Please explain four-dimensional computed tomography (4D-CT).

Four-Dimensional Computed Tomography (4D-CT)

4D-CT, also known as respiration-correlated CT, is a specialized imaging technique that captures the motion of anatomical structures throughout the respiratory cycle by acquiring multiple CT images at each couch position and retrospectively sorting them based on respiratory phase. 1

Core Concept and Definition

The "4D" refers specifically to the addition of respiratory motion as the fourth dimension to the three spatial dimensions of conventional CT imaging. 1 This technique addresses a critical limitation of standard CT scans, which capture only a random snapshot of tumor position during free breathing and can generate significant motion artifacts. 1

Technical Acquisition Process

Data Collection Method

• 4D-CT is performed using multislice CT scanners operated in cine (axial) mode at each couch position. 1, 2
• The scanner acquires multiple images (typically 12-15 consecutive scans) at each slice location over the duration of at least one complete respiratory cycle plus one full CT gantry rotation. 3, 2
• An oversampled spiral CT scan can be acquired using a pitch of 0.5 with scanner rotation times of 0.5-1.5 seconds. 4, 2
• CT scans should be acquired with slice thickness of 2-3 mm for optimal resolution. 1

Respiratory Tracking

The patient's breathing pattern must be monitored simultaneously during CT acquisition using either:

• External respiratory signals (such as infrared cameras tracking chest wall motion or abdominal surface movement) 4, 2
• Quantitative spirometry measurements that directly measure tidal volume 3
• Internal fiducial markers (though less commonly used for initial imaging) 1

The respiratory tracking provides precise temporal correlation that serves as the sorting metric for image reconstruction. 3

Image Reconstruction and Sorting

Retrospective Binning Process

• Each acquired CT image is time-stamped and sorted into specific "bins" corresponding to different phases of the respiratory cycle based on the concurrent respiratory signal. 4, 2
• Images from different couch positions acquired at similar respiratory phases are combined to create complete volumetric datasets. 2
• Typically, 8-10 complete 3D CT volumes are reconstructed, each representing a different phase of the breathing cycle. 2
• Phase tolerances during binning must be carefully chosen to ensure complete volumetric coverage while maintaining temporal coherence. 2

Quality Metrics

The quality of 4D-CT reconstruction can be quantified by correlating the respiratory metric with internal anatomical motion. 3 Studies using internal lung air content as a surrogate for motion have demonstrated excellent correlations (r>0.99) with spirometry-measured tidal volume, with reconstruction precision typically better than 8% (mean 5.1%). 3

Clinical Applications and Advantages

Radiotherapy Treatment Planning

4D-CT is strongly preferred over conventional CT for treatment planning in thoracic oncology, particularly for lung cancer radiotherapy. 1 The European Organisation for Research and Treatment of Cancer (EORTC) explicitly recommends this approach in their guidelines published in the Journal of Clinical Oncology. 1

Key Clinical Benefits

• Allows for individualized, smaller treatment margins even for tumors with limited motion (the majority of primary lung tumors), resulting in reduced radiation exposure to normal tissues. 1
• Provides critical information about motion of mediastinal structures (lymph nodes, esophagus) that may differ significantly from primary tumor motion. 1
• Enables evaluation and implementation of motion-management strategies for highly mobile tumors. 1
• Reduces systematic errors that occur when planning is based on tumor positions at respiratory cycle extremes. 1
• Improves tumor delineation accuracy by reducing partial volume effects. 1
• Allows better quantification of radiotracer uptake when combined with PET imaging. 1

Important Limitations and Caveats

Single Scan Sufficiency

A critical caveat: Analysis of repeat 4D-CT scans has shown that a single 4D-CT may not be sufficient to capture all thoracic motion throughout an entire treatment course. 5 While some studies suggested a single scan was adequate 1, more recent research demonstrated volume differences up to 33.4% between planning 4D-CT and evaluation scans during treatment. 5 This highlights the potential need for repeat imaging during prolonged treatment courses.

Residual Artifacts

• 4D-CT images may still contain residual motion artifacts due to partial projection effects and variations in breathing patterns during acquisition. 2, 6
• Irregular breathing patterns can compromise image quality and introduce geometric errors. 1
• The temporal coherence within reconstructed 4D-CT volumes depends on the number of images reconstructed per slice—more reconstructed images allow smaller phase tolerances and better quality. 2

Technical Considerations

• Intravenous contrast use with 4D-CT scans of the thorax is not well established, though contrast improves delineation of centrally located tumors and lymph nodes on conventional CT. 1
• Various techniques are being investigated to reduce respiratory artifacts that can affect 4D-CT quality. 1
• Differences in breathing patterns between planning 4D-CT and actual treatment delivery can lead to geometric errors, though volumetric imaging during treatment can minimize this risk. 1

Integration with Other Imaging Modalities

4D-CT can be combined with 3D or 4D PET scanning for improved functional and anatomical correlation. 1 The impact of additional 4D PET information compared to 3D PET is promising and under active investigation in prospective stereotactic body radiotherapy (SBRT) trials. 1

Practical Implementation

From phantom studies using typical respiratory periods, 4D-CT demonstrates a precision of approximately 2.5 mm for quasi-regular motion. 2 The technique requires patient cooperation and the ability to breathe naturally during the extended acquisition time (typically several minutes to cover the entire scan length). 1