Magnetic Resonance Imaging in the Evaluation of Abdominal Aortic Aneurysm

Magnetic Resonance Imaging in the Evaluation of Abdominal Aortic Aneurysm

Martin R. Prince

Magnetic resonance imaging (MRI) provides a comprehensive preoperative evaluation of abdominal aortic aneurysms (AAAs) without requiring ionizing radiation, arterial catheterization, or nephrotoxic contrast (Figure 1). It is also useful in evaluating aortic dissection and inflammatory aneurysms. Images are formed by first placing the patient within a magnetic field that is strong enough to cause a small fraction of the atomic nuclei to orient with the magnetic field. A radio frequency pulse wiggles these nuclei so they emit a signal. By collecting signals from hundreds of pulses, it is possible to build up enough signal-to-noise ratio to create an image.

The radio frequency pulse must be tuned to a specific atomic nucleus. Each type of nucleus has a frequency at which it resonates. Because hydrogen is by far the most prevalent atom in the human body, virtually all imaging is performed by tuning to the hydrogen frequency. It is possible to sample a variety of nuclei with a process known as MR spectroscopy or even to obtain images based on nuclei other than hydrogen, but because the hydrogen in water and fat is abundant in blood and perivascular tissues, the standard hydrogen imaging is ideal for evaluating vascular pathology, including AAAs. Hydrogen imaging is sometimes referred to as proton imaging because the hydrogen nucleus is a proton.

The imaging process is characterized by two important time constants, known as T1 relaxation and T2 relaxation. The T1 relaxation time constant characterizes the time required for protons to recover after a pulse so that another pulse can be given. Fat has a fast T1 relaxation and thus is bright on T1-weighted images. Muscles and organs are intermediate in signal intensity on T1-weighted images. Blood tends to flow away in between the pulse and the collection of the data and thus appears as a blackflow void. When the blood flow is not very fast, which is common in aortic aneurysms, there may be an incomplete vascular flow void, and the blood may be gray.

The T2 relaxation time for each tissue represents how long the signal lasts after each pulse. Typically, water and water-rich tissues, including tumors, organs, inflammation, and so on, hold their signal for a long time and appear bright on T2-weighted images. Normally, blood with a lot of water would be expected to be bright on T2-weighted images; however, blood tends to flow away in between the pulse and collection of signal, so it actually appears as a blackflow void.

With very fast pulse sequences, called gradient echo pulse sequences, it is possible to capture bright signal from blood. These fast images can be further enhanced with injection of gadolinium contrast. The gadolinium enhances T1 relaxation, allowing pulses to be given more rapidly to achieve even higher signal-to-noise ratio for higher image quality, often within a comfortable breath-hold. The contrast also helps to eliminate flow artifacts and allows use of a fast three-dimensional (3-D) gradient echo pulse sequence that minimizes metallic surgical clip artifacts. Bright blood imaging is also possible with fast gradient echo sequences with balanced gradients that allow leftover signals from each echo to superimpose, thereby building up high signal-to-noise ratio and T2 weighting while compensating for blood motion. These sequences, known as steady-state free precession, are commonly used for the noncontrast MRA techniques.

Evaluation of AAAs is performed with a series of MR images that take advantage of the multiplanar capability of MR and the multiple contrast mechanisms available with MR. T1-weighted images can depict the aneurysm with a blackblood effect in sagittal, coronal, or axial planes (Figure 2). High-resolution evaluation of the luminal anatomy of the aneurysm and its relationship to renal, splanchnic, iliac, and common femoral arteries is performed with a high-resolution, high-speed, 3-D gadolinium-enhanced magnetic resonance angiography (MRA) sequence (Figure 3). This high-resolution, high-speed 3-D gadolinium-enhanced MRA sequence can also assess for branch vessel stenoses and for enhancement associated with inflammatory aneurysms. Post-gadolinium (Figure 4) and/or T2-weighted images in the axial or sagittal plane can also help identify inflammatory aneurysms.

Jul 15, 2018 | Posted by in CARDIOLOGY | Comments Off on Magnetic Resonance Imaging in the Evaluation of Abdominal Aortic Aneurysm
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