Takemasa Tanaka

Assessment of image quality in dental radiography, part 2 Optimum exposure conditions for detection of small mass changes in 6 intraoral radiography systems

OBJECTIVE The purpose of this study was to compare 2 film systems and several digital intraoral systems with regard to visual image quality through use of a test phantom developed for this purpose. STUDY DESIGN The detectors used for digital imaging were as follows: Computed Dental Radiography (CDR), Digora, Dixel, and Sens-A-Ray without scintillator layer. Two types of digital images were prepared for the observer performance test: one with original gray scales and another with contrast enhancement. Images with and without enhancement from the 4 systems were displayed to 7 observers. The change in the average number of perceptible holes was plotted against exposure, and modified perceptibility curves were created and compared with curves for the film systems. The exposure level at which the maximum number of holes was perceived was defined to be optimum. The optimum exposure levels were determined for each digital system and compared with that of the film systems. At the optimum exposure, the average maximum numbers of perceptible holes in each digital system with and without contrast enhancement were compared with the maximum numbers for the film systems. The minimum exposure levels were determined to be those at which the number of perceptible holes exceeded the number for film, and the possibility of exposure reduction was evaluated. RESULTS All digital systems except the Digora system showed lower optimum exposures than E-speed film. In all digital images without enhancement, however, the maximum number of perceptible holes was significantly lower than that for the film systems at that exposure. With contrast enhancement, all digital systems except the Sens-A-Ray system showed visibility superior to that of the film systems. With the CDR, Digora, and Dixel systems, exposures could be further reduced by a considerable amount, with greater retention of information than was associated with film. CONCLUSIONS Our results strongly suggest that digital systems, if properly used, can exceed film systems in the detection of small mass changes.

THE COMBINED USE OF US AND MR IMAGING FOR THE DIAGNOSIS OF MASSES IN THE PAROTID REGION

Purpose: To evaluate the usefulness of the combination of the two non-invasive modalities US and MR imaging to diagnose masses in the parotid region. Material and Methods: The US and MR findings of 21 patients with parotid masses were analyzed retrospectively by two radiologists without any clinical or histopathological information. The specific points evaluated were location, shape, margin, internal architecture, and intensity level on both US and MR, posterior echo enhancement on US, and capsule-like lining of the tumor on MR. Results: The findings concerning the shape and margin on US and MR were in fairly good agreement. Concerning the findings of the internal architecture, US could reveal the minute structures of the tumor while MR demonstrated differences in the signal intensities of histological tissue types of the various tumors. The posterior echo enhancement on US and the capsule-like lining on MR of the tumors were also useful for the diagnosis. Conclusion: Our results suggest that the combination of US and MR is useful for examining soft tissue masses in the parotid region to make a more accurate diagnosis, and not just differentiate malignant lesions from those which are benign.

Assessment of image quality in dental radiography, part 1 ☆ ☆☆ ★ ★★: Phantom validity

OBJECTIVE The purpose of this study was to describe and validate an image-quality phantom to be used in dental radiography for comparison of film and digitally acquired images. STUDY DESIGN An aluminum block of 12 steps, with 7 holes in each step, was covered by acrylic blocks. This phantom was radiographed with Kodak Ultra-speed and Ektaspeed Plus films at 70, 65, and 60 kVp with the whole exposure range available. All together, 50 dental films were randomly sequenced and presented to 7 observers. The average number of perceptible holes from all steps was plotted against exposure for each tube voltage and film type, generating a modified perceptibility curve. The tentative optimum exposure level was determined from perceptibility curves in each experimental condition and compared with that determined by means of the standard aluminum stepwedge and the preset time of the x-ray machine. The density range of this phantom at the optimum exposure was compared with that of clinical dental radiographs. Validity of the phantom was evaluated according to the optimum exposure level from the modified perceptibility curves and the overall density range. Finally, the average maximum numbers of perceptible holes at the tentative optimum exposure level were compared for each tube voltage and film type. The statistical test used was a 2-way factorial analysis of variance. RESULTS The exposure at the perceptibility curve peak approximated that obtained by means of the standard aluminum step-wedge and the time preset by the manufacturer. The overall density range at the perceptibility curve peak covered the clinical density range for each tube voltage and film type. There were no statistically significant differences between film types or among tube voltages. CONCLUSIONS The x-ray attenuation range for this phantom seemed to approximate clinical conditions. In addition, differences in image quality could be quantitatively evaluated by means of the number of the holes seen in the phantom.

Magnetic resonance imaging of oral and maxillofacial angiomas

Eleven patients with oral and maxillofacial angiomas (seven hemangiomas and four lymphangiomas) were evaluated with magnetic resonance imaging using a 0.2-T permanent system and spin-echo pulse sequences. These lesions typically had signal intensities that were iso T1-weighted, similar to muscle, and high T2-weighted, greater than subcutaneous fat. Nine tumors had well- or relatively well-defined margins, and seven cases had curvilinear structures of low signal intensities in the masses on T2-weighted images. It was impossible to distinguish hemangiomas from lymphangiomas on MR images. Our experience suggested that most angiomas of oral and maxillofacial regions present special characteristics on magnetic resonance images. It is thought that information obtained with magnetic resonance images can contribute significantly to the evaluation of the extent of these lesions.

A follow-up of rhabdomyosarcoma of the infratemporal fossa region in adults based on the magnetic resonance imaging findings: Case reports

Two adult patients with rhabdomyosarcoma of the infratemporal fossa region were evaluated by computed tomography and magnetic resonance imaging both before and after treatment. Successful treatment accompanied by reduction of the tumor mass as a result of chemotherapy was demonstrated in one patient. In the other patient a subsequent increase in the tumor mass was documented. Information on not only the size but also the internal architecture of the tumor may be useful when tumor response to therapy is being investigated. Magnetic resonance imaging is recommended as a method for outcomes evaluation in adult rhabdomyosarcoma cases.

Imaging features of maxillary osteoblastoma and its malignant transformation

We report two cases of osteoblastoma, one of them an unusual case in a 32-year-old woman in whom a maxillary tumor was confidently diagnosed as an osteoblastoma at the time of primary excision and subsequently transformed into an osteosarcoma 7 years after the onset of clinical symptoms. The other patient developed osteosarcoma arising in the maxilla, which was diagnosed 3 years after the primary excision and is very suggestive of malignant transformation in osteoblastoma. We present the radiological features, including computed tomographic and magnetic resonance imaging studies, of this unusual event of transformed tumor and compare imaging features of benign and dedifferentiated counterparts of this rare tumor complex.

Three-dimensional imaging of teeth and jaw bones using fluoroscopic computed tomography (FCT)

Introduction The appearance of CT has made it possible to get three-dimensional images of the human body, and recently it has come into use in dental medicine. Although CT will have an increasing role in dental diagnostic imaging, its use in this area is quite limited because of a large exposure dose, long examination time, and high cost. Owing to the developments in new technology, there is a possibility of realizing a very compact, low exposure CT system which can provide three-dimensional images in a single scan. This new type of CT, which utilizes an image intensifier and a TV camera as a two-dimensional detector system, is often called X-ray video CT or fluoroscopic CT (FCT) 1-3). We have developed a prototype of a microcomputer-based low dose fluoroscopic CT system for scanning the dento-maxi l lo fac ia l region. 4,~) Va r i ous images (such as transverse axial sections, longitudinal sections, and 3D display) were obtained, and the image quality as well as the exposure dose was compared between the present fluoroscopic CT and conventional CT. Method Conventional CT uses a one dimensional X-ray detector (NaI, BGO, Xe etc.) with a fan beam geometry, while fluoroscopic CT uses an image intensifier as a two-dimensional X-ray detector with a cone beam geometry. Therefore, it is nesessary to scan many times to get three-dimensional images in the case of conventional CT, but on the other hand, it is only needed a single scan for threedimensional imaging using FCT. Fig. 1 shows the difference between fluoroscopic and conventional CT schematically.

Computer simulation of an intraoral radiography using perspective projection of CT data

Though the intraoral radiography is most frequently and widely used in clinical dentistry, this method is very special comparing with other extraoral radiography for jaw bones. The radiographer must consider a lot of factors, such as, the anatomical form of maxilla or mandible unique to each patient, direction and size of the tooth, the position of the film, and the position and angle of the cone-head. It is not easy to master this technique because of these factors associated with this technique. Computer-assisted training system may be useful to learn the effects of these factors on the radiograph. The purpose of this study was to develop a computer simulation system for intraoral radiography by a projection from point focus using the three dimensional data of the head with CT scanner.