A large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle, the arteries effectively act as anastomoses for each other. This means that if any one of the communicating arteries becomes blocked, blood can flow from another part of the circle to ensure that blood flow is not compromised.
The circle of Willis is formed by both the internal carotid arteries, entering the brain from each side and the basilar artery, entering posteriorly. The connection of the vertebral arteries forms the basilar artery. The basilar artery divides into the right and left posterior cerebral arteries. The internal carotid arteries trifurcate into the anterior cerebral artery, middle cerebral artery, and posterior communicating artery. The two anterior cerebral arteries are joined together anteriorly by the anterior communicating artery. The posterior communicating arteries join the posterior cerebral arteries, completing the circle of Willis.
The time of flight angiography MRI technique allows imaging of the circle of Willis without the need of a contrast medium (best results with high field MRI). A cerebrovasular contrast enhanced magnetic resonance angiography (MRA) depicts the circle of Willis in addition to the vessels of the neck (carotid and vertebral arteries) with one bolus injection of a contrast agent.

For Ultrasound Imaging (USI) see Cerebrovascular Ultrasonography at Medical-Ultrasound-Imaging.com.

(MRI-CA) Coronary angiography with dobutamine stress tagging (MR images are taken after the heart has been stressed by using a medication called dobutamine). Investigational noninvasive imaging as a diagnostic tool for evaluating stenosis, anatomy and flow effects in coronary arteries with dobutamine stress.

For Ultrasound Imaging (USI) see Stress Echocardiogram at Medical-Ultrasound-Imaging.com.

The Esaote Group, founded in the 1980's, is a global leader in research, production, and marketing of medical diagnostic equipment and related services. Esaote offers integrated diagnostic solutions for ultrasound electromedical diagnostic systems, and MRI. The Esaote MR equipment is dedicated for specific clinical applications, as distinguished from conventional whole body MRI systems. This MRI product family is based on a unique, proprietary technology developed specifically for musculoskeletal imaging.
The Italian based company is a member of the Bracco Group.
Esaote North America is the exclusive U.S. distributor of MRI devices manufactured by Esaote.

MRI Scanners:

0.2T to 0.31T:
Vet-MR (Based on the E Scan XQ, dedicated to the veterinarian practice.)

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Hitachi Medical Systems America, Inc. (HMSA), was a major provider of magnetic resonance imaging systems in the United States. Hitachi had more than 2300 installed permanent magnet MR imaging systems worldwide. As a full-line supplier of medical imaging equipment in Japan, Hitachi Medical Corporation (HMC) founded HMSA to provide a direct link to the U.S. marketplace. Altaire™ , the open MR system from Hitachi, extends the family of open MRI products.

In December 2019 Japan's Fujifilm announced the acquisition of Hitachi's diagnostic imaging business for 179 billion yen ($1.63 billion). This includes Hitachi’s CT, MRI, X-ray, and ultrasound imaging operations, also its electronic health record business. Fujifilm expects the deal to close in July 2020 subject to regulatory clearances.

MRI Scanners:

0.3T to 0.7T:
1.5T:

Lantheus Medical Imaging is a worldwide leader in medical imaging, with headquarters in North Billerica, Massachusetts, and nearly 700 employees worldwide with offices in Puerto Rico, Canada, and Australia.
The product portfolio includes ABLAVAR™ (gadofosveset trisodium) a blood pool contrast agent for magnetic resonance angiography MRA, Cardiolite® (Kit for the preparation of Technetium Tc99m Sestamibi), a cardiac perfusion imaging agent and DEFINITY® Vial for (Perflutren Lipid Microsphere) injectable suspension, the leading cardiac ultrasound contrast agent in the US.

MRI Contrast Agents: