The CEBAF large acceptance spectrometer (CLAS) is used to study photo- and electro-induced nuclear and hadronic reactions by providing efficient detection of neutral and charged particles over a good fraction of the full solid angle. A collaboration of about 30 institutions has designed, assembled, and commissioned CLAS in Hall B at the Thomas Jefferson National Accelerator Facility. The CLAS detector is based on a novel six-coil toroidal magnet which provides a largely azimuthal field distribution. Trajectory reconstruction using drift chambers results in a momentum resolution of 0.5% at forward angles. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of View the MathML source. These capabilities are being used in a broad experimental program to study the structure and interactions of mesons, nucleons, and nuclei using polarized and unpolarized electron and photon beams and targets. This paper is a comprehensive and general description of the design, construction and performance of CLAS

The CEBAF Large Acceptance Spectrometer for operation at 12 GeV beam energy (CLAS12) in Hall B at Jefferson Laboratory is used to study electro-induced nuclear and hadronic reactions. This spectrometer provides efficient detection of charged and neutral particles over a large fraction of the full solid angle. CLAS12 has been part of the energy-doubling project of Jefferson Lab's Continuous Electron Beam Accelerator Facility, funded by the United States Department of Energy. An international collaboration of over 40 institutions contributed to the design and construction of detector hardware, developed the software packages for the simulation of complex event patterns, and commissioned the detector systems. CLAS12 is based on a dual-magnet system with a superconducting torus magnet that provides a largely azimuthal field distribution that covers the forward polar angle range up to 35 • , and a solenoid magnet and detector covering the polar angles from 35 • to 125 • with full azimuthal coverage. Trajectory reconstruction in the forward direction using drift chambers and in the central direction using a vertex tracker results in momentum resolutions of <1% and <3%, respectively. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of 10 35 cm −2 s −1 . These capabilities are being used in a broad program to study the structure and interactions of nucleons, nuclei, and mesons, using polarized and unpolarized electron beams and targets for beam energies up to 11 GeV. This paper gives a general description of the design, construction, and performance of CLAS12.

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We present a method to correct the geometric distortion caused by field inhomogeneity in MR images of patients wearing MR-compatible stereotaxic frames. Our previously published distortion correction method derives patient-dependent error maps by computing the phase-difference of 3D images acquired at different TEs. The time difference (delta TE = 4.9 ms at 1.5 T) is chosen such that the water and fat signals are in phase. However, delta TE is long enough to permit phase wraps in the difference images for frequency offsets greater than 205 Hz. Phase unwrapping techniques resolve these only for connected structures; therefore, the phase difference for fiducial rods may be off by multiples of 2 pi relative to the head. We remove this uncertainty by using an additional single 2D phase-different image with delta TE = 1 ms (during which time no phase-wraps are typically expected) to determine the correct multiple of 2 pi for each rod. We tested our method in a cadaver and in a patient using CT as the gold standard. Targets in the frame coordinates were chosen from CT and compared with their locations in MR. Localizing errors using MR compared with CT were as large as 3.7 mm before correction and were reduced to less than 1.11 mm after correction.

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