Urinary Tract Imaging : Basic Principles of CT, MRI, and Obviously Moving-picture show Imaging

Alan W. Partin Doctor, PhD , in Campbell-Walsh-Wein Urology , 2021

Renal Magnetic Resonance Imaging

Simple cysts have similar characteristics on ultrasound, CT, and MRI. Complex cysts can be differentiated or characterized using MRI. Hemorrhage within a cyst results in a high signal on T1-weighted images because of the paramagnetic effects of the byproducts from the breakdown of blood (hemosiderin) (Fig. 3.26) (Roubidoux, 1994).

Proteinaceous contents within a cyst demonstrate high bespeak on T1-weighted images. Chronic hemorrhage results in a blackness ring along the cyst wall on T2-weighted images.For benign and circuitous cysts there should be no enhancement (Israel and Hindman, 2004).

When independent risk factors were evaluated for RCC, enhancement of the cyst wall had college sensitivity and specificity than calcifications on the cyst wall. Calcifications can cause artifacts that may decrease the ability to appreciate enhancement of small nodules within the wall of a circuitous cyst on CT imaging. MRI has the advantage of non being influenced by calcifications within the wall of a complex cyst. Calcifications appear every bit a void on MRI. Therefore MRI is more likely to detect enhancement of a renal cell carcinoma in the wall of a circuitous cyst compared with CT imaging when mural calcifications are nowadays (Israel and Bosniak, 2003).

MRI offers a distinct advantage over CT imaging in the evaluation of the pseudocapsule, which appears on T1- and T2-weighted images every bit a low signal surrounding the lesion. The lack of pseudocapsule surrounding a renal mass had an accuracy of 91% in predicting pT3a disease (Roy and El Ghali, 2005).

MRI allows differentiation of different subtypes of RCC by using a multiparametric approach. These sequences can include T1-weighted images, multiplanar T2-weighted sequences (with and without fat suppression), dynamic contrast-enhanced (DCE) sequences (with arterial, corticomedullary, nephrogenic, and excretory phases), diffusion-weighted images (DWIs) (with respective credible diffusion coefficient [ADC] maps), and CSI. Using these unique features, we are amend able to differentiate the subtypes of renal masses compared with CT imaging.

Renal cell carcinoma clear cell blazon (ccRCC) is the nigh common type of RCC. It is characterized by a heterogeneous high signal on T2-weighted sequences because of the presence of hemorrhage, necrosis, and/or cysts (Pedrosa and Chou, 2008). Papillary RCC (pRCC), when compared with ccRCC, exhibits a homogenous lower bespeak intensity on T2-weighted images, which is secondary to hemosiderin degradation (histiocytes) inside the tumor (Pedrosa and Chou, 2008) (Fig. 3.27). Hemorrhagic cysts with an enhancing peripheral wall growth and/or a solid hypoenhancing mass with depression signal intensity (SI) on T2-weighted images resulted in 80% sensitivity and 94% specificity in differentiating pRCC from other types of RCC (Fig. three.28) (Pedrosa and Chou, 2008).

Magnetic Resonance Imaging

J.O.Due south.H. Cleary , A.R. Guimarães , in Pathobiology of Human Disease, 2014

Introduction

Magnetic resonance imaging (MRI) is a noninvasive modality, which produces multiplanar and true 3D datasets of subjects in vivo. It achieves high spatial resolution, typically of the order of millimeters in the clinical setting. Crucially, it differs from other techniques such equally computed tomography (CT) by producing excellent soft tissue contrast without harmful ionizing radiation. MRI has transformed the function of radiology in medicine since its initial applications in structural imaging in the early 1980s and now encompasses wider areas of functional and molecular imaging. In the kickoff part of this article, we give an overview of the principles of MRI and some common uses in the diagnosis of pathologies such every bit stroke and cancer. We keep to talk over the role of MR contrast agents, including their awarding to the exciting new areas of molecular and cellular imaging. Next, we accost the office of MR spectroscopy, a technique often complementary to MRI for the identification of illness processes through the assessment of metabolites. Finally, nosotros and then wait at an emerging awarding of MRI – high-resolution MR histology – as an adjunct to pathology studies.

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Diagnostic Kidney Imaging

Alan Southward.L. Yu MB, BChir , in Brenner and Rector'south The Kidney , 2020

Diagnostic Magnetic Resonance Imaging Technique

Routine MRI evaluation of the kidneys includes axial and coronal T1-weighted and T2-weighted sequences. Dynamic contrast media-enhanced T1-weighted sequences with fatty suppression are also routinely obtained. Due to excellent tissue differentiation provided past MRI, the renal cortex and medullary pyramids are easily differentiated on sequences that are not enhanced by contrast media. On T1-weighted sequences the renal cortex has higher signal intensity than practise the medullary pyramids. On T2-weighted sequences the renal cortex has lower indicate intensity than do the medullary pyramids (Fig. 25.16). With kidney injury, this corticomedullary differentiation disappears. 45,110 Urine, similar h2o, normally appears black on T1-weighted sequences and white on T2-weighted sequences (Fig. 25.15 andFig. 25.17).

CE-MRI allows for dynamic evaluation of the kidneys and surrounding structures. Serial acquisitions are obtained after bolus injection of Gd-C (0.one–0.2 mmol per kilogram of body weight) at ii mL/sec. 111,112 The injection should exist administered by means of an automatic, MRI-compatible ability injector to ensure accurateness of the timed bolus, including book and rate of injection. 112,113 The corticomedullary-arterial phase (approximately 20 seconds after injection) is all-time for evaluating the arterial structures and corticomedullary differentiation. In the nephrographic phase (seventy–xc seconds after injection), tumor detection is maximized, and the renal veins and surrounding structures are best demonstrated (Fig. 25.18). Imaging can be performed in any airplane, only the coronal plane is used most frequently for dynamic imaging because it allows imaging of the kidneys, ureters, vessels, and surrounding structures in the fewest number of images. The characteristics of parenchymal enhancement are similar to those observed on CE-CT.

The claret vessels can be variable in point intensity on routine MRI that is not enhanced by contrast media, ranging from white to blackness. This is due to many factors, including, but not limited to, flow-related parameters, location and orientation of the imaged vessel, and choice of pulse sequence. By taking advantage of some of these factors, diagnostic angiography and venography may exist performed without the use of intravenous contrast; these sequences are sometimes called "vivid-blood" sequences. Although contrast media-enhanced magnetic resonance angiography (CE-MRA) remains the preferred method of vascular imaging, loftier-quality MRA that is not enhanced by contrast media has regained popularity because of the advocacy in MR hardware and imaging sequences, also every bit the risk of NSF in patients with poor renal role. MRA not enhanced by contrast media is especially attractive for evaluating the renal arteries in patients with severe renal dysfunction or those with a relative contraindication for CE-CMA. The most robust sequences are based on inversion recovery, and counterbalanced steady-state free precession techniques. 114,115 Older, less robust techniques include time-of-flight MRA, which is based on flow-related enhancement, and phase-contrast MRA, which is based on velocity and management of period. Phase-dissimilarity MRA can be used in conjunction with CE-MRA to discover turbulent flow and loftier velocities associated with stenoses. Unlike MRA that is non enhanced past contrast media, CE-MRA minimizes flow-related enhancement and motion. The success of CE-MRA depends on the T1-shortening properties of gadolinium, which allow for faster imaging, increased coverage, and improved resolution. 77,116 Accurate timing of the bolus injection is critical in CE-MRA. The fourth dimension at which the bolus arrives at the renal arteries may be determined with a bolus injection of 1 mL of Gd-C, followed by a saline flush. A three-dimensional T1-weighted gradient-echo MRI pulse sequence is then obtained in the coronal plane during the injection of approximately 15 to xx mL of Gd-C at 2 mL/sec, timed to capture the arterial phase. 111,112 Sequential three-dimensional sequences are obtained to capture the venous phase (magnetic resonance venography). The information sets tin be postprocessed into multiple formats, improving ease and accuracy of interpretation (Fig. 25.19). 117–119

Volume 2

Rachel W. Chan , ... Angus Z. Lau , in Encyclopedia of Biomedical Engineering, 2019

Introduction

Magnetic resonance imaging (MRI) is a noninvasive imaging technique that enables the observation of anatomic structures, physiological functions, and molecular limerick of tissues. MRI is based on nuclear magnetic resonance (NMR), whose name comes from the interaction of certain atomic nuclei in the presence of an external magnetic field when exposed to radiofrequency (RF) electromagnetic waves of a specific resonance frequency. This commodity is intended to give an overview of selected topics in MRI beginning with a brief history.

NMR was kickoff documented in 1939 in a molecular beam by Isidor Rabi, who received the Nobel Prize in Physics in 1944. In 1946, techniques were developed independently by Felix Bloch and Edward Purcell that extended NMR to liquids and solids. Bloch and Purcell shared the Nobel Prize in Physics in 1952 for other important contributions to methods in magnetic resonance. Information technology was not until 1973 that Paul Lauterbur devised a technique to create the first 2-D image from NMR signals. This is now known as MRI. Strategies to improve imaging speed were introduced by Peter Mansfield in 1978. For their contributions, Lauterbur and Mansfield were awarded the Nobel Prize in Physiology or Medicine in 2003. Raymond Damadian also made significant contributions to the evolution of MRI for man imaging by demonstrating that tumors and normal tissue could exist distinguished. Since then, MRI has become a vital imaging modality for clinical use.

The basis of MRI is that certain atomic nuclei, typically those of hydrogen, in the tissue, become magnetized when placed in an external magnetic field. This produces, in the tissue, a net magnetization, M, that is initially aligned with the direction of the main magnetic field, B 0. A typical MRI experiment starts with the transmission of an RF pulse, B one, to perturb this magnetization. This is termed RF excitation and requires hardware called transmit coils. The excitation process involves 'tipping' the magnetization away from the longitudinal axis (i.eastward., parallel to B 0, where signal cannot be detected) to the transverse airplane (i.e., orthogonal to B 0), where it can then be detected past hardware known every bit receiver coils. After the RF pulse is turned off, the magnetization undergoes processes called relaxation and precession every bit it returns to its thermal equilibrium configuration. It is possible to observe the magnetization because the transverse component of processing magnetization induces an electromotive strength in the receiver whorl. This is detected as the NMR signal. In MRI, the received bespeak tin be spatially encoded by the application of magnetic field gradients that are superimposed on the uniform, chief magnetic field. Excitation and detection modules are repeated until all information are nerveless. The data are recorded and processed to form an image.

MRI is able to produce cross-sectional images of the body with excellent soft tissue dissimilarity. MRI operates in the RF range, so it does non have any harmful ionizing radiation. The versatility of MRI is illustrated in Fig. 1, which shows several of many possible types of images that tin be produced, where each type of prototype has a unlike image contrast or 'weighting.' Each contrast mechanism offers some unique information for the noninvasive detection, diagnosis, and characterization of illness.

Fig. 1

Fig. 1. Dissimilar types of image contrast produced by MRI. (A) T 2, (B) T 1, and (C) proton-density-weighted images are shown of the same slice of the brain. (D) Also shown is an epitome acquired with the fluid-attenuated inversion-recovery (FLAIR) sequence, which nulls the signal from cerebrospinal fluid. Arrows indicate the presence of a silent brain infarct (SBI).

 Reproduced from Zhu YC, Dufouil C, Tzourio C, and Chabriat H. (2011). Silent Brain Infarcts. Stroke 42(4):1140–1145.

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Imaging

John Feehally DM, FRCP , in Comprehensive Clinical Nephrology , 2019

Magnetic Resonance Imaging

Although it only rarely should be the beginning examination used to evaluate the kidneys, MRI is typically an adjunct to other imaging. The major advantage of MRI over other modalities is directly multiplanar imaging. CT is limited to piece acquisition in the axial plane of the belly, and coronal and sagittal planes are acquired only by reconstruction, which tin can lead to loss of information.

Tissues comprise an abundance of hydrogen, the nuclei of which are positively charged protons. These protons spin on their axis, producing a magnetic field (magnetic moment). When a patient is placed in a strong magnetic field in an MRI scanner, some of the protons align themselves with the field. When a radiofrequency pulse is applied, some of the protons aligned with the field will absorb free energy and reverse direction. This captivated free energy is given off as a radiofrequency pulse every bit the protons relax (return to their original alignment), producing a voltage in the receiver curlicue. The coil is the hardware that covers the region of interest. For renal imaging, a body coil or torso coil is used. Relaxation is a 3D event giving ascension to two parameters: T1 relaxation results in the recovery of magnetization in the longitudinal (spin-lattice) plane, whereas T2 results from the loss of transverse (spin-spin) magnetization. A rapid-sequence variant of T2 in common use is fast spin echo (FSE). Hydrogen ions move at slightly different rates in the dissimilar tissues. This difference is used to select imaging parameters that can suppress or aid in the detection of fat and water. Fluid, such as urine, is night or low in bespeak on T1-weighted sequences and bright or high in signal on FSE sequences. Fat is bright on T1 and non every bit bright on FSE sequences (Fig. v.24). The sequences and imaging planes selected must be tailored to the individual MR study. Diffusion-weighted imaging (DWI) evaluates the liberty of water molecules to lengthened in tissues; restriction of improvidence is imaged as bright areas on the DWI image ready. Using the DWI information, regional apparent diffusion coefficients of the tissue tin can be calculated and an image of the distribution of the coefficients of the tissue can be produced. Night areas on what is known as an apparent diffusion coefficient map are seen in infection, neoplasia, inflammation, and ischemia (Fig. v.25).

Standard MR images usually include T1, T2, or FSE sequences and oft boosted dissimilarity-enhanced T1 images. The imaging plane varies according to the clinical concerns. Usually, at least one sequence is performed in the axial plane. Sagittal and coronal images embrace the entire length of the kidney and can make some subtle renal parenchymal abnormalities more conspicuous (Fig. five.26).

On T1-weighted sequences, the normal renal cortex is college in betoken than the medulla, producing a distinct corticomedullary differentiation, which becomes indistinct in parenchymal renal disease. It is analogous to the echogenic kidney seen on ultrasound. On FSE sequences, the corticomedullary distinction is not as abrupt just should nonetheless be present.

Magnetic Resonance Imaging

John A. Detre MD , in Neurobiology of Illness, 2007

I. History of Magnetic Resonance Imaging

Clinical MRI is the result of an boggling number of scientific and engineering advances [1]. The first successful nuclear magnetic resonance (NMR) spectroscopy experiments were independently demonstrated in the 1945 by Felix Bloch and Edward Purcell, who shared the Nobel Prize in Physics in 1952 for the finding. For the adjacent few decades, NMR experiments were mainly used for chemical and physical assay of small samples that could be fit into small-bore NMR spectrometers. Loftier-resolution NMR continued to evolve into a powerful modality for detailed chemical analysis of molecules, but NMR imaging of spatially resolved signals developed in a different direction. Spatial encoding in NMR is accomplished through the apply of magnetic field gradients, which can innovate spatial variations in the primary magnetic field. The concept of generating images with NMR arose from Paul Lauterbur'due south 1972 idea of applying field gradients in all 3 dimensions, using dorsum-projection methods borrowed from CT scanning to generate images. This inspired evolution of Fourier transform reconstruction by Richard Ernst in 1974, which is the predominant approach used today. Around that time, wide-bore NMR systems capable of imaging living animals and human limbs were available, and larger magnets capable of accommodating a human being trunk were being considered. Peter Mansfield reported the first in vivo image of man anatomy in 1977, a cross-sectional image through a finger. The potential diagnostic value of changes in NMR relaxation was suggested by Raymond Damadian and others and further motivated the development of MRI for clinical use. Nuclear magnetic resonance imaging was renamed magnetic resonance imaging to avoid the undesirable connotations of the word nuclear among the lay public. In 2003, Lauterbur and Mansfield shared the Nobel Prize in Medicine for MRI.

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Magnetic Resonance Imaging

Eric T. Chou , John A. Carrino , in Pain Management, 2007

▪ DESCRIPTION OF MODALITY

Magnetic resonance imaging (MRI) is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used to obtain microscopic chemical and physical information about molecules. MRI is based on the absorption and emission of energy in the radiofrequency (RF) range of the electromagnetic spectrum. Information technology produces images based on spatial variations in the stage and frequency of the RF energy being captivated and emitted by the imaged object. A number of biologically relevant elements, such as hydrogen, oxygen-16, oxygen-17, fluorine-19, sodium-23, and phosphorus-31 are potential candidates for producing MR images. The human body is primarily fat and h2o, both of which have many hydrogen atoms, making the human torso approximately 63% hydrogen atoms. Hydrogen nuclei have an NMR betoken, and then for these reasons clinical MRI primarily images the NMR signal from the hydrogen nuclei given its affluence in the human torso. Protons comport like small bar magnets, with north and south poles within the magnetic field. The magnetic moment of a single proton is extremely pocket-sized and not detectable. Without an external magnetic field, a group of protons assumes a random orientation of magnetic moments. Nether the influence of an practical external magnetic field, the protons assume a nonrandom alignment, resulting in a measurable magnetic moment in the direction of the external magnetic field. By applying RF pulses, images can and then be created based on the differences in signal from hydrogen atoms in unlike types of tissue. A variety of systems are used in medical imaging ranging from open MRI units with magnetic field force of 0.3 Tesla (T) to extremity MRI systems with field strengths up to 1.0 T and whole-torso scanners with field strengths upwardly to 3.0 T (in clinical use). Considering of its superior soft tissue dissimilarity resolution, MRI is all-time suited for evaluation of internal derangement of joints, central nervous system abnormalities, likewise as other pathologic processes in the patient with pain.

The advantages of MRI over other imaging modalities include absenteeism of ionizing radiations, superior soft tissue contrast resolution, high-resolution imaging, and multiplanar imaging capabilities. The fourth dimension to acquire an MRI epitome has been a major weakness and continues to be so with the advent of faster CT scanners (with multislice CT). However, newer imaging techniques (e.thou., parallel imaging), faster pulse sequences, and higher field strength systems are addressing this event.

A number of pulse sequences accept been invented to highlight differences in signal of various soft tissues. The most common and near basic of pulse sequences include T1-weighted and T2-weighted sequences. T1-weighted sequences have traditionally been considered good for evaluation of anatomic structures. Tissues that evidence a high signal (bright) and T1-weighted images include fat, blood (methemoglobin), proteinaceous fluid, some forms of calcium, melanin, and gadolinium (a contrast amanuensis). T2-weighted sequences accept by and large been considered fluid-conspicuity pulse sequences, useful for identifying pathologic processes. Tissues that show a high signal on T2-weighted images include fluid-containing structures (i.due east., cysts, joint fluid, cerebrospinal fluid) and pathologic states causing increased extracellular fluid (i.east., sources of infection or inflammation).

Advanced imaging techniques used in medical imaging include magnetic resonance angiography (MRA), diffusion weighted imaging, chemical shift imaging (fatty suppression), functional imaging of the brain, and MR spectroscopy (MRS). Many of these techniques are specially useful in brain imaging. MRA (either time-of-flight or stage contrast) and improvidence weighted imaging are useful for the detection and characterization of ischemic insults in the brain. MRS uses the differences in chemical composition in tissues to differentiate necrosis or normal brain matter from tumor.

In musculoskeletal imaging, MR arthrography is a technique bachelor to augment the delineation of internal derangements of joints. ane Arthrography tin be either indirect (intravenous gadolinium is administered and immune to diffuse into the articulation) or direct (a dilute gadolinium solution is percutaneously injected into the joint) to provide distention of a joint, profitable in the evaluation of ligaments, cartilage, synovial proliferation, or intraarticular bodies. MR arthrography has been most extensively used in the shoulder to outline labral-ligamentous abnormalities as well as to distinguish partial-thickness from full-thickness tears in the rotator cuff. It is also helpful in demonstrating labral tears in the hip, partial- and total-thickness tears of the collateral ligament of the elbow, and bands in the elbow. This technique is also useful in patients after meniscectomy in the knee to notice recurrent or residual meniscal tears, evaluate perforations of the ligaments and triangular fibrocartilage in the wrist, and appraise the stability of osteochondral lesions in the articular surface of joints. T1-weighted images are often employed with MR arthrography to bring out the T1 shortening effects of gadolinium. Fat saturation is also added to help differentiate fat from gadolinium. A T2-weighted sequence in at to the lowest degree one aeroplane is likewise necessary to observe cysts and edema in other soft tissues and bone marrow.

Patients in whom MRI is contraindicated include those who have the following: cardiac pacemaker, implanted cardiac defibrillator, aneurysm clips, carotid avenue vascular clench, neurostimulator, insulin or infusion pump, implanted drug infusion device, bond growth/fusion stimulator, and a cochlear or ear implant. In addition, patients who accept a history of metalworking should have a pre-MRI screening radiograph of the orbits to evaluate for radiopaque strange bodies near the ocular globe.

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Magnetic Resonance Imaging

Cihan Duran , ... Frank J. Rybicki , in Vascular Medicine: A Companion to Braunwald's Heart Disease (2d Edition), 2013

Bones Principles

Magnetic resonance imaging (MRI) relies upon the inherent magnetic backdrop of homo tissue and the ability to employ these properties to produce tissue contrast. Magnetic resonance imaging detects the magnetic moment created by unmarried protons in omnipresent hydrogen atoms. Considering any moving electrical charge produces a magnetic field, spinning protons produce minor magnetic fields and tin can be thought of as little magnets or "spins." When a patient is placed in the bore of a large magnet (i.due east., MRI scanner), hydrogen protons align with the externally applied static magnetic field (B 0) to create a net magnetization vector. On a quantum level, most protons will distribute randomly, either with or against the scanner's B0. However, a slight excess of spins aligns with the field, causing net tissue magnetization. The time required for this alignment is denoted by the longitudinal relaxation time, T1. T1 variations between tissues is used to provide dissimilarity.

Spinning protons wobble or "precess" about the axis of B0. The frequency of the wobble is proportional to the forcefulness of B0. If a radiofrequency (RF) pulse is practical at the resonance frequency of the wobble, protons can absorb energy and spring to a higher free energy state. This RF pulse deflects the protons, creating a new net magnetization vector distinct from the major centrality of the practical magnetic field. The internet magnetization vector tips from the longitudinal to the transverse plane (transverse magnetization). The protons are "flipped" by the RF pulse, and the internet magnetization vector is defined by a "flip angle." The stronger the RF pulse applied, the greater the angle of deflection for the magnetization. Mutual flip angles for spin echo are ninety° and 180°. For slope repeat (GRE) MRI, flip angles typically range between 10° and 70°. After the RF pulse tips the spinning protons out of alignment with the primary magnetic field, new protons begin to align with the primary magnetic field at a charge per unit determined by the T1 relaxation time.

Free energy is given off as the spins move from high to low energy states. The absorbed RF energy is retransmitted at the resonance frequency and can exist detected with RF antennas or "coils" placed around the patient. These signals are compiled, and after mathematical processes get the MR images. Proton excitation with an externally applied RF field is repeated at short intervals to obtain signals. This MR parameter is referred to as repetition fourth dimension (TR). For conventional MRI, TR is typically 0.v to 2 seconds, whereas for MRA, TR ranges from xxx to less than v milliseconds. When the spins are tipped to the transverse plane, they all precess in phase. The speed of wobbling depends on the strength of the magnetic field each proton experiences. Some protons spin faster while others spin slower, and they quickly leave of phase relative to one another. Throughout the dephasing process, the MR point decays. This loss of phase is termed T2 relaxation time or transverse relaxation. T2, like T1, is unique among tissues and is used for image contrast. In addition to the intrinsic T2 of tissue, inhomogeneity of B0 results in rapid loss of transverse magnetization. The relaxation time that reflects the sum of these random defects with tissue T2 is chosen T2*. To obtain an MRI point, these spins must be brought back in phase and produce a indicate or repeat. The fourth dimension at which it happens is referred to every bit repeat fourth dimension (TE). In spin repeat imaging technique, the repeat is obtained by using a refocusing 180° RF pulse, later which the spins begin to dephase. Another 180° RF pulse can be applied to generate a second echo and and then on. Signal loss at longer echo times reflects tissue T2. In GRE imaging, the repeat is obtained by gradient reversal rather than RF pulse. Because this includes furnishings from tissue homogeneity, TE-dependent signal loss reflects T2*. Recently, GRE sequences (balanced GRE steady-land free precession [SSFP]) accept been adult that are insensitive to magnet field inhomogeneities and reflective of actual tissue T2.

Longitudinal and transverse magnetizations occur simultaneously but are two different processes that reflect properties of various tissues in the body. Since T1 measures betoken recovery, tissues with short T1 are bright, whereas tissues with long T1 are nighttime. Fatty has a very short T1. In contrast, T2 is a measure of signal loss. Therefore, tissues with short T2 are dark, and those with long T2 are bright. Simple fluids, such equally cerebrospinal fluid and urine, have long T2. To differentiate between the tissues based on these relaxation times, MR images tin be designed to be T1-weighted, T2-weighted, or proton-density weighted. Exogenous contrast such as gadolinium-based agents are routinely used to alter tissue conspicuity. Spatial encoding of signals obtained from tissues is required for imaging. Boosted external time-varying magnetic fields are applied to spatially encode the MR indicate. Spatially dependent gradients are used to locate the MR betoken in space. In ii-dimensional (2nd) MRI, these are piece-choice, frequency-encoding, and stage-encoding gradients. In three-dimensional (3D) MRI, the slice-option gradient is replaced by a 2d stage-encoding slope.

Magnetic resonance echoes are digitized and stored in "k-space" composed of either 2 axes (for 2D imaging) or three axes (for 3D imaging). K-infinite represents frequency data and is related to paradigm infinite by Fourier transformation. An of import feature of k-infinite is that tissue dissimilarity is adamant by the heart of k-space (cardinal phase encoding lines), whereas the periphery of the k-space encodes the prototype detail. The order in which 1000-space lines are collected can be varied, strongly influencing tissue contrast. For example, in CE-MRA, the central dissimilarity-defining portion of chiliad-space may be acquired early in the scan (centric acquisition) during meridian intraarterial contrast concentration to maximize arterial contrast. In addition to elementary line-by-line g-space acquisition schemes, more complex schemes take been described. In spiral imaging, data acquisition begins at the eye of k-space and spirals to the periphery. Slice-selective gradients applied along the z-centrality will form axial images. Those along the y-centrality will yield coronal images, and the x-axis gradients will provide sagittal images. An oblique slice tin can exist selected by a combination of two or more gradients.

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Magnetic Resonance Imaging

In Imaging of Pain, 2011

Concept

•

Magnetic resonance imaging (MRI) uses the movement of protons inside a magnetic field to generate an prototype.

•

Within the constant magnetic field of an MRI scanner, tissues that contain gratuitous hydrogen nuclei (protons) generate varying signals when pulses of radiofrequency (RF) energy are applied to them.

•

These signals, which depend on the type of tissue and the speed at which the tissue "relaxes" or gives up its motion, are then mathematically converted into an paradigm.

•

The contrast of the image thus depends on the betoken intensity (SI) of different tissues. Certain tissues that are rich in gratis protons, such as water and fatty, are very responsive to the RF pulses. Other tissues with fewer free protons, such as cortical bone and air, are less responsive and generate much less signal.

•

Different tissue contrasts can exist determined, depending on the strength and timing of the RF pulse; this parameter is known every bit an MR sequence. The most basic forms of MR sequences include:

•

T1-weighted (T1W) imaging, on which fluid appears dark and fat appears bright.

•

T2-weighted (T2W) imaging, on which both fluid and fat appear bright.

•

Proton density (PD) imaging, on which fluid appears intermediate-SI and fat appears bright.

•

Manipulating the MR sequences allows the demonstration of different tissue characteristics. For example, the signal from fatty tin can be cancelled out (made dark) using a technique known equally fat suppression. Fat suppression with T2 weighting is very useful in musculoskeletal imaging to increase contrast betwixt bright pathologic tissue and fatty. Common fatty suppression techniques include:

•

Short TI inversion recovery (STIR) imaging.

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Fat suppression with T2 weighting (FST2W imaging).

•

Intravenous contrast agents such equally gadolinium can exist administered to raise the visualization of vessels and inflammatory tissue. T1W with fat suppression (FST1W) images are oftentimes used to ameliorate contrast betwixt enhanced tissue and adjacent fatty structures.

•

Intra-articular dissimilarity agents may besides exist administered, producing an MR arthrogram result to enhance the evaluation of intra-articular structures such as articular cartilage, fibrocartilage, and ligaments. This method is often employed in shoulder, wrist, elbow, and hip imaging.

•

Certain metals, such as stainless steel and cobalt-chrome, distort the magnetic field and thus produce epitome artifact. Other metals, such as titanium, produce much less image distortion. Such distortion may dethrone the image quality and is an of import consideration in referring patients with metallic devices such as orthopaedic hardware for evaluation past MRI.

•

Implantable electronic devices, such as cardiac pacemakers and neural stimulators, are affected by the magnetic field and are also incompatible with MRI evaluation.

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