How to Understanding MRI Pulse Sequence Part 1/ការយល់ដឹងពីការប្រើ MRI Pulse sequence https://youtu.be/f87ElhJXjcg

History of MRI

Timeline of MRI :

1. 1946 MR Phenomenon by Bloch & Purcell. 

2. 1950 NMR develoed as ananlytical tool

3. 1952 Nobel Prize of Bloch and Purcell.

4. 1972  Computerized Tomography .

5. 1973 Backprojection MRI -by Lauterur.

6. 1975 Fourier Imaging by Ernst .

7. 1980 MRI Spin Warp imaging .

8. 1986 Gradient Echo Imaging , NMR Microscope .

9. 1988 Angiography .

10. 1991 Nobel Prize R.R Ernst .

11. 1992 Fuctional Imaging 

Paramagnetic Contrast

Gadolinium

gadolinium (Gd) are the most widely used of all MR contrast agents. Because of its unique electronic structure (described below), Gd is strongly paramagnetic.

Paramagnetism is an intrinsic property of certain materials to become temporarily magnetized when placed in an external magnetic field. In fact, Gd is one of only four elements that can be magnetized at room temperature (the other three being iron, nickel, and cobalt).

The powerful paramagnetic properties of Gd make it extremely useful as an MR contrast agent. Gadolinium is not directly seen in an MR image, but manifests its presence indirectly by facilitating the relaxation of nearby hydrogen protons. Gd preferentially shortens T1 values in tissues where it accumulates rendering them bright on T1-weighted images.

Paramagnetism may exist over a wide dimensional range — from subatomic particles to atoms to entire molecules. Nuclear paramagnetism, the form responsible for the NMR phenomenon, is extremely weak except in the immediate vicinity of the nucleus. It plays little or no role in determining the gross paramagnetic properties of entire atoms like gadolinium.

The form of paramagnetism exhibited by gadolinium compounds derives from electrons, not protons, and is known as Curie paramagnetism. Because of electrons have the same spin (½) but a much smaller size than protons, their gyromagnetic ratios are 657 times larger. If these electrons remain unpaired in shells or bonding orbitals, the unbalanced spins produce a strong magnetic moment capable of inducing magnetic relaxation in nearby nuclei. This is the origin of the bulk paramagnetism possessed by elements such as gadolinium. 

Gadolinium has atomic number 64 on the periodic table. It occupies the central position in the lanthanide series of elements. Lanthanides are rare-earth metals grouped chemically because they possess partially filled inner shells of electrons (4f and 5d subshells). 

The electronic structure of the neutral Gd atom is shown right. Note the 7 unpaired electrons in its 4f subshell that account for the element's strong paramagnetism. In its ionized state, Gd⁺³ donates its 6s² and 5d¹electrons for bonding, leaving its 4f⁷electron shell intact. The powerful magnetic moment of Gd is therefore largely maintained even when chelated to a ligand such as DTPA in a contrast agent formulation.

What's diagnosis?

What is it in this picture?

Functional MRI (fMRI)

Blood oxygen level dependent functional MRI, or BOLD fMRI, is an advanced MRI technique in which level of oxygen present in an area of the brain is used to map out what parts of the brain are activated in specific tasks. In this method, repeated imaging of the brain can be performed while the patient performs a task, and the level of oxygenation changes, showing which parts of the brain are most activated.

MRI Diffusion Tensor Imaging (DTI)

Diffusion tensor imaging, or DTI, is an advanced MRI technique in which the asymmetric motion of water is used to map out specific properties in the brain. One application of DTI is called tractography, or identifying the specific tracts of neurons which pass through the brain.

Diffusion Module

Process and visualize diffusion images, with the diffusion weighted imaging (DWI) model, the diffusion tensor imaging (DTI) model, and the new constrained spherical deconvolution (CSD) model, addressing the issue of crossing fibers.

constrained spherical deconvolution (CSD)

The corticospinal tract (part of the motor network) and arcuate fasciculus (part of the language network) were tracked, bilaterally, with constrained spherical deconvolution (CSD). The fan shape of the corticospinal tract, in its upper portion, as well as the C-shape of the arcuate fasciculus, can be recognized.

fMRI

Functional MRI (fMRI), also called BOLD imaging, is a magnetic resonance imaging-based neuroimaging technique that makes it possible to detect the brain areas that are involved in a task, a process or an emotion.

MRI Brain Perfusion

Perfusion MRI is based on the analysis of the contrast enhancement of MRI images after a peripheral injection of a contrast agent, e.g. Gd-DTPA. The injection of 0.1 mmol/kg is usually quick (i.e. a bolus), at a rate of 5-10 ml/s, and is followed by a saline flush.

Dynamic Susceptibility Contrast (DSC) perfusion MRI uses a GRE-EPI (T2*-weighted) sequence in which the contrast agent induces an hypointensity. For example, 40 volumes can be acquired every 1.5 seconds during 1 minute. Concentration-versus-time curves are calculated from the variation of the MR image signal induced by the contrast agent. The tissue curves are sometimes deconvolved by the arterial input function (AIF) in order to eliminate the influence of the injection kinetics. From the curves, several parameters can be calculated: cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), time to peak (TTP), time to peak of the residue function (Tmax), etc.

Dynamic Contrast Enhanced (DCE) perfusion MRI uses a spoiled fast gradient echo (T1-weighted) sequence in which the contrast agent induces an hyperintensity. Conversion from image signal to concentration-versus-time curves requires a calibration procedure, e.g. a measurement of pre-injection T1 relaxation time by relaxometry. Taking into account the arterial input function (AIF), tracer kinetics models applied to the curves lead to a measurement of the transfer constant between the intra-vascular and extra-vascular spaces Ktrans (a marker of vessel permeability) and of the fractional volume of the extra-vascular extra-cellular space.

DSC perfusion MRI is often used in combination with diffusion MRI for the imaging of ischemic stroke. The mismatch between perfusion and diffusion MRI is a marker of the ischemic penumbra (i.e. the zone which can still be preserved by an appropriate treatment).

In brain tumors, DSC and DCE perfusion can be used to assess the tumoral vascularization, as a marker of the tumor's grade.

Is it ok for patients to have both an MR and a CT with contrast on the same day

This question was addressed on the RSNA site and there is no issue doing both MRI and CT the same day with Gadolinium and Iodinated Contrast.

Here is more detailed information courtesy of Richard A. Vitti MD from Medical Affairs at GE Healthcare.

CT contrast, or X-ray contrast for IV use, is usually an iodinated compound, whereas most MRI contrast media contains a heavy metal ion usually gadolinium. Iodine provides contrast by virtue of absorption of X-rays at the K-edge. Gd provides contrast in MR by changing the magnetic moment. Generally these agents distribute themselves from the vascular space to the interstitial space of soft tissues, and are elminated by glomerular filtration by the kidneys.

TR and TE

TR and TE are basic pulse sequence parameters and stand for repetition time and echo timerespectively. They are typically measured in milliseconds (ms).

Pulse Sequence Part I

• Without pulse sequences we can‟t do MRI. Our life depends on it in terms of which kind of image contrast we want to see or, even, which kind of pathology we want to detect.
• Understanding what a pulse sequence is and how it influences the image is vitally important.
• A pulse sequence is a sequence of events, which we need to acquire MRI images.
• These events are: RF pulses, gradient switches and signal collecting.

Pulse Sequence Overview a

Pulse Sequence Overview

• An MRI sequence is a number of radiofrequency pulses and gradients that result in a set of images with particular appearance.
• This section presents a simplified approach to recognising and thinking about common MRI sequences, but does not concern itself with the particulars of each sequences.