PubMed · 6913768
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L A Kunichev. 1981. [Machine massage].. https://pubmed.ncbi.nlm.nih.gov/6913768/
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Distortion of the static magnetic field inside the human head is dependent on regional tissue susceptibility variations and geometrical shape. These effects result in resonance line broadening and frequency shifts and consequently, intensity and spatial errors in both magnetic resonance imaging (MRI) and magnetic resonance (MR) spectroscopy. To calculate the field distortion due to the susceptibility's geometry, two dimensional (2D) finite element analysis was applied to simulate the field distribution in a 2D model of the human head, placed in a uniform magnetic field. The model contains air-filled cavities and sinuses, and the remainder is treated as water. The magnetic field deviation was evaluated using gray scale plots and histograms of the magnetic field. The shifts in parts/million and broadening of the histograms correspond to the NMR of the sampled region. The field distribution of the human head was also experimentally mapped using the DANTE tagging sequence. The calculated and experimental field maps are in good agreement. Thus, geometric considerations with uniform susceptibilities are sufficient to explain most of the static magnetic field distribution in the human head.
A fluid transport model study was used to evaluate the sealing ability of five root canal sealers, AH 26, Sealapex, Ketac Endo, Roth, and Kerr EWT, and a dental bonding agent at thicknesses of 0.05 mm (thin layer) and 0.3 mm (thick layer) with 270 standard human root sections obturated with sealer combined with standard gutta-percha cylinders. AH26 and Sealapex sealed more tightly in thick layers, whereas Ketac Endo, Johnson and Johnson Bonding Agent, Roth, and Kerr EWT sealed more tightly in thin layers. In thin layers J&J Bonding sealed more tightly than any other sealer tested. In thick layers AH 26 and Sealapex sealed more tightly than Ketac Endo and J&J Bonding Agent, and these sealed more tightly than Roth and Kerr EWT. These findings indicate that the thickness of the sealer layer significantly influences the sealing of a root canal filling and that the influence of thickness varies depending on the type of sealer.