Megavoltage computed tomography

Megavoltage Computed Tomography
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Megavoltage Computed Tomography (MVCT) is an advanced imaging technique used primarily in the field of radiation therapy. It utilizes high-energy X-rays, typically in the megavoltage range, to create detailed cross-sectional images of the body. This technology is particularly useful for image-guided radiation therapy (IGRT), allowing for precise targeting of tumors while minimizing exposure to surrounding healthy tissues.

Principles of Megavoltage Computed Tomography

Megavoltage CT operates on the same basic principles as conventional computed tomography (CT), but with some key differences due to the higher energy levels involved. In MVCT, X-rays are generated by a linear accelerator, which is also used for delivering therapeutic radiation doses. The X-rays pass through the patient's body and are detected by an array of detectors on the opposite side, creating a series of images that are reconstructed into a three-dimensional representation of the patient's anatomy.

X-ray Generation

In MVCT, X-rays are produced by accelerating electrons to high energies using a linear accelerator. These electrons collide with a heavy metal target, typically tungsten, producing X-rays in the megavoltage range (4-25 MV). This is in contrast to diagnostic CT, which uses kilovoltage X-rays (typically 80-140 kV).

Image Reconstruction

The data collected by the detectors is processed using complex algorithms to reconstruct cross-sectional images. The higher energy X-rays in MVCT result in lower contrast images compared to kilovoltage CT, but they provide better visualization of dense structures such as bones and are less affected by artifacts from metal implants.

Applications in Radiation Therapy

Megavoltage CT is primarily used in the context of radiation therapy, where it plays a crucial role in treatment planning and delivery.

Image-Guided Radiation Therapy (IGRT)

In IGRT, MVCT is used to verify the position of the patient and the tumor immediately before or during radiation treatment. This ensures that the radiation is delivered precisely to the target area, improving treatment efficacy and reducing side effects.

Adaptive Radiation Therapy

MVCT can also be used in adaptive radiation therapy, where treatment plans are modified based on changes in the patient's anatomy over the course of treatment. This is particularly important for tumors that may shrink or shift position during therapy.

Advantages and Limitations

Advantages

• Reduced Metal Artifacts: MVCT is less susceptible to artifacts caused by metal implants, making it useful for patients with prosthetics or dental fillings.
• Bone Visualization: The high-energy X-rays provide excellent visualization of bony structures, aiding in accurate patient positioning.

Limitations

• Lower Soft Tissue Contrast: Compared to kilovoltage CT, MVCT has lower contrast for soft tissues, which can make it challenging to distinguish between different types of soft tissue structures.
• Higher Radiation Dose: MVCT involves higher radiation doses than diagnostic CT, although the doses are still within safe limits for therapeutic applications.

Comparison with Kilovoltage CT

While both MVCT and kilovoltage CT are used in radiation therapy, they serve different purposes and have distinct advantages and disadvantages. Kilovoltage CT provides better soft tissue contrast and is often used for initial diagnostic imaging and treatment planning. MVCT, on the other hand, is more suited for treatment verification and adaptive therapy due to its reduced susceptibility to metal artifacts and better bone visualization.

Future Directions

Research is ongoing to improve the image quality of MVCT and reduce the radiation dose. Advances in detector technology and image reconstruction algorithms hold promise for enhancing the utility of MVCT in clinical practice.

Conclusion

Megavoltage Computed Tomography is a valuable tool in modern radiation therapy, providing critical information for accurate treatment delivery. Its ability to visualize bony structures and reduce metal artifacts makes it an essential component of image-guided and adaptive radiation therapy.