Recent Updates Advances in MRI 2022 | Major Points Covered

In this post 'Recent Advances in MRI' is explained, Topics such as:

1. Diffusion Tensor Imaging
2. Intraoperative MRI
3. CSF Flow study in spine
4. MR Elastography

  Magnetic Resonance Imaging (MRI) diagnostic imaging has enjoyed tremendous growth since it was introduced to clinical practice in the early 1980s. By now it is firmly established as one of the preferred diagnostic imaging tools. After thirty years of intense research and development, the recent advances in MRI technology have matured and the pace of technological changes introduced to the marketplace has slowed.   

 However, recently the medical equipment manufacturing industry introduced new lines of MRI systems that represent a significant improvement over the exciting systems. These systems are about to cause another paradigm shift in MRI practices by altering clinical applications that were thus far difficult, time-consuming, or erratic in performance. With new systems, they become so robust that they get rapidly incorporated into the MRI standards of practice.

Diffusion Tensor Imaging

  Our brain contains more than 100 billion neurons that communicate with each other via axons for the formation of complex neural networks. The structural mapping of such networks during health and disease states is essential for understanding brain functions. However, our understanding of brain structural connectivity is surprisingly limited, due in part to the lack of non-invasive methodologies to study axonal anatomy. Diffusion tensor imaging (DTI) is a recently developed MRI technique that can measure macroscopic axonal organization in nervous system tissues.   Thus Diffusion Tensor Imaging (DTI) can be defined as an MRI-based neuroimaging technique that makes it possible to estimate the location, orientation, and anisotropy of the brain's white matters tracts. It is an extension of diffusion-weighted imaging (DWI).  

Indications (DTI)

The most common indication for DTI may be broadly categorized as either: tract/lesion localization or tissue characterization. These are distinct applications, each with its own approaches and potential pitfalls, they are not mutually exclusive.   a. Tract/lesion localization: Mapping of specific white matter fiber tracts (by directional color mapping and/or tractography), most commonly to depict the locations and spatial relationships of lesions with respect to functionally critical tracts for purposes of surgical risk assessment and treatment planning.   b. Tissue characterization: The use of any DTI metric to discriminate normal from abnormal tissue or to determinate one abnormal tissue from another.  

Physics and Principle

  The goal of diffusion MRI and specifically diffusion tensor imaging (DTI) is to image the diffusion of water in the brain. DTI uses the fact that water is always moving (due to Brownian motion), this leads to natural diffusion of water at all times.   This is because as water moves in different directions these gradient fields can level that movement differentially. 

Thus the water molecules that move during the magnetization than their neighboring water molecules which leads to a change in image contrast.   When this is done in more than six directions, one can actually be able to calculate the direction of water diffusion Red for example means that the diffusion direction is in the left-right direction, green indicates front-back, and blue up-down.   Apart from the direction of water diffusion, the amount of diffusion can also be measured. When the direction and amount of diffusion are combined, one can compute what they call a tensor, these are ellipsoids with a direction. 

 The value of the tensor at each point in the brain can be quantified using different methods. One method uses fractional anisotropy (FA), which quantifies the relative length of the tensor compared to the width, or a different way to think about it is that it quantifies the fraction of the tensor that is non-isotropic.   The microstructure of the brain determines the diffusion of water. The water inside white matter tracks preferentially diffuse along the same direction as the actual white matter fibers and myelin. While in gray matter the microstructure is less organized but is more isotropically (ball) shaped. Models of water diffusion in the white matter will look like ellipsoids that are more anisotropic, as in the picture above.   

 While tensors in grey matter and cerebral spinal fluid are more isotropic (ball-shaped). Each point (or volume pixel, aka voxel) has a tensor with a direction and a relative anisotropy. In addition to running whole-brain statistics, the individual tensor can be used to run fiber tractography. Deterministic tractography helps to get a rough impression of how white matter pathways run through the brain. Mathematical algorithms include fiber assignment by continuous tracking (FACT) and tensor deflection (TEND) to follow the direction of each tensor and then step to the next voxel in order to tractography through the whole brain.  

The technique of DTI imaging

MR scanner axes X, Y, and Z are never perfectly parallel to the white matter tracts at every point in the image. In DTI, images are acquired in at least six, usually 12-24 directions instead of three in the usual trace diffusion. A DTI image (Fractional Analysis image) in one direction (anterior to posterior) shows bright genu of the corpus callosum and optic radiations because these fibers are perpendicular in this direction. 

Such images are obtained in 12 or 24 directions to get the tractography.   The pure apparent diffusion coefficient for each pixel is calculated from these images in multiple directions. This is called as 'principle eigen value'. The principal eigenvalue is calculated along the true axis of diffusion called as 'Eigen vector'. The image formed with principal eigenvalue is called a diffusion tensor image that gives the orientation of fiber tracts.  

Clinical Application in Recent Advances in MRI

Diffusion tensor measures the magnitude of the ADC in the preferred direction of water diffusion and also perpendicular to the direction. The resultant image shows white matter tracts very well. Hence this technique is also called 'tractography'.   

Various maps are used to indicate the orientation of fiber tracts including FA (fractional anisotropy), RA (regional anisotropy), and VA (volume ratio) maps.   The clinical Application of Diffusion Tensor Imaging or Tractography is that they are useful for the assessment of the relationship of tracts with tumors, tumor invasion of tracts, and preoperative planning. It is also used to evaluate white matter tracts in various congenital anomalies and dysplasias.  

Intraoperative MRI (Recent Advances)

Intraoperative magnetic resonance imaging (iMRI) is a procedure to create images of the brain during surgery.   Precision and accuracy are two essential components when accurately targeting regions of interest in the brain. 

Avoidance and preservation of eloquent cortex such as motor, speech, and visual areas depend on precise identification of these regions during the procedure. In most operating rooms, surgeons must make their best estimation in the OR based on MRI scans performed well before the patient enters the operating room. The problems with such approaches are factors like:  

• "Brain shift": It is the term applied to the dynamic change that intracranial anatomy undergoes after craniotomy, burr hole placement, drainage of cerebrospinal fluid, or resection of a lesion. This compromises the localization of neural structures in space relative to where they were when preoperative images were required
• Gliomas (particularly low-grade gliomas) pose a particular challenge to surgeons because many of these tumors do not possess distinct capsules. Thus, even well-trained human eyes are incapable of discerning where the border of the lesion ends and the viable brain begins.

Uncertainty due to such factors leads to two problems: 1) Inadequate resection secondary to the surgeon stopping at what appears to be grossly abnormal tissue (so as to avoid neurologic damage) and, 2) Neurologic damage caused by aggressive surgery in which resection ends only when clearly normal brain tissue is visualized.   

Hence intraoperatively acquired images are used to provide neurosurgeons with the information needed to perform real-time, image-guided surgery. The boundary between tumor and viable neural tissue is often difficult to see with the naked eye, So the Superimposition of functional MRI, diffusion tensor imaging, and cortical mapping images eliminates a surgeon's uncertainty in determining tumor boundary and shifting brain structures.   

Thus, it helps to obtain maximal lesion resection, while minimizing untoward neurologic damage. Such maximal lesion resection directly impacts the survival time of patients with low- and high-grade gliomas.  

Indications for iMRI (recent advances)

During Intraoperative MRI Doctors use iMRI to assist in surgery for treatment of:

• Brain tumor
• Dystonia
• Epilepsy
• Essential tremor
• Glioma
• Neuropsychiatric
• Parkinson's disease
• Pediatric brain tumor
• Pituitary tumors

 

iMRI Principle and Planning Recent Advances

The iMRI (Intraoperative magnetic resonance imaging) technology is used by surgeons during surgery, as:

• Portable iMRI devices: These are MRI devices that are moved into the operating room to create images.
• Nearby iMRI devices: These are kept in a room adjacent to the operating room so that doctors can easily move the patients to the adjacent room for imaging during the surgery.

 At certain points in your operation, the surgeon may request imaging with iMRI. Intraoperative MRI is generally performed after the surgeon has removed as much of the tumor as possible with no neurological deficit. 

The sequences utilized include those which best depicted the lesson on preoperative scanning, maybe with/ without contrast depending on the lesion type. If further resection, under the same anesthesia and surgical setting   If not, the operating is complicated and no further imaging is required. 

Thus it allows better and more complete tumor resection without the need for a second surgery.   The intraoperative Mri (also other imaging systems like CT, PET, etc.) can be computer-assisted, to form the so-called "neuronavigation". This system can lead surgical procedures to the images obtained both pre and intraoperatively.  

Clinical application iMRI

1. Removal of tumors (primary and secondary). The surgical removal of glioma and glioblastomas is a common indication. These malignant brain tumors are best treated by the most complete resection possible. 

2. Pituitary adenomas are also a common indication for intraoperative MRI. These are usually approached by a trans-sphenoidal route. 

3. Epileptic foci can be obliterated. 

4. Vascular malformations such as cavernous malformations, AV malformations, and AV fistulae can be removed. 

5. Intraoperative MRI can also be used to check for complications such as bleeding or ischemia.  

CSF Flow Study in Spine

The CSF (cerebrospinal fluid) comprises all intracerebral ventricles, spinal and brain subarachnoid spaces, and the central canal of the spinal cord. The rate of CSF formation In humans is about 0.3-0.4 ml min-1 (about 500 ml day-). Total CSF volume is 90-150ml in adults and 10-60 ml in neonates. 

Potential sites of CSF origin include the choroid plexus, parenchyma of the brain and the spinal cord, and ependymal lining of the ventricles.   CSF flow studies are performed using a variety of MRI techniques and are able to qualitatively assess and quantify pulsatile CSF flow. The most common technique used is time-resolved 2D phase-contrast MRI with velocity encoding.   

It is important to remember, when referring to CSF flow in the setting on imaging we are referring to pulsatile to-and-fro flow due to vascular pulsations rather than bulk transport of CSF (the mechanism by which produce CSF is absorbed, via absorption at arachnoid granulations and via the lymphatic pathway). The latter is too slow to be easily assessed clinically.   There are several disorders such as communicating and non-communicating hydrocephalus, Chiari malformation, syringomyelic cyst, and arachnoid cyst that can change the CSF dynamics.   

CSF Principle and planning (recent advances)

Time-resolved 2D phase-contrast imaging with velocity encoding is the most widely used method for CSF flow studies. It relies upon the location-specific sequential application of a pair of phase encoding pulses in opposite directions. The stationary protons will experience the same pulse at both times and therefore return no signal. Protons that have moved will experience different phase encoding pulses and will thus be visible. 

The expected velocity of flow is taken into account to avoid aliasing artifacts. This expected velocity of flow is taken into account to avoid aliasing artifacts.   These expected velocities can be encountered relying on higher VENCs (up to 25 cm/s). Typical CSF flow is 5-8 cm/s. In patients with hyperdynamic circulation much higher velocities can be encountered relying on higher VENCs (up to 25 cm/s). Images are typically presented in sets of 3 for each plan and VENC obtained, similar to susceptibility-weighted imaging (SWI). The set comprises of: 1. Re-phased image (magnitude of flow compensated signal)

• the flow of high signal
• background is visible

2. Magnitude image (magnitude of the different signal)

• the flow of high signal (regardless of direction)
• background is suppressed

3. Phase image (phase of different signal)

• a signal is dependent on direction: forward flow is of high signal; reverse flow is of low signal
• background is mid-gray

 

CSF flow classification

Qualification of flow can be generated by defining a region of interest (e.g. cross-sectional area of the aqueduct of Sylvius) and charting velocity versus time, which is typically pulsatile (e.g. forward during systole and backward during diastole).  

CSF clinical Application

The following clinical situations can benefit from CSF flow studies:  

CSF Aqueduct stenosis

It is the most common cause of congenital hydrocephalus 3 It occurs when the long, narrow passageway between the third and fourth ventricles (the aqueduct of Sylvius) is narrowed or blocked, which can be because of infections, hemorrhage, or a tumor. 

Fluid accumulates "upstream" from the obstruction, providing hydrocephalus. An MRI better delineates the extent of obstructive hydrocephalus, with an enlargement (often marked) of the lateral and third ventricles.   Recent advances in MRI CSF flow study are helpful, and the absence of aflow-void signal intensity on sagittal T2 images at the aqueductal level has been suggested as a sign of aqueductal stenosis.  

Normal Pressure Hydrocephalus (NHP)

Normal-pressure hydrocephalus (NPH) is a state of chronic hydrocephalus in which the CSF pressure is in the physiological range, but a slight pressure gradient persists between the ventricles and the brain parenchyma. This pathology is described in elderly patients and has classic symptoms triad of ain't disturbance, urinary incontinence, and dementia.   

 In properly selected patients, ventricular shunting is used for the resolution of symptoms and slow progressive deterioration. Phase-contrast MRI is useful in the selection of patients for shunt placing. Increased pulsatility throughout the cerebral aqueduct has been correlated with a favorable response to shunting.  

Patency of Third VentriculostomY

Neuroendoscopic third ventriculostomy (NTV), is increasingly used as an alternative treatment for obstructive hydrocephalus. It has become the most common neuroendoscopic procedure. This procedure restores the pulsatile bidirectional CSF motion. 

The morbidity associated with this technique is low, and the success rates are high. A range of image parameters has been assessed to evaluate the permeability of the NTC. PC flow-sensitive MRI techniques offer more physiological data than structural MRI and qualitative assessment of the patency of ventriculostomy. In addition, the measurement of stroke volume in ventriculostomy.   Flow chart Cerviomedullary junction (Foramen Magnum)

Chiari I malformation

  The Chiari 1 malformation, also known as the Arnold-Chiari malformation, is called displacement of the cerebellar tonsils through the posterior foramen magnum. As the herniated tonsils fill the foramen magnum in the setting of Chiari, CSF flow is reduced at the herniated tonsils fill the foramen magnum in the setting of Chiari, CSFfkow is reduced at the craniovertebral junction.   Thus a compensatory can effectively plug the CSF partway at the foramen magnum. The degree of CSF flow instruction is used to select patients who are most responsive to surgery.  

CSF Achondroplasia

Achondroplasia is a congenital genetic disorder resulting in dwarfish and is the most common skeleton dysplasia. In this condition, the cine mode MR imaging demonstrates CSF flow disturbance at the corticospinal junction resulting from foramen magnum stenosis and medullary compression. 

Attenuated flow is often noted between the suboccipital subarachnoid spaces and the craniocervical junction, which improves after craniotomy   Cerebrospinal fluid (CSF) flow studies may be useful in the differentiation of an arachnoid cyst from the mega cisterna magna. Arachnoid cyst will not show CSF movement during systole and diastole and will show different flow than surrounding CSF.  

MR Elastography Principle

Magnetic resonance elastography (MRE) is a dynamic elasticity imaging technique that uses mechanical waves to quantitatively assess the shear modulus (or stiffness) of tissues. It is considered to be an imaging-based counterpart to palpation, which is commonly used by physicians to diagnose and characterize diseases.   

The success of palpation as a diagnostic tool is based on the fact that the mechanical properties of tissues are often dramatically affected by the presence of disease processes such as cancer, inflammation, and fibrosis MRE obtains information about the stiffness of tissue by assessing the propagation of mechanical waves through the tissue with a special magnetic resonance imaging (MRI) technique.   The three basics of MRI are:

• Shear waves with frequencies ranging from 50-500 Hz are induced in the tissue using an external driver,
• Waves are imaged inside the body using a special MRI technique and,
• The resulting data are processed to generate quantitative images displaying the stiffness of tissue. These are called electrograms.

  These three waves propagate more rapidly in stiffer tissue and more slowly in softer tissue. If the waves are applied continuously, the speed of propagation is reflected in the wavelength. Hence, as tissue stiffness increases, the wavelength becomes longer. These low-frequency mechanical shear waves are generated with a special acoustic driver system and propagated into the body. A modified phase-contrast pulse sequence with cyclic motion encoding gradients synchronized to the mechanical waves is used to image the micron-level displacements associated with wave propagation.   

 The imaging process can be accomplished in one or more breath-holds and shows images depicting the pattern of propagation waves in the liver. The wave images are then processed with specialized software (called an inversion algorithm) to generate quantitative cross-sectional images which depict the stiffness of tissue. The tissue with shear waves of longer wavelengths are represented as areas of higher stiffness as compared to those with a shorter wavelength.  

Technique for MR Elastography

The technique can be readily implemented on a conventional MR system with added hardware to generate mechanical waves, and special software for acquisition and processing in commercially-available implementations of MRE, the hardware typically consist of an active acoustic driver, located outside the magnet, which is coupled via plastic tubing to a disc-shaped non-metallic passive driver that is placed against organ i.e. for liver diagnosis, on around the body.   

In the case of liver evaluation, a continuous acoustic vibration at typically 60Hz is transmitted into the abdomen via a pasive driver. The applied vibration are well-tolerated and do not cause any discomfort.   Depending on the specific application, the phase-contrast MRE sequence may be based on gradient-recalled echo (GRE), spin-echo (SE), or echo-planar imaging (EPI) sequences, with added cyclic motion-encoding gradients (MEGs) which allow shear waves with amplitudes in the micron range to be readily imaged.  

Generating Elestogram

After the acquisition is complete, the wave images are automatically processed by the scanner to generate images that depict tissue stiffness called electrograms. Several different types of inversion algorithms have been used, including spatial frequency estimation, and analytic solutions to the wave equation. These quantitative images typically depict shear stiffness in units of kilopascals (kPa) and may be displayed in grayscale or with a color scale.  

Clinical Application for MR Elastography

MRE is already being used clinically for the assessment of patients with chronic liver diseases and is emerging as a safe, reliable, and non-invasive alternative to liver biopsy for staging hepatic fibrosis. MR elastography can be indirectly used to evaluate and stage liver fibrosis and is also used for the characterization of liver tumors. MRE is also being investigated for application to pathologies of other organs including the brain, breast, blood vessels, heart, kidneys, lungs, and skeletal muscle. This method is still in its infancy but the initial result is encouraging.