A method for the reconstruction of four‐dimensional synchronized CT scans acquired during free breathing

Abstract

Breathing motion is a significant source of error in radiotherapy treatment planning for the thorax and upper abdomen. Accounting for breathing motion has a profound effect on the size of conformal radiation portals employed in these sites. Breathing motion also causes artifacts and distortions in treatment planning computed tomography (CT) scans acquired during free breathing and also causes a breakdown of the assumption of the superposition of radiation portals in intensity-modulated radiation therapy, possibly leading to significant dose delivery errors. Proposed voluntary and involuntary breath-hold techniques have the potential for reducing or eliminating the effects of breathing motion, however, they are limited in practice, by the fact that many lung cancer patients cannot tolerate holding their breath. We present an alternative solution to accounting for breathing motion in radiotherapy treatment planning, where multislice CT scans are collected simultaneously with digital spirometry over many free breathing cycles to create a four-dimensional (4-D) image set, where tidal lung volume is the additional dimension. An analysis of this 4-D data leads to methods for digital-spirometry, based elimination or accounting of breathing motion artifacts in radiotherapy treatment planning for free breathing patients. The 4-D image set is generated by sorting free-breathing multislice CT scans according to user-defined tidal-volume bins. A multislice CT scanner is operated in the ciné mode, acquiring 15 scans per couch position, while the patient undergoes simultaneous digital-spirometry measurements. The spirometry is used to retrospectively sort the CT scans by their correlated tidal lung volume within the patient's normal breathing cycle. This method has been prototyped using data from three lung cancer patients. The actual tidal lung volumes agreed with the specified bin volumes within standard deviations ranging between 22 and 33 cm3. An analysis of sagittal and coronal images demonstrated relatively small (<1 cm) motion artifacts along the diaphragm, even for tidal volumes where the rate of breathing motion is greatest. While still under development, this technology has the potential for revolutionizing the radiotherapy treatment planning for the thorax and upper abdomen.

Comparative evaluation of image quality from three CT simulation scanners

“…The collection of volumetric patient data over time (not possible with single‐slice scanners) can then be used to model tumor and lung motion during breathing or to determine the extent of tumor motion for margin determination in the treatment‐planning process. ( 17 ) Cardiac imaging has also benefited from the capabilities of multislice scanning, which has been used to measure coronary motion, which is then correlated with cardiac image quality. In fact, ECG‐gated spiral CT using multislice scanners has made it possible to scan the entire heart in a single breath hold, while continuous data acquisition makes it possible to reconstruct the multiple phases of the cardiac cycle.…”

Attenuation Correction Strategies for Positron Emission Tomography/Computed Tomography and 4-Dimensional Positron Emission Tomography/Computed Tomography

“…[77][78][79] However, it is not clear whether the setup of 4-dimensional CT imaging primarily for the assessment of tumor motion is ideal for obtaining average CT when most PET/CT scanners are in nuclear medicine and are without the respiratory monitoring device needed for 4-dimensional CT. A practical approach is to acquire average CT without a respiratory monitoring device to improve quantification of the PET data. 51,80 The additional radiation dose for average CT can be less than 1 mSv, and the additional scan time is less than one minute and will not impact the overall scan time of a PET/CT procedure.…”

“…84 It was adopted for tumor imaging of the thorax almost a decade ago, 60,[66][67][68] about the same time when 4-dimensional CT was launched. 62,[77][78][79]85 Today, 4-dimensional CT has been accepted as a standard practice in CT imaging of tumor motion for radiation therapy. 4-dimensional PET is still not yet widely accepted as a standard practice.…”

Geometric accuracy of the MR imaging techniques in the presence of motion

“…Due to its excellent soft tissue contrast and nonionizing radiation employment, MRI is increasingly being used as an alternative modality to the four‐dimensional computed tomography (4D‐CT),1, 2 especially when determining patient‐specific breathing pattern for organs’ motion tracking during radiotherapy delivery. Furthermore, considerable effort is being invested in incorporating real‐time MRI during therapy delivery to develop new therapy techniques such as MRI‐guided high‐intensity focused ultrasound (HIFU) and MRI‐integrated Linear Accelerators (LINAC) 3, 4, 5…”