Static Versus Prospective Gated Non–breath Hold Volumetric MDCT Imaging of the Lungs1

Abstract

Rationale and Objectives: We seek to establish lung imaging methods which provide for the ability to image the lung under dynamic, non-breath hold conditions while providing "virtual breath hold," quantifiable volumetric image data sets. We use static, breath hold images as the gold standard for evaluating these virtual breath hold images in both a phantom and sheep. Materials and Methods:We have developed axial methods for gating image acquisition to multiple points in the respiratory cycle interleaved with incremental table stepping during multidetector-row CT (MDCT) scanning. Datasets are generated over multiple breaths, providing volume images representative of multiple points within a respiratory cycle. To determine the reproducibility and accuracy of the methods , 6 anesthetized sheep were studied by MDCT in non-gated and airwaypressure (P awy )-gated modes where P awy was 0, 7 and 15 cmH 2 O.Results: No significant differences were found between the coefficient of variation in air volume measured from repeated static scans (1.74±1.78%), gated scans: Inspiratory gated (1.2±0.44%) or expiratory-gated (1.39±0.98%), or between static (1.74±1.78%) and gated (1.39+/-0.98%) scanning at similar P awy (p>0.1). Measured air volumes were larger from static vs. gated scans by 5.85±3.77% at 7cmH 2 O and 4.45±3.6% at 15cmHL 2 O P awy (p<0.05) consistent with hysteresis. Differences between air volumes at 7 and 15 cmH 2 O measured from either static or gated scans or that delivered by a supersyringe were insignificant (p<0.05). Visual accuracy of 3D anatomic geometry was achieved, and landmark certainty was within 1mm across respiratory cycles. Conclusion:A method has been demonstrated which provides for accurate gating to respiratory signals during axial scanning. High resolution volumetric image datasets are achievable while the scanned subject is breathing.Images are quantitatively similar to breath hold images with differences likely explained by known P-V hysteresis effects.

Posture-dependent Human 3He Lung Imaging in an Open-access MRI System

“…Hyperpolarized 3 He gas is produced via the spin-exchange optical pumping technique using vaporized Rb as an intermediate (14). Our modular 3 He polarization apparatus includes gas storage, transport, and delivery stages (37); recent modifications are described here.…”

“…Scintigraphy (11) offers tomographic and quantitative information but uses relatively high and costly doses of nuclear tracers and suffers from poor resolution. Computed tomography provides superior anatomic detail with limited functional data (12)(13)(14). Positron emission tomography (PET) and PET/ computed tomography are used to directly measure pulmonary ventilation and perfusion and have provided the best regional quantitative detail thus far (4,15), but subjects are restricted to prone or supine orientations.…”

“…To perform MRI at such a field strength, we exploit the practicality of hyperpolarized 3 He MRI at magnetic fields Ͻ10 mT (35)(36)(37)(38). 3 He hyperpolarized to 30%-60% can be created by one of two laser-based optical pumping processes (14,15) before the MRI procedure, and then high-resolution gas space imaging can be performed without the need of a large applied magnetic field. Such highspin polarization gives 3 He gas a magnetization density similar to that of water in ϳ10 T fields, despite the drastically lower spin density of the gas.…”

Functional Imaging: CT and MRI

“…With the broader coverage, retrospective gating methods have emerged for both cardiac as well as pulmonary imaging whereby, via a very low pitch, images are gathered during the respiratory or cardiac cycles while recording the physiologic signal together with the projection images. Prospectively, (11) or retrospectively (10,(12)(13)(14)(15)(16)(17), the portions of the physiologic cycle of interest can be selected from within the slow pitch spiral data such that a volumetric image data set is reconstructed for just that location of the respiratory or cardiac cycle. Multiple portions of the physiologic cycle can thus be reconstructed to yield a dynamic image sequence of the organ of interest.…”

Hyperpolarized Media MR Imaging – Expanding the Boundaries?

“…Multidetector-row CT (MDCT) and other techniques have recently been used for the quantitative assessment of regional lung function including ventilation (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) and perfusion (31) at a level of resolution approaching the size of the pulmonary acinus which represents the functional unit of the lung. Establishment of quantitative tools for the assessment of images derived from hyperpolarized gas studies via MRI have been slow to emerge, and quantitatability of MDCT image data is one of MDCT's strengths.…”