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Department of Radiology Achieves World's Lowest Exposure Dose for Infants

From The University of Tokyo Hospital to the World
Department of Radiology

Osamu Abe, Professor and Chairman

Written by Eriko Maeda, Project Assistant Professor

The Department of Radiology handles the full spectrum of radiological examination and treatment at the University of Tokyo Hospital. The department is comprised of the division of diagnostic radiology, which undertakes accurate systemic diagnostic imaging (CT, MRI, etc.), systemic angiography, and endovascular treatment; the division of nuclear medicine, which undertakes PET and RI examination; and the division of radiotherapy, which undertakes radiotherapy for cancer and vascular malformation. This issue introduces the challenges taken on by the division of diagnostic radiology to achieve the world’s lowest level of radiation exposure in cardiac CT examination.

1. Clinical diagnostic radiology

In modern medicine, the diagnosis and follow-up of many diseases depend on diagnostic imaging. While radiologists have few opportunities to see patients face-to-face, they are deeply engaged in team care as the only specialists who diagnose all diseases affecting the entire body through imaging.
When requests for examination are received from clinical departments, we first examine appropriate test methods to match the conditions, including the necessity of contrast medium in CT; the injection rate and scanning timing if a contrast medium is used; and what types of images should be taken in MRI (diagnosis is performed by comparing images that are taken using various methods, such as T1-weighted scans and diffusion-weighted images). During tests, radiologists are focused on safe examination with low exposure. Following scanning, they proceed to the interpretation process, in which radiologists check the images for abnormalities through systematic examination of all organs. If an abnormality is found, detailed findings are presented, and around 10 disease candidates are listed. Subsequently, the list is narrowed down to the three or so that seem the most likely. After prioritizing items in the shortlist, the interpretation of systemic findings and of abnormal findings are described in writing, and communicated to the relevant clinical departments as the final step in the process. Radiologists participate in almost all clinical conferences, including those of the cancer board, and provide inputs as imaging specialists.
The University of Tokyo Hospital has more than 20 board-certified diagnostic radiologists. The division of diagnostic radiology has approximately 30 radiologists, including graduate students and medical interns. In October 2016, we welcomed new professor Dr. Osamu Abe, and we are renewing our efforts in clinical practice, education, and research (Photo 1).

2. Reduced exposure in cardiac CT

There are extremely large differences among facilities in terms of the image quality of cardiac CT and the exposure doses involved, depending on the form of participation by radiologists. In our department, experienced diagnostic radiologists specializing in cardiovascular imaging, led by Project Assistant Professor Maeda, have performed cardiac CT on patients ranging from newborn babies to the elderly, while at the same time conducting ongoing research toward reduced exposure (Photo 2).
Patients with congenital heart diseases must undergo a series of examinations accompanying radiation exposure, including catheterized examination and treatment in addition to CT scans, and infants, in particular, are highly sensitive to radiation. This is why efforts have been made to minimize the exposure dose in CT. We have succeeded in taking high-quality cardiac CT images of infants with the lowest-level dose in the world (approx. 0.2 mSv) by applying full iterative reconstruction to the state-of-the-art 320-slice CT. This is on the level of one-tenth to one-hundredth of the dose received in infant cardiac CT at a normal hospital, and is equivalent to one routine chest x-ray (or because the exposure dose in routine chest radiography is also low at our hospital, equivalent to 10 scans), or to a round-trip flight between Japan and the U.S.
While 320-slice CT scans have become popular in Japan and overseas, images at our hospital are remarkably clear despite the significantly low dose. The main reason for this is that because they have had considerable experience in the diagnosis of patients with complicated congenital heart disease, our radiologists have developed an adequate understanding of images required in clinical settings through communication with pediatricians and thoracic surgeons, and have thereby developed methods for using contrast mediums in accordance with various hemodynamic patterns. Because the heart of an infant measures just several centimeters, images are required to have an accuracy of between half to one millimeter. Furthermore, the rate of their heartbeat is fast, while blood flows vary depending on the congenital heart disease, making imaging difficult in and of itself, even before examining any possible reduction in the exposure dose. The extreme level of specialization is close to that of macro photographers or astrophotographers. Because our hospital has established basic infrastructure to cope with the highest level of difficulty, we are able to acquire images with sufficient clarity at reduced doses. Moreover, each single test can be finished without the fear of retaking, as failure is extremely rare. Because our radiologists have become accustomed to working with many images, they can perform a proper diagnosis, which contributes to an even lower dose, thereby forming a virtuous circle. This is how our hospital has achieved routine examination at a low dose that no other facility in the world can match (Photos 3 and 4). Another element that has contributed to our success is that we are able to use state-of-the-art models and image processing programs as part of joint research conducted with manufacturers.
Lower dose does not always mean better imaging. Just as is the case with underexposure in a camera, too low a dose compromises image quality. Without the requisite experience, it is difficult to reduce the dosage when working with actual patients. We therefore apply findings collected through infant cardiac CT to adults, thereby achieving a reasonable reduction in exposure (Photo 5). We invite you to come and receive radiological examination at our hospital with peace of mind.

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