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Or welcome to the I Can't Sleep Podcast,
Where I read random articles from across the web to bore you to sleep with my soothing voice.
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Benjamin Boster.
Today's episode is from a Wikipedia article titled,
Magnetic Resonance Imaging.
Magnetic Resonance Imaging,
MRI,
Is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes inside the body.
MRI scanners use strong magnetic fields,
Magnetic field gradients,
And radio waves to generate images of the organs in the body.
MRI does not involve x-rays or the use of ionizing radiation,
Which distinguishes it from computed tomography,
CT,
And position emission tomography,
PET,
Scans.
MRI is a medical application of nuclear magnetic resonance,
NMR,
Which can also be used for imaging and other NMR applications,
Such as NMR spectroscopy.
MRI is widely used in hospitals and clinics for medical diagnosis,
Staging,
And follow-up of disease.
Compared to CT,
MRI provides better contrast in images of soft tissues,
For example,
In the brain or abdomen.
However,
It may be perceived as less comfortable by patients due to the usually longer and louder measurements with the subject in a long,
Confining tube,
Although open MRI designs mostly relieve this.
Additionally,
Implants and other non-removable metal in the body impose a risk and may exclude some patients from undergoing an MRI examination safely.
MRI was originally called NMRI,
Nuclear Magnetic Resonance Imaging.
But nuclear was dropped to avoid negative associations.
Certain atomic nuclei are able to absorb radiofrequency,
RF energy,
When placed in an external magnetic field.
The resultant involving spin polarization can induce an RF signal in a radiofrequency coil and thereby be detected.
In other words,
The nuclear magnetic spin of protons in the hydrogen nuclei resonates with the RF incident waves and emit coherent radiation with compact direction,
Energy,
Frequency,
And phase.
This coherent amplified radiation is easily detected by RF antennas close to the subject being examined.
It is a process similar to masers.
In clinical and research MRI,
Hydrogen atoms are most often used to generate a macroscopic polarized radiation that is detected by the antennas.
Hydrogen atoms are naturally abundant in humans and other biological organisms,
Particularly in water and fat.
For this reason,
Most MRI scans essentially map the location of water and fat in the body.
Pulses of radio waves excite the nuclear spin energy transition,
And magnetic field gradients localize the polarization in space.
By varying the parameters of the pulse sequence,
Different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms therein.
Since its development in the 1970s and 1980s,
MRI has proven to be a versatile imaging technique.
While MRI is most prominently used in diagnostic medicine and biomedical research,
It also may be used to form images of non-living objects,
Such as mummies.
Diffusion MRI and functional MRI extend the utility of MRI to the use of non-living objects to capture neuronal tracks and blood flow respectively in the nervous system,
In addition to detailed spatial images.
The sustained increase in demand for MRI within health systems has led to concerns about cost-effectiveness and over-diagnosis.
In most medical applications,
Hydrogen nuclei,
Which consist solely of a proton that are in tissues,
Create a signal that is processed to form an image of the body in terms of the density of those nuclei in a specific region.
Given that the protons are affected by fields from other atoms to which they are bonded,
It is possible to separate responses from hydrogen in specific compounds.
To perform a study,
The person is positioned within an MRI scanner that forms a strong magnetic field around the area to be imaged.
First,
Energy from an oscillating magnetic field is temporarily applied to the patient at the appropriate resonance frequency.
Scanning with X and Y gradient coils causes a selected region of the patient to experience the exact magnetic field required for the energy to be absorbed.
The atoms are excited by a RF pulse and the resultant signal is measured by a receiving coil.
The RF signal may be processed to deduce position information by looking at the changes in RF level and phase caused by varying the local magnetic field using gradient coils.
As these coils are rapidly switched during the excitation and response to perform a moving line scan,
They create the characteristic repetitive noise of an MRI scan as the windings move slightly due to magneto-restriction.
The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state.
Exogenous contrast agents may be given to the person to make the image clearer.
The major components of an MRI scanner are the main magnet,
Which polarizes the sample,
The shim coils for correcting shifts in the homogeneity of the main magnetic field,
The gradient system,
Which is used to localize the region to be scanned,
And the RF system,
Which excites the sample and detects the resulting NMR signal.
MRI requires a magnetic field that is both strong and uniform to a few parts per million across the scan volume.
The field strength of the magnet is measured in teslas,
And while the majority of systems operate at 1.
5 teslas,
Commercial systems are able to measure up to 1.
5 teslas per million.
At 1.
5 teslas,
Commercial systems are available between 0.
2 and 7 teslas.
Whole-body MRI systems for research applications operate in 9.
4 teslas,
10.
5 teslas,
11.
7 teslas.
Even higher-fueled whole-body MRI systems,
For example,
14 teslas and beyond,
Are in conceptual proposal or in engineering design.
Most clinical magnets are superconducting magnets,
Which require liquid helium to keep them at low temperatures.
Lower field strengths can be achieved with permanent magnets,
Which are often used in open MRI scanners for claustrophobic patients.
Lower field strengths are also used in a portable MRI scanner approved by the FDA in 2020.
Recently,
MRI has been demonstrated also at ultra-low fields,
I.
E.
,
In the microtesla to millitesla range,
Where sufficient signal quality is made possible by pre-polarization and by measuring the Larmor precision fields at about 100 microteslas with highly sensitive superconducting quantum interference devices,
Squids.
Each tissue returns to its equilibrium state after excitation by the independent relaxation process of T1 spin lattice,
I.
E.
,
Magnetization in the same direction as the static magnet field.
And T2 spin-spin transverse to the static magnetic field.
To create a T1-weighted image,
Magnetization is allowed to recover before measuring the MR signal by changing the repetition time,
TR.
This image weighting is useful for assessing the cerebral cortex,
Identifying fatty tissue,
Characterizing focal liver lesions,
And in general obtaining morphological information,
As well as for post-contrast imaging.
To create a T2-weighted image,
Magnetization is allowed to decay before measuring the MR signal by changing the echo time,
TE.
This image weighting is useful for detecting edema and inflammation,
Revealing white matter lesions,
And assessing zonal anatomy in the prostate and uterus.
The information from MRI scans comes in the form of image contrasts based on differences in the rate of relaxation of nuclear spins following their perturbation by an oscillating magnet field in the form of radiofrequency pulses through the sample.
The relaxation rates are a measure of the time it takes for a signal to decay back to an equilibrium state from either the longitudinal or transverse plane.
Magnetization builds up along the z-axis in the presence of a magnetic field,
B0,
Such that the magnetic dipoles in the sample will,
On average,
Align with the z-axis,
Summing to a total magnetism,
Mz.
This magnetization along z is defined as the equilibrium magnetization.
Magnetization is defined as the sum of all magnetic dipoles in a sample.
Following the equilibrium magnetization,
A 90-degree radiofrequency,
Or f-pulse,
Flips the direction of the magnetization vector in the xy-plane,
And is then switched off.
The initial magnetic field,
B0,
However,
Is still applied.
Thus,
The spin magnetization vector will slowly return from the xy-plane back to the equilibrium state.
The time it takes for the magnetization vector to return to its equilibrium value,
Mz,
Is referred to as the longitudinal relaxation time,
T1.
Subsequently,
The rate at which this happens is simply the reciprocal of the relaxation time,
1 over T1 equals R1.
Similarly,
The time in which it takes for Mxy to return to zero is T2,
With the rate 1 over T2 equals R2.
Magnetization as a function of the time is defined by the Bloch equations.
T1 and T2 values are dependent on the chemical environment of the sample,
Hence their utility in MRI.
Soft tissue and muscle tissue relax at different rates,
Yielding the image contrast in a typical scan.
The standard display of MR images is to represent fluid characteristics in black and white images.
MRI has a wide range of applications in medical diagnoses,
And around 50,
000 scanners are estimated to be in use worldwide.
MRI affects diagnosis and treatment in many specialties,
Although the effect on improved health outcomes is disputed in certain cases.
MRI is the investigation of choice in the preoperative process.
MRI is the investigation of choice in the preoperative staging of rectal and prostate cancer,
And has a role in the diagnosis,
Staging,
And follow-up of other tumors,
As well as for determining areas of tissue for sampling and biobanking.
MRI is the investigative tool of choice for neurological cancers over CT,
As it offers better visualization of the posterior cranial fossa,
Containing the brain stem and the cerebellum.
The contrast provided between gray and white matter makes MRI the best choice for many conditions of the central nervous system,
Including demyelinating diseases,
Dementia,
Cerebrovascular disease,
Infectious disease,
Alzheimer's disease,
And epilepsy.
Since many images are taken milliseconds apart,
It shows how the brain responds to different stimuli,
Enabling researchers to study both the functional and structural brain abnormalities in physiological disorders.
MRI also is used in guided stereotactic surgery and radiosurgery for treatment of intracranial tumors,
Arteriovenous malformations,
And other surgical treatable conditions using a device known as the mLocalizer.
New tools that implement artificial intelligence in healthcare have demonstrated higher image quality and morphometric analysis in neuroimaging with the application of a denoising system.
The record of the highest spatial resolution of a whole intact brain post-mortem is 100 microns from Massachusetts General Hospital.
The data was published in Nature on the 30th of October 2019.
Though MRI is used widely in research on mental disabilities,
Based on a 2024 systematic literature review and meta-analysis commissioned by the Patient-Centered Outcomes Research Institute PCORI,
Available research using MRI scans to diagnose ADHD showed great variability.
The authors conclude that MRI cannot be reliably used to assist in making a clinical diagnosis of ADHD.
Cardiac MRI is complementary to other imaging techniques such as echocardiography,
Cardiac CT,
And nuclear medicine.
It can be used to assess the structure and the function of the heart.
Its applications include assessment of myocardial ischemia and viability,
Cardiomyopathies,
Myocarditis,
Iron overload,
Vascular diseases,
And congenital heart disease.
Applications in the musculoskeletal system include spinal imaging,
Assessment of joint disease,
And soft tissue tumors.
Also,
MRI techniques can be used for diagnostic imaging of systematic muscle diseases,
Including genetic muscle diseases.
Swallowing movement of throat and esophagus can cause motion artifact over the imaged spine.
Therefore,
A saturation pulse applied over this region of the throat and esophagus can help to avoid this artifact.
Motion artifact arising due to pumping of the heart can be reduced by timing the MRI pulse according to heart cycles.
Blood vessels flow artifacts can be reduced by blood vessels flow artifacts can be reduced by applying saturation pulses above and below the region of interest.
Epidobiliary MR is used to detect and characterize lesions of the liver,
Pancreas,
And bile ducts.
Focal or diffuse disorders of the liver may be evaluated using diffusion-weighted opposed-phase imaging and dynamic contrast enhancement sequences.
Extracellular contrast agents are used widely in liver MRI,
And newer epidobiliary contrast agents also provide the opportunity to perform functional biliary imaging.
Anatomical imaging of the bile ducts is achieved by using a heavily T2-weighted sequence in magnetic resonance cholangiopancreatography,
MRCP.
Functional imaging of the pancreas is performed following administration of secretin.
Magnetic resonance angiography,
MRA,
Generates pictures of the arteries to evaluate them for stenosis,
Abnormal narrowing,
Or aneurysms,
Vessel wall dilations at risk of rupture.
MRA is often used to evaluate the arteries of the neck and brain,
The thoracic and abdominal aorta,
The renal arteries,
And the legs,
Called a runoff.
A variety of techniques can be used to generate the pictures,
Such as administration of paramagnetic contrast agent,
Or using a technique known as flow-related enhancement,
Where most of the signal on an image is due to blood that recently moved into that plane.
Techniques involving phase accumulation,
Known as phase contrast angiography,
Can also be used to generate flow velocity maps easily and accurately.
Magnetic resonance sphenography,
MRV,
Is a similar procedure that is used to image veins.
In this method,
The tissue is now excited inferiorly,
While the signal is gathered in the plane immediately superior to the excitation plane,
Thus imaging the venous blood that recently moved from the excited plane.
MRI for imaging anatomical structures or blood flow do not require contrast agents,
Since the varying properties of the tissues or blood provide natural contrasts.
However,
For more specific types of imaging,
Exogenous contrast agents may be given intravenously,
Orally,
Or intra-articularly.
Most contrast agents are either paramagnetic,
And are used to shorten T1 in the tissue they accumulate in,
Or superparamagnetic,
And are used to shorten T2 in healthy tissues reducing its signal density.
The most commonly used intravenous contrast agents are based on chelids of gadolinium,
Which is highly paramagnetic,
And is used to shorten T2 in healthy tissues reducing its signal which is highly paramagnetic.
In general,
These agents have proved safer than the iodinated contrast agents used in x-ray radiography or CT.
Anaphylactoid reactions are rare,
Occurring in approximately 0.
03 to 0.
1%.
Of particular interest is the lower incidence of nephrotoxicity,
Compared with iodinated agents when given at usual doses.
This has made contrast-enhanced MRI scanning an option for patients with renal impairment,
Who would otherwise not be able to undergo contrast-enhanced CT.
Gadolinium-based contrast reagents are typically octadentate complexes of gadolinium-3.
The complex is very stable,
So that in use the concentration of the uncomplexed GD3 plus ions should be below the toxicity limit.
The ninth place in the metal ion's coordination sphere is occupied by a water molecule,
Which exchanges rapidly with water molecules in the reagent molecule's immediate environment,
Affecting the magnetic resonance relaxation time.
In December 2017,
The Food and Drug Administration,
FDA,
And the United States announced in a drug safety communication that new warnings were to be included on all gadolinium-based contrast agents,
GBCAs.
The FDA also called for increased patient education and requiring gadolinium contrast vendors to conduct additional animal and clinical studies to assess the safety of these agents.
Although gadolinium agents have proved useful for patients with kidney impairment,
In patients with severe kidney failure requiring dialysis,
There is a risk of a rare but serious illness,
Nephrogenic systemic fibrosis,
Which may be linked to the use of certain gadolinium-containing agents.
The most frequently linked is gadodiamide,
But other agents have been linked too.
Although a causal link has not been definitively established,
Current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential,
And that dialysis should be performed as soon as possible after the scan to remove the agent from the body promptly.
In Europe,
Where more gadolinium-containing agents are available,
A classification of agents according to potential risks has been released.
In 2008,
A new contrast agent named Gadoxatid,
Brand named Eovist US or Primavist EU,
Was approved for diagnostic use.
This has the theoretical benefit of a dual excretion path.
An MRI sequence is a particular setting of radiofrequency pulses and gradients,
Resulting in a particular image appearance.
The T1 and T2 weighting can also be described as MRI sequences.
Magnetic Resonance Spectroscopy,
MRS,
Is used to measure the levels of different metabolites in body tissues,
Which can be achieved through a variety of single-voxel or imaging-based techniques.
The MR signal produces a spectrum of resonances that corresponds to different molecular arrangements of the isotope being excited.
This signature is used to diagnose certain metabolic disorders,
Especially those affecting the brain,
And to provide information on tumor metabolism.
Magnetic Resonance Spectroscopic Imaging,
MRSI,
Combines both spectroscopic and imaging methods to produce spatially localized spectra from within the sample or patient.
The spatial resolution is much lower,
Limited by the available SNR,
But the spectra in each voxel contains information about many metabolites.
Because the available signal is used to encode spatial and spectral information,
MRSI requires high SNR levels.
Because the available signal is used to encode spatial and spectral information,
MRSI requires high SNR achievable only at higher field strengths,
3 teslas and above.
The high procurement and maintenance costs of MRI with extremely high field strengths inhibit their popularity.
However,
Recent compressed sensing-based software algorithms,
E.
G.
SAMVI,
Have been proposed to achieve super-resolution without requiring such high field strengths.
Real-time Magnetic Resonance Imaging,
RT-MRI,
Refers to the continuous monitoring of moving objects in real-time.
Traditionally,
Real-time MRI was possible only with low image quality or low temporal resolution.
An iterative reconstruction algorithm removed limitations.
Radial Flash MRI,
Real-time,
Yields a temporal resolution of 20-30 ms for images,
With an in-plane resolution of 1.
5-2.
0 mm.
Real-time MRI adds information about diseases of the joints and the heart.
In many cases,
MRI examinations become easier and more comfortable for patients,
Especially for the patients who cannot calm their breathing or who have arrhythmia.
Balanced Steady-State Free Precision,
BSSFP imaging,
Gives better image contrast between the blood pool and myocardium than Flash MRI,
At the cost of severe banding artifact when B0 inhomogeneity is strong.
The lack of harmful effects on the patient and the operator make MRI well-suited for interventional radiology,
Where the images produced by an MRI scanner guide minimally invasive procedures.
Such procedures use no ferromagnetic instruments.
A specialized growing subset of interventional MRI is intraoperative MRI,
In which an MRI is used in surgery.
Some specialized MRI systems allow imaging concurrent with the surgical procedure.
More typically,
The surgical procedure is temporarily interrupted so that MRI can assess the success of the procedure or guide subsequent surgical work.
In guided therapy,
High-intensity focused ultrasound HIFU beams are focused on a tissue that are controlled using MR thermal imaging.
Due to the high energy at the focus,
The temperature rises to above 65 degrees Celsius,
Which completely destroys the tissue.
This technology can achieve precise ablation of diseased tissue.
MR imaging provides a three-dimensional view of the target tissue,
Allowing for the precise focusing of ultrasound energy.
The MR imaging provides quantitative real-time thermal images of the treated area.
This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue,
And if not,
To adapt the parameters to ensure effective treatment.
Hydrogen has the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance,
And because its high gyromagnetic ratio gives a strong signal.
However,
Any nucleus with a net nuclear spin could potentially be imaged with MRI.
Such nuclei include Helium-3,
Lithium-7,
Carbon-13,
Fluorine-19,
Oxygen-17,
Sodium-23,
Phosphorus-31,
And Xenon-129.
Sodium and Phosphorus are naturally abundant in the body,
So they can be imaged directly.
Cassius isotopes such as Helium or Xenon must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions.
