Respiratory gating allows therapists to track the patient's respiratory cycle both at the time of the CT scan for imaging and at the time of treatment. In effect, respiratory gating facilitates isolation of the position of the target during one specific phase of the respiratory cycle, generally, during either exhale or inhale. Thus, by isolating the target position, therapists can decrease the size of the radiation fields to closely match the target size and position.
Respiratory gating is accomplished by monitoring the patient's normal breathing pattern. For example, in a known method, every time the patient exhales, the radiation beam comes on instantly for half a second. The moment the patient starts to inhale, the radiation beam is terminated. The radiation beam is pulsed repeatedly in such a manner until the entire radiation dose has been delivered. Unfortunately, such a configuration increases the total time of radiation treatment.