Beam pitch is a crucial parameter in helical (spiral) computed tomography (CT) scanning that relates the patient table movement to the X-ray beam width during data acquisition.
Defining Beam Pitch
In the context of CT scanning, the beam pitch is specifically defined as the table distance travelled in one 360° gantry rotation divided by the total thickness of all simultaneously acquired slices. This ratio is fundamental to understanding how the scanner collects data during a spiral scan.
The Beam Pitch Calculation
The definition provides a direct formula for calculating beam pitch:
Beam Pitch = (Table Distance per Gantry Rotation) / (Total Slice Thickness)
Let's break down the components:
- Table Distance per Gantry Rotation: This is the linear distance the patient table moves forward along the patient's body during a single full rotation (360 degrees) of the X-ray tube and detectors around the patient.
- Total Thickness of All Simultaneously Acquired Slices: In modern multislice CT scanners, multiple slices of data are acquired at the same time. This value is the sum of the thicknesses of all these simultaneously acquired slices.
Why Beam Pitch Matters
The value of the beam pitch directly impacts several aspects of the CT scan:
- Data Coverage:
- A beam pitch of 1 indicates that the table moves exactly the width of the total beam (total slice thickness) during one rotation, resulting in contiguous data acquisition without gaps or overlap.
- A pitch greater than 1 means the table moves faster than the beam width, creating gaps in the data coverage between rotations. This can lead to reduced radiation dose but may miss subtle anatomy.
- A pitch less than 1 means the table moves slower than the beam width, resulting in overlapping data acquisition between rotations. This increases radiation dose but provides more sampled data, which can be beneficial for certain applications and reconstructions.
- Scan Time: A higher beam pitch (pitch > 1) allows the scan to cover a larger length of the patient in less time.
- Image Quality and Dose: Pitch is a trade-off between scan time, radiation dose, and image quality. Choosing the appropriate pitch is vital for optimizing the scan for the clinical question while managing dose.
Understanding beam pitch is essential for CT technologists, radiologists, and physicists involved in optimizing CT protocols.
