Fusion modality imaging -- the combination of two imaging modalities into a single system -- is being hotly pursued by medical equipment vendors. Fusion imaging’s benefit to clinicians is the ability to perform dual-modality studies in a single exam without repositioning the patient. Recent products have included combination PET/CT systems and ultrasound/C-arm systems. On the drawing board are systems combining modalities such as PET/mammography and MR/x-ray.
A group of scientists at the department of radiology at Stanford University in Stanford, CA, are developing an x-ray fluoroscopy/MR system for interventional studies. At the 2002 RSNA meeting in Chicago, they shared some surprising findings of their research on x-ray tube behavior in a large magnetic field.
"We found that the main magnetic field modified the incident electron trajectories," said Rebecca Fahrig, Ph.D., who presented the results.
The Stanford team’s MR/fluoroscopy system consists of a fixed-anode x-ray tube with a 12° anode, a digital flat-panel x-ray detector (a GE Medical Systems prototype called Apollo), a calibrated ionization chamber (which was verified in the magnet, both horizontally and vertically, using Tc-99m as a radiation source), and a 0.5-tesla interventional magnet (GE Medical Systems Signa SP).
The group acquired measurements of tube output (in millirads per minute), half-value layer (HVL) using millimeter-thick aluminum as a filter, and focal-spot distribution as functions of field strength. The alignment between the cathode-anode axis and the main magnetic field was maintained for all experiments, noted Fahrig. The interactions and trajectories of electrons were simulated using the Electron Gamma-Ray Shower version 4 model (EGS4) to garner results.
"We found that the x-ray tube output per milliampere increased at high magnetic fields. In fact, the output increased by approximately 35% in a diagnostic-strength magnetic field," Fahrig said.
At 72 kVp, although the x-ray tube output increased by ~35%, the HVL decreased by only 6% as the field strength increased from zero tesla to 0.32 tesla, Fahrig said. The investigators also discovered that the focal-spot area increased with increasing field strength, and then decreased at higher field strength. Farig reported that at 0.32 tesla the focal-spot area was only 35% larger than it was at zero tesla.
The researcher’s EGS4 modeling showed that approximately 35% of electrons incident on the irradiated width of the target were backscattered with most of their kinetic energy.
"Normally, these backscattered electrons either re-accelerate onto the surface and contribute to off-focal radiation, or hit other parts of the x-ray tube and produce no x-rays. In a large magnetic field, backscattered electrons spiral tightly about the magnetic field lines, and are reaccelerated onto the tungsten anode close to the original scattering site, producing x-rays with similar energy, angular distribution, and source location to those produced by the incident electrons," Fahrig said.
Although there was an increase in tube output in a diagnostic-strength magnetic field, the x-ray spectrum is not significantly different at zero-tesla field strength, she said.
"We found the associated increase in instantaneous focal-spot heat loading occurred due to the magnetic field. Global heat loading of the magnet, bearings, and other tube components probably won’t increase. We’d like to do some other experiments at higher field strengths to further elucidate focal-spot plots," Fahrig said.
By Jonathan S. BatchelorAuntMinnie.com staff writer
March 5, 2003
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