Anatomic MRI

Anatomic MRI is not a single imaging method, but rather a variety of techniques for visualizing different tissues in the body (e.g., brain, lungs, heart, liver, bones). MR images can be acquired in any plane, as individual slices or a three-dimensional volume, and with high spatial resolution.

Anatomic MRI images are created using a strong magnetic field and radiofrequency energy, and they can be fine-tuned to highlight water content or fat content. Upon selecting appropriate imaging parameters, it is possible to visualize differences in tissues, including normal and abnormal tissues. Anatomic MRI does not require ionizing radiation, and can be performed repeatedly with little or no risk to pediatric patients and research participants.

Cardiac MR (CMR)

Cardiac MR (CMR) is a subset of anatomic MR that focuses on the heart. Because of the complex motion of the heart caused by both the cardiac cycle and breathing, special techniques are employed. Cardiac imaging protocols allow a series of dynamic MR images acquired over the cardiac cycle. Degradation of image quality can be minimized and avoided using high-speed imaging during a breath hold, and / or the use of strategies to monitor the location of the diaphragm to measure and compensate for breathing motion. When combined with phase-contrast techniques, quantitative measurements of physiologic parameters such as ejection fraction, valve regurgitation and blood velocity are possible.

Contrast-Enhanced MRI

Contrast agents are often useful in clinical MR imaging, particularly in the detection of highly perfused tissue (e.g., tumors) or poorly perfused tissue (e.g., necrotic tissue, fibrosis). In the research setting, contrast agents require ethics board (IRB) approval for use and require nursing of physician support for administration in the IRC. MRI contrast agents modulate the MR signal intensity of tissue by reducing the T1 and / or T2.

Diffusion-weighted MRI (DWI)

Diffusion-weighted MRI (DWI) is a type of MRI that generates images based on the diffusion of water molecules in the body. Diffusion rates for water molecules are tissue specific and can change dramatically with cellular injury and death (e.g., traumatic brain injury, stroke) as well as in a variety of disease states (e.g., tumors). Water molecules undergoing diffusion in the nerves and axons of the brain are highly constrained by the elongated shape of the cells. By acquiring directionally sensitive DWI images, Diffusion Tractography Imaging (DTI) can be performed to generate images of the brain’s white matter tracts and muscle fibers in the heart. DTI images of the brain reveal the neuronal anatomic connections within the brain and can be used to assess brain development and abnormalities.

Functional MRI (fMRI)

Functional MRI is a type of MR imaging that reveals brain activity. When neuronal activity increases in the brain, local changes in blood demand shift the deoxyhemoglobin-to-oxyhemoglobin ratio in the capillary bed. This shift causes a subtle change in the magnetic susceptibility of the blood that can be detected by MRI. This is called the Blood Oxygen Level Dependent (BOLD) effect. fMRI can be used with experimental paradigms to reveal the parts of the brain responding to sensory stimulation (e.g., touch, sight, sound, etc.). It can also be used to reveal the parts of the brain associated with different functions such as cognition, speech formation, memory, and emotion. Additionally, fMRI can be performed in the “resting state”. With this approach, temporal correlations of the BOLD signal in different parts of the brain are measured to determine which parts of the brain are functionally connected to other parts (connectome imaging).

MR Angiography (MRA)

There are several MRI methods available for visualizing blood vessels in the body. These include phase-contrast techniques which can quantify the velocity of moving blood, Time-of-Flight (TOF) techniques which rely on differences in spin-saturation to highlight moving blood, and contrast-enhanced methods in which an intravenous injection of a contrast agent is made to shorten blood’s T1 and make it more visible. All of these methods can be used with two-dimensional and three-dimensional acquisitions. Furthermore, data acquisition can be performed with cardiac synchronization to obtain information about the dynamics of blood flow (e.g., 4D flow).

MR Elastography (MRE)

MR Elastography (MRE) is used to measure the stiffness or hardness of tissues, most commonly the liver. Using a small vibrating drum (“paddle”) placed over the region of interest, shear waves can be created. These shear waves can be imaged using a modified phase-contrast pulse sequence, with increasing wave thickness associated with increasing tissue stiffness. Color stiffness maps (elastograms) can be created. MRE also can be used to evaluate other tissues, such as the pancreas and kidneys. Increasing stiffness has been shown to be a marker of increasing tissue damage and fibrosis (scarring).

MR-guided Interventions

While MRI is most often used for diagnostic imaging, its unique ability to visualize many different types of tissue make it an attractive adjunct for some interventions. MR imaging techniques can be used to visualize therapeutic and sub-therapeutic changes in temperature, follow interventional devices (e.g., needles or catheters) in three dimensions, and provide details of soft tissue response to therapy. In general MR imaging is much slower than X-ray imaging, and the confines of the magnet surrounding the patient limits physical access.

MR Spectroscopy (MRS)

MRS is a type of MR that can extract information about the chemicals (metabolites) that reside in tissues, such as the brain and liver. Using MRS, the amount of certain substances (e.g., NAA, lactate, choline, creatine, and GABA in the brain and fat in the liver) can be established. MRS is similar to NMR spectroscopy used in analytical chemistry laboratories, but is applied in vivo to acquire data from a small volume of tissue, or as an additional dimension of information in an MR image. MRS can be used to interrogate brain tumors, white matter diseases, infections, and a variety of metabolic diseases, including mitochondrial disorders. MRS can also be used to detect and quantify liver fat in the setting of non-alcoholic fatty liver disease (NAFLD).

Multinuclear MRI

Most MR images are based on the imaging of protons (hydrogen atoms). However, certain MRI scanners can acquire data from other elements in the body, such as sodium and phosphorus. In addition, multinuclear MRI can be used to image intravenous or inhaled agents, such as hyperpolarized carbon-13 pyruvate and hyperpolarized xenon or helium gas. While multinuclear MRI can be used to create anatomic images, it is more often employed to study dynamic processes, such as tissue metabolism (e.g., in tumors, liver) and lung and airway function.

Perfusion MRI

Perfusion MRI can be used to assess blood flow within tissue or an organ, most often the brain. While perfusion MRI can be performed with intravenous contrast material, it is more often performed without exogenous contrast agent using arterial spin labeling (e.g., pCASL). pCASL (Pseudo-Continuous Arterial Spin Labeling) “tags” flowing arterial blood immediately prior to entering the tissue of interest (e.g., the brain). Perfusion MRI can be used to image acute, subacute, and chronic cerebrovascular vascular abnormalities, such as stroke, as well as evaluate other disorders, including tumor blood flow before and after treatment. Perfusion MRI can also be used to evaluate other organs, such as the kidneys.

Quantitative MRI

While MRI is most often used to obtain anatomic images that are evaluated qualitatively, MR images that allow quantitative tissue characterization also can be acquired. Such quantitative MRI techniques allow discrimination of normal from abnormal tissue as well as measurement of certain tissue components (e.g., fat or iron). Quantitative MRI techniques that can be performed in the brain, heart, musculoskeletal system, abdomen (e.g., liver, kidneys, pancreas, bowel), etc. and include relaxometry (T1, T2, T2*, and T1rho), Dixon methods for measuring proton density fat fraction and T2*/R2*, and numerous other techniques (e.g., quantification of intestinal peristalsis).

Pre-Clinical MRI

The In Vivo Microimaging Laboratory (IVML) is a dedicated facility for pre-clinical animal MRI. It consists of a horizontal bore 7T system and a 9.4T vertical bore system. These scanners allow imaging with up to 150 µm isotropic resolution. In addition to anatomic imaging, a variety of advanced techniques can be performed using these scanners, including multinuclear and proton MR spectroscopy, diffusion-weighted imaging, and cardiac MRI. Pre-Clinical MRI allows for non-invasive assessment of many models of human disease (e.g., cancers, liver fibrosis, cardiac disease), including longitudinal follow-up to look for changes over time.

Temperature-sensitive Imaging

The conventional contrast mechanisms used in anatomic and other types of MR imaging (e.g., T1, T2, etc.) are temperature sensitive, and, if appropriately calibrated, MR can be used to measure small changes in temperature. For tissue containing a substantial amount of water (e.g., muscle, tumors, etc.) MR can also reveal subtle changes in temperature by measuring thermally-induced changes in the rate of hydrogen-boding exchange of water molecules. Temperature-sensitive MR imaging can be an important adjunct in MR image-guided interventions, such as HIFU and laser ablations.

Ultra-short TE (UTE)

In anatomic and functional MRI there is typically a short delay between the excitation of the MR signal and its detection. Imaging pulse sequences allow the operator to select this delay, and in many cases longer echo times (TE) are useful for highlighting tissues with long T2 (e.g., edema, tumors, CSF, etc.). However, tissues in complex magnetic environments (e.g., lung parenchyma and bone) give MR signals that rapidly lose phase coherence and thus appear to be devoid of signal by the time data is acquired. This phenomenon becomes more pronounced at higher magnetic fields. To overcome these limitations a number of MRI UTE techniques have been developed which allow preservation of signal from tissues such as lung and bone.

Ultrasound

Ultrasound imaging employs high-frequency sound waves to image tissues within the body. Images are obtained using a hand-held probe (transducer) which is placed on the skin over a thin layer of water-based gel. Ultrasound can provide anatomic images (B-mode imaging) as well as measurements of blood flow (Doppler imaging) and tissue stiffness (elastography). Ultrasound is safe, well-tolerated, and can be used at the point-of-care (e.g., Schubert Research Clinic). Most non-osseous tissues in the body can be assessed using ultrasound techniques (e.g., liver, spleen, kidneys, pancreas, bowel, muscles). Additional quantitative techniques also have recently become available that allow a variety of additional tissue characterizations, such as measurement of liver fat. Ultrasound-based contrast agents are now available allowing new methods of visualizing tissue blood flow.