Collimators Component on CT System

Collimation on CT Serves two purpose :

1. to reduce unnecessary radiation doses to patient
2. To ensure good image quality

There are two types of collimator :

1. Prepatient : the position of collimator between the xray source and the paient. Since x ray photons emitted from the x ray cover a very wide range in z, the prepatient collimator restrict the x ray flux applied to narrow region as it sis applied to the patient. For a single slice CT, it not only reduces dose to the patient. It also defines the slice thickness of t he imaging plane. For multislice CT, however, the slice thickness is defined by detector aperture instead of the collimator. Because nearly 99% of the x ray photons emitted from the xray tube blocked by pre patient collimator , x ray efficiency for CT is poor.
2. Postpatient : Typically, two kinds of collimators are used : in plane and cross plane.  The in plane collimation (grid)n is used by third generation CT scanner to reject scattered x ray photons. This technique cannot be employed effectively by foruth generation scanners due to a wide acceptance angle. This type sre place in front of the detector, focusing on the x ray source. Since the path of scateered raediation generally deviates from the original x ray photon (primary photon) path, plat block the photon from watering the detector.Cross plane collimation is employed by both third and fourth generation scanner. For 3^rd generation scanner, it servers mainly as additional z collimator to improve the slice sensitivity profile of scanner. The slice thickness is determined by combination of umbra and penumbra region. Because of the geomethric limitation, it is often difficult to design a prepatient collimator that provides very thin slices profiles for the scanner . The achieve this objective, additional collimation is employed near the surface of the detector to futher restrict the x ray beam to a narrow slice thickness. The disadvantage to approach is the penalty to the dose efficiently of the scanner because a portion of the x ray photons that pass through the patient is not used

A post patient cross plane grid is sometimes used by the fourth generation scanner to reject scattered radiation is a manner similar t the in plane collimation (grid) for the 3^rd generation scanner. Although the role of 4th generation scanners is fading, the use of the grid is likely to be employed by 3^rd generation scanner. As the volume coverage of the detector increases, the scatter to primary ratio (the amount of scattered radiation compared to primary radiation) will increase . In the not too distance future , it may become necessary for the multislice or volume metric scanners to employ suck collimation for scatter rejection