Computed tomography angiography
Computed tomography angiography is a computed tomography technique used to visualize arterial and venous vessels throughout the body. Using contrast injected into the blood vessels, images are created to look for blockages, aneurysms and stenosis. CTA can be used to visualize the vessels of the heart, the aorta and other large blood vessels, the lungs, the kidneys, the head and neck, the arms and legs. CTA can be used to examine blood vessels in many key areas of the body including the brain, kidneys and the lungs. Coronary CT angiography is the use of CT angiography to assess the arteries of the heart; the patient receives an intravenous injection of contrast and the heart is scanned using a high speed CT scanner. With the advances in CT technology, patients are able to be scanned without needing medicines by holding their breath during the scan. CTA is used to assess heart or vessel irregularities, location of stents and whether they are still open, to check for atherosclerotic disease; this method displays the anatomical detail of blood vessels more than magnetic resonance imaging or ultrasound.
Today, many patients can undergo CTA in place of a conventional catheter angiogram, a minor procedure during which a catheter is passed through the blood vessels all the way to the heart, however CCTA has not replaced this procedure. CCTA is able to detect narrowing of blood vessels in time for corrective therapy to be done. CCTA is a useful way of screening for arterial disease because it is safer, much less time-consuming than catheter angiography, is a cost-effective procedure. CTA can be used in the chest and abdomen to identify aneurysms in the aorta or other major blood vessels; these areas of weakened blood vessel walls that bulge out can life-threatening. CTA is the test of choice when assessing aneurysm before and after endovascular stenting due to the ability to detect calcium within the wall. Another positive of CTA in abdominal aortic aneurysm assessment is it allows for better estimation of blood vessel dilation and can better detect blood clots as compared to standard angiography.
CTA is used to identify arterial dissection, including aortic dissection in the aorta or its major branches. Arterial dissection is. CTA is a quick and non-invasive method of identifying dissections and can show the extent of the disease and if there is leakage. CT pulmonary angiogram is used to examine the pulmonary arteries in the lungs, most to rule out pulmonary embolism, a serious but treatable condition, it has become the technique of choice for detection of pulmonary embolism due to its wide availability, short exam time, ability to see other diseases that may present like pulmonary embolisms, a high degree of confidence in the validity of the test. In this test, a PE will appear as a dark spot inside the blood vessel or a sudden stop of the bright contrast material. CT angiography should not be used to evaluate for pulmonary embolism when other tests indicate that there is a low probability of a person having this condition. A D-dimer assay might be a preferred alternative to test for pulmonary embolism, that test and a low clinical prediction score on the Wells test or Geneva score can exclude pulmonary embolism as a possibility.
Visualization of blood flow in the renal arteries in patients with high blood pressure and those suspected of having kidney disorders can be performed using CTA. Stenosis of a renal artery can be corrected. A special computerized method of viewing the images makes renal CT angiography a accurate examination. CTA is used in the assessment of native and transplant renal arteries. While CTA is great for imaging of the kidneys, it lacks the ability to perform procedures at the same time, thus traditional catheter angiography is used in cases of acute renal hemorrhage or acute arterial obstruction. CTA can be used assess acute stroke patients by identifying clots in the arteries of the brain, it can be used to identify small aneurysms or arteriovenous malformation inside the brain that can be life-threatening. While CTA can produce high quality images of the carotid arteries for grading the level of stenosis, calcium deposits in the area where the vessels split can lead to interference with accurate stenosis grading.
Because of this, magnetic resonance angiography is used more for this purpose. CTA can be used in the legs to detect atherosclerotic disease, it can be used to image vessels in suspected blockages, trauma cases, or patients with surgical complications. CT angiography is a contrast CT where images are taken with a certain delay after injection of radiocontrast material; the contrast material is radiodense causing it to light up brightly within the blood vessels of interest. In order for the CT scanner to be able to scan the correct area where the contrast is, the scanner uses either automatic detectors which start scanning when enough contrast is present, or small test boluses. With the small test bolus a small amount of contrast is injected in order to detect the speed that the contrast will move through the blood vessels. After determining this speed, the full bolus is injected and the scan is begun at the timing determined by the test bolus. After the scan is completed the images are post-processed to better visualize the vessels and can be created in the 3D images.
Harms of overuse of CT ang
An MRI sequence in magnetic resonance imaging is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance. A multiparametric MRI is a combination of two or more sequences, and/or including other specialized MRI configurations such as spectroscopy; this table does not include experimental sequences. Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 and T2. To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time; this image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions and in general for 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; this image weighting is useful for detecting edema and inflammation, revealing white matter lesions and assessing zonal anatomy in the prostate and uterus.
The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows: Proton density weighted images are created by having a long repetition time and a short echo time. On images of the brain, this sequence has a more pronounced distinction between gray matter and white matter, but with little contrast between brain and CSF, it is useful for the detection of joint disease and injury. A gradient echo sequence is the base of many important derived sequences such as echo-planar imaging and SSFP stationary sequences, it allows to obtain short repetition times, therefore to acquire images in a short time. The gradient echo sequence is characterized by a single excitation followed by a gradient applied along the reading axis called the dephasing gradient; this gradient modifies the spin phase in a spatially dependent manner, so that at the end of the gradient the signal will be canceled because the coherence between the spins will be destroyed.
At this point the reading gradient of opposite polarity is applied, so as to compensate for the effect of the disparity gradient. When the area of the reading gradient is equal to that of the mismatching gradient, the spins will have a coherent new phase, therefore a signal will be detectable again; this signal takes the name of echo or more of gradient echo signal, because it is produced by rephasing due to a gradient. The sequences of the gradient echo type allow to achieve short repetition times, as the acquisition of an echo corresponds to the acquisition of a k-space line, this acquisition can be made quick by increasing the amplitude of the gradients of rephasing and reading. A sequence of the spin echo type must instead wait for the exhaustion of the signal, formed spontaneously after the application of the excitation impulse before it can produce an echo. For comparison purposes, the repetition time of a gradient echo sequence is of the order of 3 milliseconds, versus about 30 ms of a spin echo sequence.
At the end of the reading, the residual transverse magnetization can be maintained. In the first case there is a spoiled sequence, such as the FLASH sequence, while in the second case there are SSFP sequences. Steady-state free precession imaging is an MRI technique. In general, SSFP MRI sequences are based on a gradient-echo MRI sequence with a short repetition time which in its generic form has been described as the FLASH MRI technique. While spoiled gradient-echo sequences refer to a steady state of the longitudinal magnetization only, SSFP gradient-echo sequences include transverse coherences from overlapping multi-order spin echoes and stimulated echoes; this is accomplished by refocusing the phase-encoding gradient in each repetition interval in order to keep the phase integral constant. Balanced SSFP MRI sequences achieve a phase of zero by refocusing all imaging gradients. New methods and variants of existing methods are published when they are able to produce better results in specific fields.
Examples of these recent improvements are T*2-weighted turbo spin-echo, double inversion recovery MRI or phase-sensitive inversion recovery MRI, all of them able to improve imaging of brain lesions. Another example is MP-RAGE. In-phase and out-of-phase sequences correspond to paired gradient echo sequences using the same repetition time but with two different echo times; this can detect microscopic amounts of fat, which has a drop in signal on OOP compared to IP. Among renal tumors that do not show macroscopic fat, such a signal drop is seen in 80% of the clear cell type of renal cell carcinoma as well as in minimal fat angiomyolipoma. T2*-weighted imaging can be created as a postexcitation refocused gradient echo sequence with small flip angle; the sequence of a GRE T2*WI requires high uniformity of the magnetic field. Fluid-attenuated inversion recovery is an inversio
In scientific visualization and computer graphics, volume rendering is a set of techniques used to display a 2D projection of a 3D discretely sampled data set a 3D scalar field. A typical 3D data set is a group of 2D slice images acquired by a MRI, or MicroCT scanner; these are acquired in a regular pattern and have a regular number of image pixels in a regular pattern. This is an example of a regular volumetric grid, with each volume element, or voxel represented by a single value, obtained by sampling the immediate area surrounding the voxel. To render a 2D projection of the 3D data set, one first needs to define a camera in space relative to the volume. One needs to define the opacity and color of every voxel; this is defined using an RGBA transfer function that defines the RGBA value for every possible voxel value. For example, a volume may be viewed by extracting isosurfaces from the volume and rendering them as polygonal meshes or by rendering the volume directly as a block of data; the marching cubes algorithm is a common technique for extracting an isosurface from volume data.
Direct volume rendering is a computationally intensive task. Volume rendering is distinguished from thin slice tomography presentations, is generally distinguished from projections of 3D models, including maximum intensity projection. Still, all volume renderings become projections when viewed on a 2-dimensional display, making the distinction between projections and volume renderings a bit vague; the epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and/or observable representations. A direct volume renderer requires every sample value to be mapped to a color; this is done with a "transfer function" which can be a simple ramp, a piecewise linear function or an arbitrary table. Once converted to an RGBA value, the composed RGBA result is projected on corresponding pixel of the frame buffer; the way this is done depends on the rendering technique. A combination of these techniques is possible. For instance, a shear warp implementation could use texturing hardware to draw the aligned slices in the off-screen buffer.
The technique of volume ray casting can be derived directly from the rendering equation. It provides results of high quality considered to provide the best image quality. Volume ray casting is classified as image based volume rendering technique, as the computation emanates from the output image, not the input volume data as is the case with object based techniques. In this technique, a ray is generated for each desired image pixel. Using a simple camera model, the ray starts at the center of projection of the camera and passes through the image pixel on the imaginary image plane floating in between the camera and the volume to be rendered; the ray is clipped by the boundaries of the volume. The ray is sampled at regular or adaptive intervals throughout the volume; the data is interpolated at each sample point, the transfer function applied to form an RGBA sample, the sample is composited onto the accumulated RGBA of the ray, the process repeated until the ray exits the volume. The RGBA color deposited in the corresponding image pixel.
The process is repeated for every pixel on the screen to form the completed image. This is a technique. Here, every volume element is splatted, as Lee Westover said, like a snow ball, on to the viewing surface in back to front order; these splats are rendered as disks. Flat disks and those with other kinds of property distribution are used depending on the application; the shear warp approach to volume rendering was developed by Cameron and Undrill, popularized by Philippe Lacroute and Marc Levoy. In this technique, the viewing transformation is transformed such that the nearest face of the volume becomes axis aligned with an off-screen image buffer with a fixed scale of voxels to pixels; the volume is rendered into this buffer using the far more favorable memory alignment and fixed scaling and blending factors. Once all slices of the volume have been rendered, the buffer is warped into the desired orientation and scaled in the displayed image; this technique is fast in software at the cost of less accurate sampling and worse image quality compared to ray casting.
There is memory overhead for storing multiple copies of the volume, for the ability to have near axis aligned volumes. This overhead can be mitigated using run length encoding. Many 3D graphics systems use texture mapping to apply textures, to geometric objects. Commodity PC graphics cards are fast at texturing and can efficiently render slices of a 3D volume, with real time interaction capabilities. Workstation GPUs are faster, are the basis for much of the production volume visualization used in medical imaging and gas, other markets. In earlier years, dedicated 3D texture mapping systems were used on graphics systems such as Silicon Graphics InfiniteReality, HP Visualize FX graphics accelerator, others; this technique was first described by Dave Santek. These slices can either be aligned with the volume and rendered at an angle to the viewer, or aligned with the viewing plane and sampled from unaligned slices through the volume. Graphics hardware support for 3D textures is needed for the second technique.
Volume aligned texturing produces images of reasonable qual
Gadolinium is a chemical element with symbol Gd and atomic number 64. Gadolinium is a silvery-white, ductile rare-earth metal, it is found in nature only in oxidized form, when separated, it has impurities of the other rare earths. Gadolinium was discovered in 1880 by Jean Charles de Marignac, who detected its oxide by using spectroscopy, it is named after the mineral gadolinite, one of the minerals in which gadolinium is found, itself named for the chemist Johan Gadolin. Pure gadolinium was first isolated by the chemist Paul Emile Lecoq de Boisbaudran around 1886. Gadolinium possesses unusual metallurgical properties, to the extent that as little as 1% of gadolinium can improve the workability and resistance to oxidation at high temperatures of iron and related metals. Gadolinium as a metal or a salt absorbs neutrons and is, used sometimes for shielding in neutron radiography and in nuclear reactors. Like most of the rare earths, gadolinium forms trivalent ions with fluorescent properties, salts of gadolinium are used as phosphors in various applications.
The kinds of gadolinium ions occurring in water-soluble salts are toxic to mammals. However, chelated gadolinium compounds are far less toxic because they carry gadolinium through the kidneys and out of the body before the free ion can be released into the tissues; because of its paramagnetic properties, solutions of chelated organic gadolinium complexes are used as intravenously administered gadolinium-based MRI contrast agents in medical magnetic resonance imaging. Gadolinium is a silvery-white, ductile rare-earth metal, it crystallizes in the hexagonal close-packed α-form at room temperature, when heated to temperatures above 1,235 °C, it transforms into its β-form, which has a body-centered cubic structure. The isotope gadolinium-157 has the highest thermal-neutron capture cross-section among any stable nuclide: about 259,000 barns. Only xenon-135 has a higher capture cross-section, about 2.0 million barns, but this isotope is radioactive. Gadolinium is believed to be ferromagnetic at temperatures below 20 °C and is paramagnetic above this temperature.
There is evidence that gadolinium is a helical antiferromagnetic, rather than a ferromagnetic, below 20 °C. Gadolinium demonstrates a magnetocaloric effect whereby its temperature increases when it enters a magnetic field and decreases when it leaves the magnetic field; the temperature is lowered to 5 °C for the gadolinium alloy Gd85Er15, this effect is stronger for the alloy Gd5, but at a much lower temperature. A significant magnetocaloric effect is observed at higher temperatures, up to about 300 kelvins, in the compounds Gd54. Individual gadolinium atoms can be isolated by encapsulating them into fullerene molecules, where they can be visualized with transmission electron microscope. Individual Gd atoms and small Gd clusters can be incorporated into carbon nanotubes. Gadolinium combines with most elements to form Gd derivatives, it combines with nitrogen, sulfur, boron, selenium and arsenic at elevated temperatures, forming binary compounds. Unlike the other rare-earth elements, metallic gadolinium is stable in dry air.
However, it tarnishes in moist air, forming a loosely-adhering gadolinium oxide: 4 Gd + 3 O2 → 2 Gd2O3,which spalls off, exposing more surface to oxidation. Gadolinium is a strong reducing agent. Gadolinium is quite electropositive and reacts with cold water and quite with hot water to form gadolinium hydroxide: 2 Gd + 6 H2O → 2 Gd3 + 3 H2. Gadolinium metal is attacked by dilute sulfuric acid to form solutions containing the colorless Gd ions, which exist as 3+ complexes: 2 Gd + 3 H2SO4 + 18 H2O → 2 3+ + 3 SO2−4 + 3 H2. Gadolinium metal reacts with the halogens at temperature about 200 °C: 2 Gd + 3 X2 → 2 GdX3. In the great majority of its compounds, gadolinium adopts the oxidation state +3. All four trihalides are known. All are white, except for the iodide, yellow. Most encountered of the halides is gadolinium chloride; the oxide dissolves in acids to give the salts, such as gadolinium nitrate. Gadolinium, like most lanthanide ions, forms complexes with high coordination numbers; this tendency is illustrated by the use of the chelating agent DOTA, an octadentate ligand.
Salts of − are useful in magnetic resonance imaging. A variety of related chelate complexes have been developed, including gadodiamide. Reduced gadolinium compounds are known in the solid state. Gadolinium halides are obtained by heating Gd halides in presence of metallic Gd in tantalum containers. Gadolinium form sesquichloride Gd2Cl3, which can be further reduced to GdCl by annealing at 800 °C; this gadolinium chloride forms platelets with layered graphite-like structure. Occurring gadolinium is composed of six stable isotopes, 154Gd, 155Gd, 156Gd, 157Gd, 158Gd and 160Gd, one radioisotope, 152Gd, with the isotope 158Gd being the most abundant; the predicted double beta decay of 160Gd has never been observed. 29 radioisotopes of gadolinium have been observed, with the most stable being 152Gd, with a half-life of about 1.08×1014 years, 150Gd, with a half-life of 1.79×106 years. All of the remaining radioactive isotopes have half-lives of less than 75 years; the majority of these have half-lives of less than 25 seconds.
Gadolinium isotopes have four
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the
Susceptibility weighted imaging
Susceptibility weighted imaging called BOLD venographic imaging, is an MRI sequence, exquisitely sensitive to venous blood and iron storage. SWI uses a flow compensated, long echo, gradient recalled echo pulse sequence to acquire images; this method exploits the susceptibility differences between tissues and uses the phase image to detect these differences. The magnitude and phase data are combined to produce an enhanced contrast magnitude image; the imaging of venous blood with SWI is a blood-oxygen-level dependent technique, why it was referred to as BOLD venography. Due to its sensitivity to venous blood SWI is used in traumatic brain injuries and for high resolution brain venographies but has many other clinical applications. SWI is offered as a clinical package by Philips and Siemens but can be run on any manufacturer’s machine at field strengths of 1.0 T, 1.5 T, 3.0 T and higher. SWI uses a velocity compensated, three-dimensional, RF spoiled, high-resolution, 3D gradient recalled echo scan.
Both the magnitude and phase images are saved, the phase image is high pass filtered to remove unwanted artifacts. The magnitude image is combined with the phase image to create an enhanced contrast magnitude image referred to as the susceptibility weighted image, it is common to create minimum intensity projections over 8 to 10 mm to better visualize vein connectivity. In this way four sets of images are generated, the original magnitude, HP filtered phase, susceptibility weighted, mIPs over the susceptibility weighted images; the values in the phase images are constrained from -π to π so if the value goes above π it wraps to -π, inhomogeneities in the magnetic field cause low frequency background gradients. This causes all the phase values to increase across the image which creates phase wrapping and obscures the image; this type of artifact can be removed by phase unwrapping or by high pass filtering the original complex data to remove the low frequency variations in the phase image. The susceptibility weighted image is created by combining the magnitude and filtered phase images.
A mask is created from the phase image by mapping all values above 0 radians to be 1 and linearly mapping values from -π to 0 radians to range from 0 to 1, respectively. Alternatively, a power function can be used instead of a linear mapping from -π to 0 to increase the effect of the mask; the magnitude image is multiplied by this mask. In this way phase values above 0 radians have no effect and phase values below 0 radians darken the magnitude image; this increases the contrast in the magnitude image for objects with low phase values such as veins and hemorrhage. SWI is most used to detect small amounts of hemorrhage or calcium. Clinical applications are under research in different fields of medicine; the detection of micro-hemorrhages and diffuse axonal injury in trauma patients is difficult as the injuries tend to be small in size and can be missed by low resolution scans. SWI is run at high resolution and is sensitive to bleeding in the gray matter/white matter boundaries making it is possible to see small lesions increasing the ability to detect more subtle injuries.
Diffusion weighted imaging offers a powerful means to detect acute stroke. Although it is well known that gradient echo imaging can detect hemorrhage, it is best detected with SWI. In the example shown here, the gradient echo image shows the region of cytotoxic edema whereas the SW image shows the localization of the stroke and the vascular territory affected; the bright region in the gradient echo weighted image shows the area affected in this acute stroke example. The arrows in the SWI image may show the tissue at risk, affected by the stroke and the location of the stroke itself; the reason that we are able to see the affected vascular territory could be because there is a reduced level of oxygen saturation in this tissue, suggesting that the flow to this region of the brain could be reduced post stroke. Another possible explanation is. In either case, this image suggests that the tissue associated with this vascular territory could be tissue at risk. Future stroke research will involve comparisons of perfusion weighted imaging and SWI to learn more about local flow and oxygen saturation.
An SWI venogram of a neonate with Sturge-Weber syndrome who did not display neurological symptoms is shown to the right. The initial conventional MR imaging methods did not demonstrate any abnormality; the abnormal venous vasculature in the left occipital lobe extending between the posterior horn of the ventricle and the cortical surface is visible in the venogram. Due to the high resolution collaterals can be resolved. Part of the characterization of tumors lies in understanding the angiographic behavior of lesions both from the perspective of angiogenesis and micro-hemorrhages. Aggressive tumors tend to have growing vasculature and many micro-hemorrhages. Hence, the ability to detect these changes in the tumor could lead to a better determination of the tumor status; the enhanced sensitivity of SWI to venous blood and blood products due to their differences in susceptibility compared to normal tissue leads to better contrast in detecting tumor boundaries and tumor hemorrhage. Multiple sclerosis is studied with FLAIR and contrast enhanced T1 imaging.
SWI adds to this by revealing the venous connectivity in some lesions and presents evidence of iron in some lesions. This key new information may help under
Veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart. In contrast to veins, arteries carry blood away from the heart. Veins are less muscular than arteries and are closer to the skin. There are valves in most veins to prevent backflow. Veins are present throughout the body as tubes. Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, large vs. small. Superficial veins are those closer to the surface of the body, have no corresponding arteries. Deep veins have corresponding arteries. Perforator veins drain from the superficial to the deep veins; these are referred to in the lower limbs and feet. Communicating veins are veins. Pulmonary veins are a set of veins. Systemic veins deliver deoxygenated blood to the heart. Most veins are equipped with valves to prevent blood flowing in the reverse direction. Veins are translucent, so the color a vein appears from an organism's exterior is determined in large part by the color of venous blood, dark red as a result of its low oxygen content.
Veins appear blue because the subcutaneous fat absorbs low-frequency light, permitting only the energetic blue wavelengths to penetrate through to the dark vein and reflect back to the viewer. The colour of a vein can be affected by the characteristics of a person's skin, how much oxygen is being carried in the blood, how big and deep the vessels are; when a vein is drained of blood and removed from an organism, it appears grey-white. The largest veins in the human body are the venae cavae; these are two large veins which enter the right atrium of the heart from below. The superior vena cava carries blood from the arms and head to the right atrium of the heart, while the inferior vena cava carries blood from the legs and abdomen to the heart; the inferior vena cava is retroperitoneal and runs to the right and parallel to the abdominal aorta along the spine. Large veins feed into these two veins, smaller veins into these. Together this forms the venous system. Whilst the main veins hold a constant position, the position of veins person to person can display quite a lot of variation.
The pulmonary veins carry oxygenated blood from the lungs to the heart. The superior and inferior venae cavae carry deoxygenated blood from the upper and lower systemic circulations, respectively; the portal venous system is a series of venules that directly connect two capillary beds. Examples of such systems include hypophyseal portal system; the peripheral veins carry blood from feet. Microscopically, veins have a thick outer layer made of connective tissue, called the tunica externa or tunica adventitia. During procedures requiring venous access such as venipuncture, one may notice a subtle "pop" as the needle penetrates this layer; the middle layer of bands of smooth muscle are called tunica media and are, in general, much thinner than those of arteries, as veins do not function in a contractile manner and are not subject to the high pressures of systole, as arteries are. The interior is lined with endothelial cells called tunica intima; the precise location of veins varies much more from person to person than that of arteries.
Veins serve to return blood from organs to the heart. Veins are called "capacitance vessels" because most of the blood volume is contained within veins. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries. After taking up cellular waste and carbon dioxide in capillaries, blood is channeled through vessels that converge with one another to form venules, which continue to converge and form the larger veins; the de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is pumped through the pulmonary arteries to the lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation; the return of blood to the heart is assisted by the action of the muscle pump, by the thoracic pump action of breathing during respiration.
Standing or sitting for a prolonged period of time can cause low venous return from venous pooling shock. Fainting can occur but baroreceptors within the aortic sinuses initiate a baroreflex such that angiotensin II and norepinephrine stimulate vasoconstriction and heart rate increases to return blood flow. Neurogenic and hypovolaemic shock can cause fainting. In these cases, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness. Jet pilots wear pressurized suits to help maintain their venous blood pressure; the arteries are perceived as carrying oxygenated blood to the tissues, while veins carry deoxygenated blood back to the heart. This is true of the systemic circulation, by far the larger of the two circuits of blood in the body, which transports oxygen from the heart to the tissues of the body. However, in pulmonary circulation, the arteries carry deoxygenated blood from the heart to the lungs, veins return blood from the lungs to the heart.
The difference between veins a