INTENSIFYING SCREENS

INTENSIFYING SCREENS

Intensifying screens are used because they decrease the x-ray dose to the patient, yet still afford a properly exposed x-ray film. Also, the reduction in exposure allows use of short exposure times, which becomes important when it is necessary to minimize patient motion. The x-ray film used with intensifying screens has photosensitive emulsion on both sides. The film is sandwiched between two intensifying screens in a cassette, so that the emulsion on each side is exposed to the light from its contiguous screen. Remember, the screen functions to absorb the energy (and information) in the x-ray beam that has penetrated the patient, and to convert this energy into a light pattern that has (as nearly as possible) the same information as the original x-ray beam. The light, then, forms a latent image on x-ray film. The transfer of information from x-ray beam to screen light to film results in some loss

of information.

Construction

An intensifying screen has four layers:

1. a base, or support, made of plastic or cardboard

2. a reflecting layer (Ti02)

3. a phosphor layer

4. a plastic protective coat

The total thickness of a typical intensifying screen is about 15 or 16 mils (1 mil= 0.001in. = 0.0254 mm).Base. The screen support, or base, may

be made of high-grade cardboard or of a polyester plastic. The base of Du Pont intensifyingscreens is a polyester plastic (Mylar*)that is 1 0 mils thick ( 10 mils = 2 54f.Lm). Kodak X-Omatic and Lanex screens have a similar (Estart) base 7 mils thick.

Reflecting Coat.

The light produced by the interaction of x-ray photons and phosphor crystals is emitted in all directions. Much of the light is emitted from the screen in the direction of the film. Many light photons, however, are also directed toward the back of the screen (i.e., toward the base layer) and would be lost as far as photographic activity is concerned. The reflecting layer acts to reflect light back toward the front of the screen. The reflecting coat is made of a white substance, such as titanium dioxide (Ti02), and is spread over the base in a thin layer, about 1 mil thick. Some screens do not have a reflecting layer (e.g., Kodak X-Omatic fine and regular).

Phosphor Layer.

The phosphor layer, containing phosphor crystals, is applied over the reflecting coat or base. The crystals are suspended in a plastic (polymer)containing a substance to keep the plastic flexible. We will discuss the phosphor in more detail later. The thickness of the Protective Layer. The protective layer applied over the phosphor is made of a plastic, largely composed of a cellulose compound that is mixed with other polymers. It forms a layer about 0.7 to 0.8 mils thick. This layer serves three functions: it helps to prevent static electricity; it gives physical protection to the delicate phosphor layer; and it provides a surface that can be cleaned without damaging the phosphor layer.

Phosphor

The original phosphor used in x-ray intensifying screens was crystalline calcium tungstate (CaW04). New (since about1973) screen phosphor technology is being developed to increase screen speed over that available with calcium tungstate, and has resulted in the introduction of a bewildering variety of screens and corresponding films. Let us first consider calcium tungstate and then review some of the new phosphors. Natural calcium tungstate (scheelite) is no longer used because synthetic calcium tungstate of better quality is produced by

fusing sodium tungstate and calcium chloride under carefully controlled conditions. The first commercial calcium tungstate screens were made in England and Germany in 1896; they were first made in the United States in 1912. The calcium tungstate crystal must be absolutely free of any contaminant if it is to fluoresce properly.

An intensifying Action of Screens.

Intensifyingscreen is used because it can converta few absorbed x-ray photons into many light photons. The efficiency with which the phosphor converts x rays to light is termed the intrinsic conversion efficiency of the phosphor; this is more accurately defined as the ratio of the light energy liberated by the crystal to the x-ray energy absorbed. The intrinsic conversion efficiency of calcium tungstate is about 5%. The ability of light emitted by the phosphor to escape from the screen and expose the film is termed the "screen efficiency."The intensification factor of a screen is the ratio of the x-ray exposure needed to produce the same density on a film with and without the screen (intensification factor is commonly determined at a film density of 1.0).

Speed of Calcium Tungstate Intensifying Screens. Several factors determine how "fast" or "slow" a calcium tungstate screen will be. These include thickness of the phosphor layer, size of the phosphor crystals, presence or absence of light-absorbing dye in the phosphor layer, and phosphor conversion efficiency. Of course, the faster screen will allow a lower x-ray exposure to the patient, but a price for this speed must be paid. The speed of a calciumtungstate screen and its ability to record detail are in reciprocal relationship; that is, high speed means less detail. This statement is also true for new screen phosphors, but the subject is more complex because higher speed does not always require a thicker screen. These screens are classified as fast, medium (par speed), and slow (detail), with intensification factors in the range of 100, 50, and 25, respectively. A thicker phosphor layer will result in a faster screen because the thick layer will absorb more x-ray photons than a thin layer. Thick screens will be faster but will cause a decrease in the clarity of the image recorded on the film. This decrease in image clarity is primarily caused by diffusionof light in the phosphor layer

Screen-Film Contact

The cassette in which the intensifying screens are mounted provides a light-tight container for the film. It also serves to hold the film in tight contact with the screens over its entire surface. With good film screen contact a dot of light produced in the screen will be recorded as a comparable dot on the film. If contact is poor, this dot of light will diffuse before it reaches the film, so that its radiographic image is unsharp. There is a simple method for testing film-screen contact. A piece of wire screen is placed on the cassette, and a radiograph of the wire screen is made. The sharpness of the image of the wire in all regions of the film is compared, and any areas of poor film-screen contact become obvious because the image of the wire appears fuzzy.

Cleaning

Intensifying screens must be kept clean. Any foreign material on the screen, such as paper or blood, will block light photons and produce an area of underexposure on the film corresponding to the size and shape of the soiled area. Cleaning will eliminate the "high spots" on a screen; these high points are the major source of excessive wear. The cause of screen failure is mechanical attrition. Under normal conditions of use, x-ray photons will not damage screens. Screens are best cleaned with

a solution containing an antistatic compound and a detergent; the solution should be applied gently (never rub vigorously) with a soft lint-free cloth. The cassette should never be closed after cleaning until it is absolutely dry.

RARE EARTH MATERIAL

Chemists divide the periodic table of the elements into four basic groups: alkaline earths, rare earths, transition elements, and nonmetals. The term "rare earth" developed because these elements are difficult and expensive to separate from the earth and each other, not because the elements are scarce. The rare earth group consists of elements of atomic numbers 57 (lanthanum) through 71 (lutetium), and includes thulium (atomic number 69), terbium (atomic number 65), gadolinium (atomic number 64), and europium (atomic number 63). Because lanthanum is the first element, the rare earth group is also known as the lanthanide series . Lanthanum (La) and gadolinium (Gd) are used in the rare earth phosphors. A related phosphor, yttrium

(Y), with atomic number 39, is not a rare earth but has some properties similar to those of the rare earths.

RADIATION EXPOSURE AND UNITS

RADIATION EXPOSURE AND UNITS

The (traditional/conventional) radiation units of measurement, the

roentgen, rad, and rem, are of importance to the radiographer—the SI (Standard International) units of measure are gaining in usage and should also be used by the radiographer

The roentgen is a unit of measurement of ionization in air, and referred to as the unit of exposure. Because x-rays ionize air, all the ions of either sign (positive or negative) formed in a particular quantity of air are counted and equated to a quantity of radiation expressed in the unit roentgen. The roentgen is valid only for x and gamma radiations at energies up to3 MeV (million electron volts). The roentgen SI unit of measurement is C/kg (coulomb per kilogram).

THE RAD

The rad is an acronym for radiation absorbed dose. As radiation passes through matter, a certain amount of energy is deposited in that matter. Absorbed dose refers to the amount of energy deposited per unit mass and is strongly related to chemical change and biologic damage. The amount of energy deposited and, thus, the amount of possible biologic damage is dependent on the following:

• Type of radiation

• Atomic number of the tissue

• Energy of the radiation

The rad SI unit of measurement is the Gy (gray).

THE REM

The rem is an acronym for radiation equivalent man. The rem uses the information collected for the rad, but also uses a quality factor (QF) to predict biologic effects from different types of radiation. Thus, the equation: rad × QF = rem. Radiations having a high QF have a higher LET and greater potential to produce biologic damage. The rem is described as the unit of dose equivalency (DE) and used to express occupational exposure.

The rem SI unit of measurement is the Sv (sievert).

Personal Radiation Monitors:

• OSL

• TLD

• Film Badge

• Pocket Dosimeter

SUMMARY

The roentgen (R unit), or unit of exposure, is the unit used to

describe quantity of ionization in air.

The rad describes absorbed dose.

The rem is the unit of dose equivalency (DE), used to quantify biological effectiveness.

The OSL is the newest, most accurate personal dosimeter. It

uses a thin layer of Al2O3 to store information.

Film badges are convenient, low-cost radiation monitors that

are processed monthly.TLDs use LiF crystals to store exposure information. They are more precise and more expensive than film badges and may be processed quarterly.

Film badges and TLDs measure exposure to beta, x, and

gamma radiation.

Pocket dosimeters are thimble ionization chambers used to

monitor larger quantities of radiation exposure, up to 200

mR.

Radiographers must strive to keep their occupational dose

ALARA.

Personnel Protection

Personnel Protection

Radiographers must avoid unnecessary radiation exposure to themselves and strive to keep patient dose to an absolute minimum. The sources of radiation exposure to the radiographer are the primary beam and secondary radiation (scatter and leakage). Radiographers must never be exposed to the primary, or useful, x-ray beam.

Radiographers must follow the ALARA (as low as

reasonably achievable) principle as they carry out their tasks. New radiologic staff participates in radiation safety orientation, and regular inservice education on radiation safety is conducted. Proper radiation monitoring and review of monthly radiation reports is essential.

 SCATTERED RADIATION

When primary x-ray photons intercept an object and undergo a change in direction, scattered radiation results.

INVERSE SQUARE LAW

Reducing the length of time exposed to ionizing radiation, results in a reduction of occupational exposure. Increasing the distance from the source of radiation, as illustrated by the inverse  square law. Placing a barrier, like a lead wall or lead apron, between you and the source of radiation results in a reduction of occupational exposure. We must minimize the time of exposure to the source of radiation, provide effective shielding from the radiation source, and, most importantly, maximize the distance from the source

of radiation.

GUIDELINES

Primary radiation barriers protect against direct exposure from the primary (useful) x-ray beam and have much greater attenuation capability than secondary barriers, which protect only from leakage and scattered radiation. Examples of primary barriers are the lead walls and doors of a radiographic room, Primary protective barriers of typical installations generally consist of walls with 1/16 inch (1.5 mm) lead thickness and 7 feet high. Secondary radiation is defined as leakage and/or scattered radiation.

The x-ray tube housing protects from leakage radiation as stated previously. The patient is the source of most scattered radiation.

PROTECTIVE APPAREL AND ITS CARE

During fluoroscopic procedures requiring the radiographer’s presence in the radiographic room, the radiographer must wear protective apparel, to include a lead apron, lead aprons must provide the equivalent of at least 0.50 mm Pb, and lead gloves at least 0.25 mm Pb equivalent. Other useful protective apparel includes thyroid shields, Lead aprons, lead gloves, and other apparel are secondary barriers; they will not provide protection from the useful beam. Proper care of protective apparel is essential to ensure effectiveness. Lead aprons and gloves should be hung on appropriate racks,not dropped on the floor or folded. Careless handling can result in formation of cracks. Lead aprons and gloves should be imaged annually(either fluoroscopically or radiographically) to check for cracks.

 PROTECTIVE ACCESSORIES

Another device available for individuals required to remain in the fluoroscopy room is a mobile leaded barrier. Mobile barriers provide full body protection from scattered radiation and are available in a variety of lead equivalents.

SPECIAL CONSIDERATIONS

PREGNANCY

Deserving special consideration in protection from occupational exposure is the pregnant radiographer. As soon as the radiographer knows she is pregnant, it is advisable that she declare her pregnancy in writing. At that time, her occupational radiation history will be reviewed. The gestational dose limit to the fetus during the gestation period must not exceed 500 mrem (5 mSv)

MOBILE UNITS

Each mobile x-ray unit should have a lead apron assigned to it. The radiographer should wear the apron while making the exposure at the furthest distance possible from the x-ray tube. The mobile unit’s exposure cord must permit the radiographer to stand at least 6 feet from the x-ray tube and patient. In mobile fluoroscopic units, there must be a source to patient skin distance of at least 12 inches.

FLUOROSCOPIC UNITS AND PROCEDURES

All fluoroscopic equipment must provide at least 12 inches (30 cm), and preferably 15 inches (38 cm), between the x-ray source (focal spot) and the x-ray tabletop (patient). The tabletop intensity of the fluoroscopic beam must not exceed10 R/min or 2.1 R/min/ma. With under table  fluoroscopic tubes, a Bucky-slot closer/cover having at least the equivalent of 0.25 mm Pb must be available to attenuate scattered radiation. Fluoroscopic mA (milliamperes) must not exceed 5, image-intensified fluoroscopy usually operates between 1 and 3 mA. Because the image intensifier functions as a primary barrier, it must have a lead equivalent of at least 2.0 mm. A cumulative timingdevice must be available to signal the fluoroscopist (audibly, visibly, or both) when a maximum of 5 minutes of fluoroscopy time has elapsed.Beam collimation must be apparent through visualization of unexposed borders on the TV monitor, and total filtration must be at least 2.5 mm Al equivalent. Because occupational exposure to scattered radiation is of considerable importance in fluoroscopy, a protective curtain/drape of at least 0.25 mm Pb equivalent must be placed between the patient and fluoroscopist. As in radiography, high kV andlow mAs(milliampere-seconds) values are preferred in an effort to reduce dose.

SPECIAL SEQUENCES OF MRI

SPECIAL SEQUENCES OF MRI

FLAIR : Fluid attenuated inversion Recovery sequence

(Flair = T2 minus water)

It’s a type of T2 weight MRI where the water is suppressed to make pathology more visible.  It is predominantly used at places where water is the confounding factor and limiting the contrast  eg: Brain

Uses :

. For better appreciation of pathologies in brain being white

.To differentiate arachnoids’ cyst from epidermoid cyst

STIR (Short Tau Inversion Recovery Sequence)

STIR  can be easily remembered as T2- Fat means pathology will remain white whereas Fat will become black.

     It is predominantly used at places where fat is the confounding factor and limiting the contrast, e.g. rest of the body and musculoskeletal imaging

Uses

. Identification of bone marrow edema in diseases like stress fracture ,  avascular  necrosis,  perthes  disease .

.For better appreciation of disease in body where presence of Fat is reducing the contrast due  to background whiteness.

Contrast Enhanced MRI (CEMRI)

Gadolinium containing compounds are most commonly used contrast agents in MRI.

     Presence of gadolinium causes reduction in both T1 and T2 relaxation time which leads to increases in signal on T1- weighted MRI and decreases in signal on T2 weighted MRI. As human eyes appreciate white color better,post contrast MRI images are always acquired as T1-weighted MRI.

MRCP

MRCP images are heavily T2- weighted sequences which demonstrate fluid-filled structures as areas of every high signal intensity and are very commonly used to show the biliary and pancreatic ducts in magnetic resonance cholangiopancreatography(mrcp),being non-invasive and no need of external contrast,it is considered as a preferred investigation to diagnose pathologies of biliary duct and pancreatic duct in comparison to diagnostic endoscopic retrograde cholangiopancreatogrphy (ERCP)

MR Angiography

Visualization of vessels using MRI

   This technique is unique due to the fact that the beautiful angiographic images can be obtained without external administration of contrast. Technique used in MRI for angiography is called TOF(TIME OF FLIGHT).it is a very commonly used angiographic technique now days and is the preferred imaging technique to assess renal artery stenosis and an intracerebral aneurysm.it is also preferred investigation to look for carotid artery stenosis

Diffusion Weighted MRI(DWI)

It is a special type of  MRI technique which assesses the random motion of the water molecule, which is a normal phenomenon. In some disease, there is reduction in the Brownian motion of  the water molecule, which is called as restricted diffusion. Area of restricted diffusion are seen as white on DWI

Common causes of Restricted Diffusion

. Acute ischemic infarct

.Abscess

.High cellular tumors

.Epidermoid cyst

Gradient Echo Imges

Used to see calcification and hemorrhage on MRI which show blooming on gradient images

Spectroscopy

It is a new technique of MRI to assess    certain metabolites in the body tissues. Metabolic changes occur earlier than the structural changes and since routine MRI picks up structure changes, new techniques have been developed for the early diagnosis of the diseases.Commonly studied metabolites are

. N acetyl aspirate

.choline

.creatine

. lipid peak

MRI is Usually a Preferred Investigation for

. For all brain tumors (contrast enhanced MRI)

.For extent of pancoast tumor

.ventricular fuction

.Posterior mediastinal mass

. Bony metastasis in spine

.Chronic subarachnoid hemorrhage 

.Traumatic paraplegia

.Extent of pott’s spine

.Stress fracture,  Perthe’s  disease. avascular necrosis, early osteomyelitis

.Pregnancy after first trimester

.Spinal cord pathologies

.Gynecological malignancies

.Uterine abnormalities

MRI

MRI

Magnetic resonance imaging (MRI) is a non- invasive method of mapping structure and various aspects of function within the body by producing images by the virtue of gyromagnetic property of protons, with the greatest advantage of not using ionizing radiation for imaging

Principle of MRI        :  NMR given by Purcell and Bloch (1952)

Invention of MRI       : Lauterbeur and Mansfield (2003)

Functional MRI          : Ogawa and Rosen

Magnetic resonance is a phenomenon whereby the nuclei of certain atoms, when placed in a magnetic field ,absorb and emit energy at a specific or resonant frequency.

          Nuclei suitable for MRI contain odd no of protons and neutrons .Almost all clinical MR images are produced using the simplest of all nuclei that of hydrogen, Which is present in virtually all biological material and exhibits relatively high MR sensitivity.

PARTS OF MRI

. Maxwell Coil –Gradient coil used to create magnetic field gradients  along the direction of the main magnetic field

. Radio frequency coils : to generate stronger magnetic field along the scan field

.Faraday cage/shield/Hoffman Box  - blocks out external static electric fields. Made up of copper

.Superconductors – Electromagnets measured in Tesla

HOW IMAGE IS ACQUIRED IN MRI

.In the absence of magnetic field all protons have random movement

. Under the influence of magnetic field, alignment of proton takes place

. Realignment occurs under the influence of radiofrequency pulse

. After switching off the radiofrequency pulse, the atoms return to their original position and release energy in the form of electrical voltage signal

.When proton releases energy to surrounding lattice, it is called as spin lattice relaxation or T1 relaxation

. When proton releases energy  to  the surrounding spin ,it is called as  spin-spin relaxation or T2 relaxation

. This electrical signal (produced by receiver coils) is digitized and analyzed in a computer, to produce MR images.

          Using T1 and T2 relaxation time , two types of images are broadly acquired using this principal; T1 weighted MR and T2 weighted MR.

CONTRAINDIVATIONS OF MRI

              It is a common saying that MRI means  “  metal results in injury”  however it is not true in current scenario an there are many metallic devices compatible with MRI and students should be aware of those , particularly devices made up of TITANIUM which are very much compatible with MRI.

       Few important devices or conditions where MRI is usually contraindicated are  :

. Cardiac pacemakers

. Cochlear implants

. Intraoccular  Metallic foreign body

. Electronically, magnetically and mechanically activated implants

. Ferromagnetic or electronically operated active devices like defibrillators

. Aneurysmal clips

. Prosthetic heart valves- metallic

. Insulin pumps and nerve stimulators

. stapedial implants

. Claustrophobia

. First trimester of pregnancy

MRI can be safely performed in

. Orthopedic implants (usually made of titanium). In cases of implants made of stainless steel, MRI can still be done however, heating is a problem so it is usually recommended for a shorter time and on a lower strength magnet. In permanent tattoos also because of the heavy metal dyes, heating may be an issue.

.Pregnancy after 1^{s t}  trimester. Safety profile of MRI is not established in first trimester so, should not be done in 1^st trimester

.cholecystectomy clips, sternal sutures

.IUCD

.Non metallic foreign bodies

.Breast implants

.Pediatric patients   ( to avoid for ionizing radiation)

.Coronary stents usually drug eluting stents are used and MRI can be done after 3 months.