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Partial volume averaging (CT artifact)

Partial volume artifact occurs when tissues of widely different absorption are encompassed on the same CT voxel producing a beam attenuation proportional to the average value of these tissues. The latest generation of CT scanners with an associated reduction in the volume of a voxel has substa...

Digital radiography

Digital radiography is based on the use of discrete values in comparison to conventional radiography which uses analog/continuous values. It removes the requirement of dark room procedures. Digital radiography does not use cassettes. It uses direct or indirect flat panel detectors or charge cou...

Intensifying screen

Intensifying screens are used in the x-ray cassette to intensify the effect of the x-ray photon by producing a larger number of light photons. It decreases the mAs required to produce a particular density and hence decreases the patient dose significantly. It also reduces motion blur and x-ray t...

Grids

Grids are placed between the patient and the x-ray film to reduce the scattered radiation reaching the detector (produced mainly by the Compton effect) and thus improve image contrast. They are made of parallel strips of high attenuating material such as lead with an interspace filled with low ...

Beam collimators

Beam collimators are 'beam direction' devices used in the x-ray tube housing, along with an arrangement of mirrors and lights, in such a way that the light and x-ray fields match each other. They are made of lead shutters which completely absorb the photons, and thus reduce the patient dose as w...

Compton effect

Compton effect or Compton scatter is one of principle forms of photon interaction. It is the main cause of scattered radiation in a material. It occurs due to the interaction of the photon (x-ray or gamma) with free electrons (unattached to atoms) or loosely bound valence shell (outer shell) ele...

CT colonography (protocol)

Computed tomographic (CT) colonography, also called CTC, virtual colonoscopy (VC) or CT pneumocolon, is a powerful minimally invasive technique for colorectal cancer screening. Indications screening test for colorectal carcinoma colon evaluation after incomplete or unsuccessful optical (conv...

Space charge

Space charge refers to the collection of electrons which are emitted from the metal surface, after the application of tube current, at a short distance away from the metal surface. These electrons collect and form a cloud of negative charge around the metal surface. This space charge repels and...

Anode heel effect

Anode heel effect refers to the lower field intensity towards the anode in comparison to the cathode due to lower x-ray emissions from the target material at angles perpendicular to the electron beam. Basic concept The conversion of the electron beam into x-rays doesn’t simply occur at the sur...

Focal spot

Focal spot is the area of the anode surface which receives the beam of electrons from the cathode. It is the apparent source of x-rays. Basic concept Size and shape of the focal spot is determined by the size and shape of the electron beam when it strikes the anode 1. Size and shape of the e...

Line focus principle

The line focus principle in radiography explains the relationship between the actual focal spot on the anode surface and the effective focal spot size. Basic concept The focal spot is the area of the target upon which the electron beam impinges. The energy of the electrons in the electron beam...

Filament circuit

The tungsten cathode needs to be heated for thermionic emission to take place. Thus a 10 voltage potential difference and 3-6 amperes of filament current is supplied, which forms the filament circuit. This should not be confused with tube current which determines the flow of electrons from the...

Filters

Filters are metal sheets placed in the x-ray beam between the window and the patient that are used to attenuate the low-energy (soft) x-ray photons from the spectrum. Filtering is the removal of these low energy x-rays from the beam spectrum which would otherwise not contribute to image quality ...

CT scanner (evolution)

CT scanners were first introduced in 1971 with a single detector for brain study under the leadership of Sir Godfrey Hounsfield, an electrical engineer at EMI (Electric and Musical Industries Ltd). Thereafter, it has undergone multiple improvements with an increase in the number of detectors and...

Positron emission tomography

Positron emission tomography (PET) is a modern non-invasive imaging technique for quantification of radioactivity in vivo. It involves the intravenous injection of a positron-emitting radiopharmaceutical, waiting to allow for systemic distribution, and then scanning for detection and quantificat...

Thermionic emission

Thermionic emission is the emission of electrons from a heated metal (cathode). This principle was first used in the Coolidge tube and then later in the modern day x-ray tubes. Before the discovery of the principle, gas tubes were used for x-ray production. The cathode has its filament circuit ...

Photoelectric effect

The photoelectric effect, a.k.a. photoelectric absorption, is one of the principal forms of interaction of x-ray and gamma photons with matter. A photon interacts with an inner shell electron in the atom and removes it from its shell. Probability of photoelectric effect The probability of this...

Kilovoltage peak

Kilovoltage peak (kVp) is the peak potential applied to the x-ray tube, which accelerates electrons from the cathode to the anode in radiography or computed tomography. Tube voltage, in turn, determines the quantity and quality of the photons generated. An increase in kVp extends and intensifies...

Hyperintense on T1-weighted images (mnemonic)

Mnemonics for hyperintense T1-weighted lesions include: My Best Friend is Pretty Cool 3 Fs and 4 Ms Mnemonic My Best Friend is Pretty Cool: M: melanin B: blood (i.e. methemoglobin in subacute hemorrhage) F: fat and slow flow P: protein; paramagnetic substances (e.g. manganese, copper); p...

T2 washout

T2 washout is a phenomenon encountered on diffusion weighted imaging (DWI) which results in DWI images (e.g. b = 1000) appearing normal despite abnormal ADC maps. For the phenomenon to occur a particular combination of ADC and T2 signal intensity is required: increased T2 signal facilitated ...

Citrate peak

Citrate is a compound examined in MR spectroscopy in the setting of possible prostate carcinoma. Citrate resonates at 2.6 ppm and is decreased in prostate cancer. For more information go to: MR spectroscopy in prostate cancer

Lactate peak

Lactate is one of the more important compounds assessed on MR spectroscopy, and resonates at 1.3 ppm chemical shift, with a characteristic double peak at long TEs. Lactate is, however, superimposed on the lipid band. Using an intermediate TE (e.g. 144 ms) will invert only lactate, allowing it t...

Lipids peak

Lipids are a collection of related compounds examined in MR spectroscopy. They resonates at 1.3 ppm chemical shift, and are markers of severe tissue damage with liberation of membrane lipids, as is seen in cerebral infarction or cerebral abscesses. It is also encountered in cerebral metastases a...

N-acetylaspartate (NAA) peak

N-acetylaspartate (NAA) is one of the more important compounds assessed on MR spectroscopy, and resonates at 2.0 ppm chemical shift (its concentration in healthy adults is 8-10 mM) 1. The synthesis of NAA, adenosine diphosphate-dependent, occurs in the neuronal mitochondria 2. NAA is the acetyl...

Glutamine-Glutamate peak

Glutamate-Glutamine (Glx) peak is one of the regions assessed on MR spectroscopy, and resonates between 2.2 and 2.4 ppm chemical shift. It overlaps with the GABA peak and cannot be routinely separated from each other. The concentration of these two brain metabolites increases in hepatic and hypo...

Gamma-aminobutyric acid (GABA) peak

Gamma-aminobutyric acid (GABA), produced by the decarboxylation of glutamate 4, is the principle inhibitory neurotransmitter in the central nervous system 1 and as such, is one of the compounds examined in MR spectroscopy. It is present in the human brain at a concentration of about 1 mM, a who...

Creatine peak

Creatine is one of the compounds examined in MR spectroscopy. It resonates at 3.0 ppm chemical shift (with a second usually smaller peak at 3.95 ppm 2). It is found in metabolically active tissues (brain, muscle, heart), where it is important in the storage and transfer of energy. It tends to b...

CT stair-step artifact

The CT stair-step artifact is found in straight structures which are oriented obliquely with respect to movement of the table and appear around the edges of sagittal and coronal reformatted images when wide collimations and non-overlapping reconstruction intervals are used. It is also seen in c...

Contrast-enhanced ultrasound

Contrast-enhanced ultrasound (CEUS) involves the administration of intravenous contrast agents consisting of microbubbles/nanobubbles of gas. Usecases liver hepatic metastasis cystadenoma/cystadenocarcinoma cholangiocarcinoma hepatocellular carcinoma hepatic adenoma focal nodular hyperpl...

Diffusion kurtosis imaging

Diffusion kurtosis imaging (DKI) is an advanced neuroimaging modality which is an extension of diffusion tensor imaging by estimating the kurtosis (skewed distribution) of water diffusion based on a probability distribution function. It provides a high order diffusion of water distribution and a...

X-ray artifacts

X-ray artifacts can present in a variety of ways including abnormal shadows noted on a radiograph or degraded image quality, and have been produced by artificial means from hardware failure, operator error and software (post-processing) artifacts. There are common and distinct artifacts for fi...

Step-and-shoot tomosynthesis (breast)

Step-and-shoot is a technology of image acquisition in digital breast tomosynthesis characterized by stop scanning at every single angle during images acquisition. Step-and-shoot technology allows advantages in microcalcifications conspicuity, spatial resolution, signal-to-noise Ratio improveme...

Flying focus tomosynthesis

Flying focus is a technology of image acquisition in digital breast tomosynthesis characterized by a continuous sweep during shooting. Sharpness in digital systems is determined by the modulation transfer function (MTF), which determine contrast transfer as a function of spatial frequency. Sin...

b values

b value measures the degree of diffusion weighting applied, thereby indicating the amplitude (G), time of applied gradients (δ) and duration between the paired gradients (Δ) and is calculated as: b = γ² G² δ² (Δ−δ/3) Therefore, a larger b value is achieved by increasing the gradient amplitude ...

High-resolution CT

High-resolution CT (HRCT) is a scanning protocol in which thin sections (usually 0.625 to 1.25 mm) are acquired and reconstructed using a sharp algorithm (e.g. bone algorithm). It has been classically used for: interstitial lung disease imaging: HRCT chest temporal bone imaging

Mirror image artifact

Mirror image artifact in sonography is seen when there is a highly reflective surface (e.g. diaphragm) in the path of the primary beam. The primary beam reflects from such a surface (e.g. diaphragm) but instead of directly being received by the transducer, it encounters another structure (e.g. ...

Flat panel detector

Flat panel detectors (FPD) are used in digital radiography (DR) for the conversion of x-rays to light (indirect conversion) or charge (direct conversion) which is read out using a thin film transistor (TFT) array. Types indirect conversion FPDs outermost layer is scintillator: phosphor screen...

Herringbone artifact

Herringbone artifact, also known as spike artifact, crisscross artifact, or corduroy artifact, is an MRI artifact related to one or few aberrant data point(s) in k-space. In image space, the regularly spaced stripes resemble the appearance of a fabric with a herringbone pattern. The artifact cov...

Relaxometry

Relaxometry is measurement of relaxation times from MR images. T1, T2 and T2* can be estimated using the appropriate pulse sequence and parameters. T2 relaxometry has found useful in quantitating signal changes on T2-weighted images as in evaluating mesial temporal sclerosis. Details T2 relaxo...

Negative enhancement integral

The negative enhancement integral in MR perfusion is used to calculate the relative cerebral blood volume (rCBV). It represents the area described by the baseline and the signal loss due to passage of contrast bolus in tissue.

CT artifacts

CT artifacts are common and can occur for various reasons. Knowledge of these artifacts is important because they can mimic pathology (e.g. partial volume artifact) or can degrade image quality to non-diagnostic levels. CT artifacts can be classified according to the underlying cause of the art...

Gallium-67 scintigraphy

Gallium-67 (Ga-67) is a photon-emitting radiotracer used for scintigraphy which is used in the form of various salts like citrate and nitrate. Once administered, imaging may consist of planar (2 dimensional), SPECT, and SPECT-CT acquisitions. Once injected it binds to plasma proteins (especially...

Characteristic radiation

Characteristic radiation is a type of energy emission relevant for X-ray production. This energy emission happens when a fast-moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of the incident electron is greater than the binding energy ...

X-ray production

X-rays are produced due to sudden deceleration of fast-moving electrons when they collide and interact with the target anode. In this process of deceleration, more than 99% of the electron energy is converted into heat and less than 1% of energy is converted into x-rays. Definitions Generator ...

Image plate artifact

Image plate artifact is caused by backscatter radiation. Backscatter radiation is transmitted through the back of the cassette to the cassette hinge where the lead coating gets weakened or cracked. To reduce backscatter, the radiographer should collimate where possible.

Technetium 99m-methyl diphosphonate

Technetium 99m-methyl diphosphonate (99mTc MDP) is a radiotracer used in nuclear medicine especially for bone scintigraphy. Any disease process which results in extracellular fluid expansion will lead to accumulation of this tracer. Radionuclide profile photon energy: 140 keV physical half-li...

Wilhelm Roentgen

Wilhelm C Roentgen (1845-1923) was a German physicist who is celebrated globally for his discovery of x-rays on 8 November 1895. Early life Wilhelm Conrad Roentgen (Röntgen in German) was born on 27 March 1845 in Lennep, Germany. He attended the primary and secondary school run by Martinus Her...

Elastography

Elastography is a newer technique that exploits the fact that a pathological process alters the elastic properties of the involved tissue. This change in elasticity is detected and imaged using elastography. Radiographic technique Sono-elastography Sono-elastography is the term used when ultr...

Doppler shift

Doppler shift or Doppler effect is defined as the change in frequency of sound wave due to a reflector moving towards or away from an object, which in the case of ultrasound is the transducer. Terminology When sound of a given frequency is discharged and subsequently reflected from a source th...

Harmonic imaging

Harmonic imaging is a technique in ultrasonography that provides images of better quality as compared with conventional ultrasound technique. Physics Harmonic imaging exploits non-linear propagation of ultrasound through the body tissues. The high pressure portion of the wave travels faster th...

Acoustic shadowing

Acoustic shadowing (sometimes referred to as posterior acoustic shadowing) is a form of ultrasound artifact. It is characterized by the apparent lack of signal deep to an imaged tissue interface, due to all (or nearly all) of the transmitted sound wave being being reflected back to the transduce...

Radioisotope spills

Radioisotope spills are classified as major or minor. Examples of major spills include quantities equal to or larger than: 1mCi I-131 100 mCi Tc-99m or Tl-201 10 mCi Ga-67, In-111, or I-123

Thallium-201 scintigraphy

Thallium-201 (Tl-201) is a radiopharmaceutical used for scintigraphy, primarily of the myocardium. The element thallium is treated by the body as an analog of potassium; it is produced in a cyclotron by bombarding thallium-203 with protons. Characteristics thallium is a monovalent cation usua...

Godfrey Hounsfield

Sir Godfrey N Hounsfield (1919-2004) pioneered the CT scanner making him one of the greats in the history of radiology and medicine in general. For his work, he received the Nobel Prize in Physiology or Medicine in 1979. This was remarkable because he had had no previous experience of working in...

Picture archiving and communication system

Picture archiving and communication system (PACS) is a modality of imaging technology which helps in image transmission from the site of image acquisition to multiple physically-disparate locations. This technology not only is economical (film-less department), but also convenient to access mult...

MRI contrast agent safety

Though considered safer than the frequently used iodinated contrast agents used in x-ray and CT studies, there are safety issues with MRI contrast agents as well. Paramagnetic metal ions suitable as MRI contrast agents are all potentially toxic when injected IV at or near doses needed for clinic...

Spiral pulse sequences

Spiral scanning on MRI is unlike spiral scanning on CT where the x-ray tube is continuously rotating and data is continuously being acquired. On MRI the word "spiral" refers to the pattern of sampling k-space. On conventional imaging sequences including spin echo and gradient echo and on fast im...

Inversion recovery sequences

Inversion recovery pulse sequences are a type of MRI sequence used to selectively null the signal for certain tissues (e.g. fat or fluid). Inversion recovery can also generate heavily T1-weighted images and was originally developed for this purpose. Physics Basically, an inversion recovery (I...

Diastolic pseudogating

Diastolic pseudogating appears as periodic bright and dark signal in arteries such as the aorta as one progresses through a series of images. Synchronisation of the cardiac cycle and the pulse sequence results in high signal in the artery during diastole when blood is relatively stationary and l...

Zebra stripes

Zebra stripes, a.k.a. zebra artifacts, appear as alternating bright and dark bands in a MRI image. The term has been used to describe several different kind of artifacts causing some confusion. Artifacts that have been described as a zebra artifact include the following: moire fringes 1,2 spi...

Metal artifact reduction sequence

A metal artifact reduction sequence (MARS) is intended to reduce the size and intensity of susceptibility artifacts resulting from magnetic field distortion. A variety of techniques are used for reducing metal artifacts at MRI, both for addressing artifacts due to the presence of metal in the i...

Diffusion tensor imaging and fiber tractography

Diffusion tensor imaging (DTI) is an MRI technique that uses anisotropic diffusion to estimate the axonal (white matter) organization of the brain. Fiber tractography (FT) is a 3D reconstruction technique to assess neural tracts using data collected by diffusion tensor imaging. Diffusion-weigh...

Central point artifact

The central point artifact is a focal dot of increased signal in the center of an image. It is caused by a constant offset of the DC voltage in the receiver. After Fourier transformation, this constant offset gives the bright dot in the center of the image as shown in the diagram. The axial MRI...

Magnets (types)

Magnets used for MRI are of three types: permanent, resistive and superconductive. Permanent MRI magnets Permanent MRI magnets use permanently magnetized iron like a large bar magnet that has been twisted into a C-shape where the two poles are close together and parallel. In the space between ...

MRI physics

The physics of MRI are complicated and much harder to understand than those underpinning image generation in plain radiography, CT or ultrasound. What follows is a very abbreviated, 'broad strokes' description of the process. Essentially, the process can be broken down into four parts: prepar...

Relaxation

Relaxation is the process in which spins release the energy received from a radiofrequency pulse. MRI signal is influenced, among other factors, by different types of relaxation: T1 relaxation (spin-lattice or longitudinal relaxation) T2 relaxation (spin-spin or transverse relaxation) T2* rel...

Acquisition time

The time of acquisition for a conventional spin echo or gradient echo sequence is the product of the repetition time, phase encoding steps, and number of averages (TR x phase steps x NEX). For example, with a one second TR, 128 phase steps, and two averages we would get an acquisition time of ab...

MRI electronics and data processing

As an introduction to the electronics and data processing of the MRI scanner, a schematic diagram has been provided (figure 1). Starting from the right hand side, we have the computer that directs all of the action in the MRI acquisition and acquires and processes the data. The computer tells t...

Superparamagnetism

Superparamagnetic materials consist of individual domains of elements that have ferromagnetic properties in bulk. Their magnetic susceptibility is between that of ferromagnetic and paramagnetic materials. The figure illustrates the effect of a superparamagnetic material (grey circle) on the ma...

Paramagnetism

Paramagnetic substances include oxygen and ions of various metals like iron, magnesium and gadolinium. These ions have unpaired electrons, resulting in a positive magnetic susceptibility. The magnitude of this susceptibility is less than 0.1% of that of ferromagnetic materials. The effect on MR...

Half-value layer

Half-value layer (HVL) is the width of a material required to reduce the air kerma of an x-ray or gamma ray to half its original value. This applies to narrow beam geometry only. With broad-beam geometry, a greater amount of scatter will reach the detector, falsely overestimating the degree of a...

Spin echo sequences

Spin-echo pulse sequences are one of the earliest developed and still widely used (in the form of fast spin echo) of all MRI pulse sequences. The pulse sequence timing can be adjusted to give T1-weighted, proton density, and T2-weighted images. Dual echo and multiecho sequences can be used to ob...

Saturation recovery sequences

Saturation recovery (SR) sequences are rarely used for imaging now. Their primary use at this time is as a technique to measure T1 times more quickly than an inversion recovery pulse sequence. Saturation recovery sequences consist of multiple 90 degree RF pulses at relatively short repetition ti...

Zero fill artifact

Zero fill artifact is one of many MRI artifacts and is due to data in the K-space array missing or set to zero during scanning. The abrupt change from signal to no signal results in artifacts in the images showing alternating bands of shading and darkness, often in an oblique direction. A spike...

Phase contrast imaging

Phase contrast imaging is an MRI technique that can be used to visualize moving fluid. Basic principle Spins that are moving in the same direction as a magnetic field gradient develop a phase shift that is proportional to the velocity of the spins. This is the basis of phase-contrast angiograp...

Resonance and radiofrequency

Protons in a magnetic field have a microscopic magnetization and act like tiny toy tops that wobble as they spin. The rate of the wobbling or precession is the resonance or Larmor frequency. In the magnetic field of an MRI scanner at room temperature, there is approximately the same number of pr...

RF overflow artifact

RF overflow artifact causes a nonuniform, washed-out appearance to an image. This artifact occurs when the signal received by the scanner from the patient is too intense to be accurately digitized by the analog-to-digital converter. Autoprescanning usually adjusts the receiver gain to prevent th...

Entry slice phenomenon

Entry slice phenomenon occurs when unsaturated spins in blood first enter into a slice or slices. It is characterized by the bright signal in a blood vessel (artery or vein) at the first slice that the vessel enters. Usually, the signal is seen on more than one slice, fading with distance. This ...

Moiré fringes

Moiré fringes are an interference pattern most commonly seen when acquiring gradient echo images using the body coil. Because of lack of perfect homogeneity of the main magnetic field from one side of the body to the other, aliasing of one side of the body to the other results in superimpositio...

Diamagnetism

Diamagnetism is the property of materials that have no intrinsic atomic magnetic moment, but when placed in a magnetic field weakly repel the field, resulting in a small negative magnetic susceptibility. Materials like water, copper, nitrogen, barium sulfate, and most tissues are diamagnetic. I...

MRI pulse sequences

An MRI pulse sequence is a programmed set of changing magnetic gradients. Each sequence will have a number of parameters, and multiple sequences grouped together into an MRI protocol. Parameters A pulse sequence is generally defined by multiple parameters, including: time to echo (TE) time ...

Intravenous MRI contrast agents

Intravenous MRI contrast agents include chelates of paramagnetic ions, both ionic and non-ionic. Gadolinium-based contrast agents (GBCAs) are the most common type. The particulates, sequestered in the liver, spleen, and lymph nodes, the intravascular agents, confined to the blood pool, and tumor...

MRI safety

MRI scanners, although free from potentially cancer-inducing ionizing radiation found in plain radiography and CT, have a host of safety issues which must be taken very seriously. MRI safety can be divided into: main magnetic field varying magnetic (gradient) fields radiofrequency M...

Twinkling artifact

Twinkling artifact is seen with color flow Doppler ultrasound 1. It occurs as a focus of alternating colors on Doppler signal behind a reflective object (such as a calculus), which gives the appearance of turbulent blood flow 2. It appears with or without an associated color comet tail artifact ...

Tumor specific MRI contrast agents

Tumor specific MRI contrast agents are pharmaceuticals that are targeted to tumors, either specifically or nonspecifically. Monoclonal antibodies are targeted to specific tumors such as adenocarcinoma of the colon. Metalloporphyrins exhibit affinity for many tumor types including carcinoma, sarc...

Reticuloendothelial MRI contrast agents

Reticuloendothelial MRI contrast agents can best be discussed in terms of those used for liver and spleen imaging and those for lymph node imaging. Liver and spleen The use of Gd-DTPA with routine imaging sequences of the liver is unsatisfactory. Particulate contrast agents targeted to the ret...

Chromium-labeled red blood cells

Chromium-labeled red blood cells is an intravascular MRI contrast agent. The use of 51Cr-labeled RBCs in nuclear medicine suggested the use of paramagnetic Cr(III)-labeled RBCs as an intravascular contrast agent for MRI. In dogs, significant enhancement of the liver and spleen is noted with mini...

Intravascular (blood pool) MRI contrast agents

Intravascular MRI contrast agents normally remain confined to the intravascular space, compared to Gd-DTPA which distributes throughout the extracellular fluid space. This is a result of intravascular agents having a molecular weight of approximately 70,000 and above, compared to a molecular wei...

Gastrointestinal MRI contrast agents

Gastrointestinal MRI contrast agents may be helpful in certain clinical scenarios in distinguishing bowel from intra-abdominal masses and normal organs. The contrast agents can be divided into positive agents (appearing bright on MRI) or negative agents (appearing dark on MRI). Positive contras...

MRI contrast agents

MRI contrast agents have become an indispensable part of contemporary magnetic resonance imaging. Although MRI was initially hoped to provide a means of making definitive diagnoses without administering contrast media, it has been found that the addition of contrast agents in many cases improves...

Radiofrequency coils

Radiofrequency coils (RF coils) are the "antennae" of the MRI system, broadcasting the RF signal to the patient and/or receiving the return signal. RF coils can be receive-only, in which case the body coil is used as a transmitter; or transmit and receive (transceiver). Surface coils are the si...

Gradient coils

Gradient coils are used to produce deliberate variations in the main magnetic field (B0). There are three sets of gradient coils, one for each direction. The variation in the magnetic field permits localization of image slices as well as phase encoding and frequency encoding. The set of gradient...

Black boundary artifact

Black boundary artifact, also known as India ink artifact or type 2 chemical shift artifact, is an artificially-created black line located at fat-water interfaces such as those between muscle and fat. This results in a sharp delineation of the muscle-fat boundary lending the image an appearance ...

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