Abstract:
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Purpose: A hybrid imaging system has been developed that combines x-ray
fluoroscopy and MR imaging. This allows image-guided interventions where x-ray
imaging provides precise, high-contrast, real-time projections and MRI
generates tomographic data, soft-tissue contrast and physiological information.
The present study assesses the effect of the magnetic field on x-ray image
quality.
Methods and Materials: The fluoroscopy system consists of a fixed-anode x-ray
tube (Brand X-ray Co., Addison IL) and a flat-panel detector (FPD) (GE Medical
Systems, Milwaukee, WI) and is placed within an interventional MRI unit (0.5 T
GE Signa SP). The x-ray tube is attached to a bar connecting the
"double-donuts" of the open MR unit and is aligned to the main magnetic
field B0. The FPD is placed below the patient couch and experiences 0.5T field
perpendicular to the readout direction. The modulation transfer function (MTF)
and the noise power spectrum (NPS) of the detector were measured for this setup
with a source to image distance (SID) of 107 cm, for two values of peak voltage
(70 and 85 kVp) and 1.55 mm aluminum beam filtration. The detector NPS was
evaluated for entrance exposures ranging from about 100 to 350 μR/frame.
The MTF was measured at the detector and also at the patient location 21 cm
above the FPD. The x-ray tube and FPD were then moved outside the magnetic
field and mounted on a test stand while maintaining the same geometry and the
measurements were repeated with the tube current adjusted to provide the same x-ray
exposure. This was done to account for the previously reported and modeled
increase in the x-ray tube output, by about 22%, due to the effect of the
magnetic field on the path of the recoil electrons. This phenomenon also
increases the focal spot size.
Results: The detector MTF was not affected by the magnetic field or the kVp
setting. The MTF at the patient position was lower inside the field than
outside resulting in FWHM difference of about 40% between the line spread
function (LSF) at the two locations and was accounted for by the focal spot
size increase. The NPS values did not show any dependence on the magnetic field
across all spatial frequencies.
Conclusion: The x-ray system characteristic measurements show that the detector
image quality is not altered by the magnetic field. However, the magnetic field
does affect the x-ray generation process, causing changes in the output
characteristics of the tube. But overall, the x-ray system in the present setup
functions with image quality losses that are within acceptable limits
particularly at lower magnifications.
(N.P. is a shareholder in GE.)
Questions about this event email: aganguly@stanford.edu
Ganguly PhD, A,
X-ray Image Quality Assessment in X-ray/MR Hybrid System. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3102107.html
