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Archive for July, 2010

Radiology

Posted by Dr KAMAL DEEP on July 25, 2010

 

anatomy_of_a_CT_scan

CT SCANNING:-

COMPUTED TOMOGRAPHY
TECHNIQUE
The CT image is a cross-sectional representation of anatomy created
by a computer-generated analysis of the attenuation of x-ray beams
passed through a section of the body. As the x-ray beam, collimated to
the desired slice width, rotates around the patient, it passes through
selected regions in the body. X-rays that are not attenuated by the
body are detected by sensitive x-ray detectors aligned 180° from the
x-ray tube. A computer calculates a “back projection” image from
the 360° x-ray attenuation profile. Greater x-ray attenuation, e.g., as
caused by bone, results in areas of high “density,” while soft tissue

[Collimation The use of lead shutters to control the effective thickness of the
X-ray beam and thus the slice thickness. Prepatient collimation is next to
the X-ray tube. Positioning of the shutters requires high precision and accuracy
as the beam will diverge, thus the shutter collimation has to be less
than the nominal slice thickness. Prepatient collimation markedly affects
patient radiation dose. Postpatient collimation is next to the detectors. This
will reduce scatter and also controls the nominal slice thickness. Thinsection
CT can be more easily acquired with postpatient collimation, but
accurate alignment between pre- and postpatient collimation is required to
optimise radiation]

structures, which have poor attenuation of x-rays, are lower in density.
The resolution of an image depends on the radiation dose, the detector
size or collimation (slice thickness), the field of view, and the matrix
size of the display. A modern CT scanner is capable of obtaining
sections as thin as 0.5–1 mm with submillimeter resolution at a speed
of 0.5–1 s per rotation; complete studies of the brain can be completed
in 2–10 s.
Helical or multidetector CT (MDCT) is now standard in most radiology
departments. Continuous CT information is obtained while the
patient moves through the x-ray beam. In the helical scan mode, the
table moves continuously through the rotating x-ray beam, generating
a “helix” of information that can be reformatted into various slice
thicknesses. Single or multiple (from 4 to 256) detectors positioned
180 degrees to the x-ray source may result in multiple slices per revolution
of the beam around the patient. Advantages of MDCT include
shorter scan times, reduced patient and organ motion, and the ability
to acquire images dynamically during the infusion of intravenous contrast
that can be used to construct CT angiograms of vascular structures
and CT perfusion images.

polls_spiralCT_3949_676812_answer_3_xlarge

HELICAL CT SCANNING

 

CT scanning produces cross-sectional images of a patient rather than the conventional shadow images of conventional radiography. Figure I-23 schematically illustrates CT scanner operation. Confusing and distracting overlying structures are eliminated. In x-ray CT scanning, a fan x-ray beam from a source rotating about the patient passes through the patient, and the exit transmission intensity is monitored by a series of detectors. Spiral CT scanners move the patient through the x-ray beam while the tube continuously rotates around the patient. The x-ray beam “cuts a slice” about 10-mm thick through the patient. The transmission at any angle can be used to calculate the average attenuation coefficient along the length of the x-ray beam. By measurement of the transmission at many angles around the patient, a complex group of mathematical equations can be solved to calculate and determine the mass attenuation coefficient of small (about 1 × 1 × 10 mm) volume elements (voxels). The final cross-sectional image is then made up of a display of the gray scale value of every voxel. For historical reasons and convenience, the attenuation coefficients are reported in terms of Hounsfield units. In Hounsfield units, bone and other dense materials are +1,000, water is equal to 0, and air is equal to –1,000. CT scanning, like digital radiography, can separate spatial and contrast resolution.

Over the last decade, computed tomography has developed rapidly,from conventional, single-slice machines through helical (spiral)
CT to the current multidetector (multislice) scanners. A number of advances in hardware and software have enabled this: in particular,
the use of slip rings to allow the scanner gantry to rotate continuously in one direction; the development of X-ray tubes with great heat capacity for long continuous X-ray exposure (up to 60 s);

Three key advances enable CT data to be acquired continuously with on-going patient movement. These are slip-ring technology, precise patient table transport, and software reconstruction algorithms.

Slip-ring technology was the fundamental step that allowed volume data acquisition and was necessary for subsequent developments described below. In older (conventional) CT systems, there was an inherent delay of 3–5 seconds between each exposure. This arose from the physical need to have cables connecting the stationary gantry and the rotating X-ray tube, detector systems, and controls (tube–detector assembly). After 1–3 exposures, depending on the cable length and other factors, the cables became wound and rotation of the tube–detector assembly had to stop, change direction, and then unwind while further exposures were taken. Not only did this result in interrupted data acquisition but occasionally led to mechanical problems. Slip-ring technology brought about a major change. It abolished the physical need for the presence of an electrical cable between the ‘on the ground’ generator and the moving tube–detector assembly. Instead, power from the generator is connected to a large stationary ring. Other large rings that house the X-ray tube, detector systems, and controls move around within the stationary ring. Power is transmitted between the stationary and moving rings by means of brushes. Hence the scene is set for continuous X-ray rotation (rather than back and forth) and continuous data acquisition.

2. Interaction of electromagnetic radiation with matter
The detection of X-rays and gamma-rays depends on the interaction of their electromagnetic fields with matter. There are three forms of photon interaction with matter that are most important in nuclear medicine: photoelectric absorption, Compton scattering, and pair production.
In a photoelectric interaction, an atom absorbs all the energy of the incident photon. The photon ceases to exist but the energy is transferred to an inner-shell electron which is ejected and is then referred to as a photoelectron. This creates a vacancy in an orbital shell which eventually leads to the emission of a characteristic X-ray ( Fig. 6.1A ).photoelectric-effect
In a Compton interaction, a photon interacts with an orbital electron and loses some of its energy which is then transferred to the electron. The photon as a result is ‘scattered’ or deflected off in a new direction with a lower energy ( Fig. 6.1B ).

In pair production, high-energy photons interact with the electric field of a nucleus ( Fig. 6.1C ); the photon transfers its energy to create a positron and an electron. Since each particle has the rest mass of an electron, 0.511 MeV, this requires a minimum photon energy of 2 × 0.511 = 1.022 MeV. Radionuclides with photons exceeding 1.02 MeV are not generally used in nuclear medicine.

PHOTOELECTRIC—DIAGNOSTIC RADIOLOGY

COMPTON-THERAPEUTIC RADIOLOGY

 

2.     ULTRASONOGRAPHY:-Ultrasound is generated by piezoelectric materials which have the property of changing thickness when a voltage is applied across them. Lead zirconate titanate (PZT) is the most widely used. The crystal is mounted in a conveniently shaped holder which contains the electrodes and any associated electronics, as well as the lenses and matching layers required to improve the beam shape (see later). The whole assembly is known as the probe or transducer, although strictly the latter term should be reserved for devices that change one form of energy into another, in this case electrical to acoustic energy.

High-frequency ultrasound gives good resolution because of its short wavelength, but the rapid attenuation of high-frequency ultrasound by tissue is the limiting factor to the maximum frequency that can be used in any given clinical application. Frequencies as high as 20 MHz can be used when only a few millimetres of tissue are to be traversed, such as for imaging the eye and skin and for intravascular ultrasound (IVUS). For superficial tissues, such as the thyroid, breast and scrotum, 7–15 MHz is appropriate. For the heart, abdomen and second and third trimester obstetrics, 3–7 MHz is optimal, while for some difficult applications, such as the abdomen in obese subjects, and for transcranial studies (most of which use Doppler), one has to resort to 1.5 or 2.5 MHz transducers. This frequency limitation is being reduced by the use of coded transmit pulses, which essentially impose a signature on the pulse (for example, by making its frequency slide upwards during the pulse, so called chirp encoding) so that the receiver circuitry can detect true echoes even when they are weaker than the noise floor.

INTERPRETATIVE PRINCIPLES
SHADOWING AND INCREASED THROUGH TRANSMISSION
Acoustic shadowing and increased through transmission of sound (often referred to as enhancement although this term is better reserved for the signal-augmenting effects of microbubble Contrast agents) are important components of the ultrasound image. Shadowing occurs when little or no ultrasound can penetrate an interface and results in a dark band over the deeper tissues, bounded by the ultrasound beam lines, which are parallel for a linear transducer and radiating for a sector transducer ( Fig. 3A.17 ).

Acoustic Shadowing

Absorption and reflection are the two main causes of shadowing. If a portion of the tissue being imaged absorbs ultrasound faster than the background, the chosen TGC(TIME-GAIN COMPENSATION) will be insufficient to compensate properly, and therefore tissues deeper to the highly attenuating region are undercorrected and appear darker than adjacent tissue. Fibrous tissue and, to a lesser extent, fat attenuate at a higher rate than the usual 1 dB MHz-1 cm-1 and are common causes of acoustic shadowing, for example in a fatty liver, behind scars and behind scirrhous breast carcinomas. High attenuation also partly accounts for the shadows seen deep to calcific lesions such as biliary and renal stones, but here intense reflection is also a factor. Whatever proportion of the incident ultrasound is reflected is not available to continue through for imaging. For stones this amounts to about 60% of the incident energy, but for tissue–gas interfaces almost all 100% of the incident ultrasound is reflected and these produce dense shadows. In this case, the shadows are often partially filled in by reverberant echoes, which form because of the efficiency of these gas–tissue boundaries as reflectors, so that reverberation artefacts commonly occur. The noise in gas shadows has given rise to their designation as ‘dirty shadows’ in stones, as compared to the ‘clean shadows’ behind stones, and this is sometimes a useful differential diagnostic feature.
Whether a stone or gas bubble actually casts a shadow depends on its size relative to the ultrasound beam width. A significant fraction (about three quarters) of the beam must be obstructed to cause a shadow. If a stone is smaller than this, or lies away from the central axis of the beam, enough ultrasound passes beside it to insonate the deeper tissues. In practice, shadowing is usually apparent behind renal and biliary stones of 5 mm or more in diameter, and much smaller calcifications may also shadow if high-resolution transducers are used. Groups of fine calcifications can also shadow if their aggregate size and density is high enough; for example, in nephrocalcinosis.

 

A third important type of shadowing, ‘edge shadows’, are sometimes known as ‘refractive shadows’, a description based on one explanation of their origin. They are seen as fine, dark lines extending deep to strongly curved surfaces and do not imply attenuation or strong reflections. Cysts and the fetal skull are typical examples and fascial sheaths are often also responsible, for example the fine shadows seen beyond Cooper’s ligaments in the breast and those caused by the neck of the gallbladder. These edge shadows must be recognized as being different from attenuating and reflective shadows to avoid errors in their interpretation.

Increased through transmission is the exact opposite of attenuation shadowing: here, a region of tissue has a lower than average attenuation and so the TGC (which is adjusted to compensate for the average attenuation) is inappropriately high for that region. Thus the echoes from deeper tissues are overamplified. The phenomenon is the hallmark of cystic spaces and the ‘bright up’ is often accentuated by the darker banding lines of the edge shadows typically formed from the cyst wall ( Fig. 3A.18 )

 

Increased Sound Transmission

Some solid tissues also show increased sound transmission, however, usually because they have a high proportion of fluid. Many tumours fall into this category, especially fibroadenomas in the breast, and lymphomatous deposits and inflammatory masses may behave in the same way. Even those fluid cavities that contain echogenic material, such as suspended crystals in the gallbladder, pus, blood or necrotic tissue, usually still produce increased transmission, depending largely on the proportion of fluid present.

 

ECHOGENICITY
The prime determinant of the strength of ultrasonic echoes is the impedance mismatch (Z) between adjacent tissue components. At the risk of being somewhat simplistic, in practical clinical terms this may be understood as interfaces between tissues of different densities. The larger the mismatch the stronger the echo, so that interfaces between soft tissues and bone, for example, give very strong echoes and, within soft tissues, the most significant components are fibrous tissue (often in the form of the perivascular microskeleton) and fatty tissue. Thus, while uniform regions of fibre or fat are echo-poor (subcutaneous fat and retroperitoneal fibrosis are examples), admixtures between them and watery tissues give stronger echoes.
A second important factor is the concentration of the scatterers: for a given impedance mismatch, a region that contains a large number of scatterers is more echogenic than one where they are spread out. Commonly, the ‘dilution’ of scatterers is caused by an increase in water content. The low reflectivity of the congested liver in right heart failure is an example. Malignant tumours are a general case: until they grow large enough to undergo necrosis or calcification (which produce new reflectors) they tend to be echo-poor ( Fig. 3A.19 ).

Echogenecity

 

Similarly, the oedematous tissues in acute inflammation give low-level echoes; examples include the echo-poor pancreas in acute pancreatitis and the segmental echo-poor regions of acute lobar nephronia in reflux nephropathy. On the other hand, the high concentration of reflectors is the cause of the echogenic kidneys in recessive (infantile) polycystic renal disease (the interfaces between the innumerable cysts cause strong echoes ( Fig. 3A.20 ));

PCKD ECHOGENECITY

strong echoes are also obtained from the multiple interfaces of the vessel walls of haemangiomas, and even stronger ones from angiomyolipomas, in which there is also admixed fatty tissue.

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ENT IMPORTANT POINTS & MCQS CONTINUED

Posted by Dr KAMAL DEEP on July 5, 2010

26. Subglottic Hemangioma:-Although rare in adults, subglottic hemangioma is the most common laryngeal and upper tracheal neoplasm in the newborn and infant. The lesion typically appears as a well-defined mass in the posterior or lateral portion of the subglottic airway ( Fig. 99–11 ). [127] [138] Although the subglottic narrowing is usually eccentric, circumferential narrowing suggestive of croup may be seen. Hemangiomas may occur on the skin or elsewhere in the body.

“Squamous papillomas, the most frequent laryngeal tumors in children, have also been reported in adults.”

Current management options that have been reported include tracheotomy, laser partial excision, open surgical resection, systemic or intralesional steroids, and systemic interferon alfa-2A

Infantile subglottic hemangiomas occur in children a few months of age and present as a lateral subglottic submucosal bluish mass, causing respiratory symptoms. These lesions, if mildly symptomatic, are managed conservatively with corticosteroids and observation. They usually involute spontaneously with time; however, a tracheotomy is occasionally needed when severe airway compromise is present. Healy and others[3] reported the use of the carbon dioxide laser for the management of this condition.[3] The carbon dioxide laser is used to vaporize the tumor until an adequate airway is achieved.
The Nd:YAG laser, although ideal for low-flow venous malformations, is not recommended for subglottic hemangiomas because these lesions are more compact capillary-type vascular lesions. The depth of penetration of the Nd:YAG laser presents a serious risk to the infant’s larynx and trachea, with potential stricture formation or tracheal perforation, and is not recommended for the management of these lesions.

27. Management Of early Glottic Cancer:- Recent advances in microlaryngeal laser excision (laser microscopic excision, LME) of early glottic lesions and in laryngeal reconstruction after vertical partial laryngectomy (VPL) have enhanced the quality of life for patients cured of their cancer.

Curative radiotherapy is reserved for early lesions which neither impair cord mobility nor invade cartilage or cervical nodes. Cancer of the vocal cord without impairment of its mobility gives a 90% cure rate after irradiation and has the advantage of preservation of voice. Superficial exophytic lesions, especially of the rip of epiglottis, and aryepiglottic folds give 70-90% cure rate. Radiotherapy does not give good results in lesions with fixed cords, subglottic extension, cartilage invasion, and nodal metastases. These lesions require surgery.

The anterior commissure has been considered as a barrier to tumor spread or as an early pathway for cancer extension into the laryngeal framework.[29] Shvero and others[55] related that surgical treatment is preferred for cancers arising in this region because of a higher local recurrence rate and an increased risk for distant metastasis. They contend that the behavior of small cancers in this location is much different from that of other early glottic cancers.[56] Other investigators have attributed the higher rates of failure after radiotherapy for anterior commissure lesions to problems with adequate dosing.[31] [71] The distance from the anterior commissure to the skin varies greatly among patients. This variability and the thick overlying thyroid cartilage have been cited as impediments to consistent dosing of radiation to this region. Some investigators relate that modern radiotherapy techniques with improved dosimetry have adequately addressed these concerns.Radiotherapy more frequently fails to control the cancer when there is involvement of the anterior commissure, impaired vocal cord mobility, or subglottic extension.

Indications for Vertical Partial Laryngectomy and Laryngoplasty ; VPL and laryngoplasty are anterior commissure involvement, extension to the vocal process of the arytenoid, selected superficial transglottic lesions, and recurrent cancer after radiation therapy. Partial laryngectomy for recurrent cancer after radiation therapy must meet the following criteria: (a) lesion limited to one cord (may involve the anterior commissure); (b) body of arytenoid free of tumor; (c) subglottic extension no more than 5 mm; (d) mobile cord; (e) no cartilage invasion; (f) recurrence correlating with initial tumor; and (g) early complications after partial laryngectomy, including subcutaneous emphysema, bleeding, and tracheotomy tube occlusion.

Early glottic carcinoma limited to the membranous portion of the vocal fold can be cured using endoscopic surgical excision, thyrotomy with cordectomy, hemilaryngectomy, VPL with laryngoplasty, and/or radiation therapy.

About one in six patients with severe dysplasia or carcinoma in situ will develop invasive carcinoma if the only therapy used is a single vocal cord stripping or biopsy.

Microinvasive carcinoma of the true vocal fold can be managed by sequential endoscopic excisional biopsy, endoscopic laser excision, or radiation therapy

The management issues in glottic carcinoma are local control, effective management of suspected or known metastatic cervical lymph nodes, patient education concerning carcinogenic substances (usually smoking cessation therapy), and patient follow-up for the possibility of residual laryngeal cancer or second primary lesions. Tumors arising on the arytenoid, in the subglottic region, or in the supraglottic portion of the larynx do not cause hoarseness and often are diagnosed at a later stage. They have a lower cure rate because delayed diagnosis is associated with diminished therapeutic effectiveness of surgery or radiation therapy.

In general, early glottic carcinoma can be managed without total laryngectomy and without the procedures that limit vocal quality or rely on radiation therapy with its inherent problems. A broad surgical armamentarium now includes endoscopic excision, laser resection, and surgical excision, with a number of reconstructive/rehabilitative techniques to enhance postoperative function.

The relative contraindications for LME as anterior commissure involvement, subglottic extension, T3 glottic cancer, and posterior commissure involvemen

Indications for VPL and laryngoplasty are tumor involvement of the anterior commissure, extension to involve the vocal process of the arytenoid, selected superficial transglottic lesions, and carcinoma recurring after radiation therapy. The contraindications for any of these procedures include a fixed vocal cord, involvement of the posterior commissure, invasion of both arytenoid cartilages, bulky transglottic lesions, and lesions invading the thyroid cartilage

when a diagnosis of severe dysplasia or carcinoma in situ is made and when the site of the lesion involves the true vocal cord, microscopic suspension laryngoscopy with stripping of the epithelium and a closely monitored program of follow-up are indicated. The patient must be convinced of the necessity to discontinue smoking and ethanol intake and to maintain a schedule of regular visits for indirect laryngoscopy following a pattern of careful assessment every 2 or 3 months for at least 5 year

Microinvasive carcinoma can be managed by endoscopic excisional biopsy (vocal cord stripping), by laser excision endoscopically, or by radiation therapy. We prefer a protocol consisting of microscopic suspension laryngoscopy and sequential vocal cord stripping every 3 months until two consecutive epithelial stripping specimens can be confirmed to be free of malignant cells. We then monitor these patients with indirect laryngoscopy every 2 to 3 months. If any suspicious epithelial changes or significant voice changes are noted, we repeat the suspension microlaryngoscopy and biopsy.

Early invasive glottic carcinoma can be treated by endoscopic excision, laser excision, thyrotomy with cordectomy, hemilaryngectomy, VPL with laryngoplasty, or radiation therapy. Traditionally, radiation therapy has been offered as the preferred treatment for invasive epidermoid carcinoma involving the membranous portion of the mobile true vocal cord. Recently, some studies have challenged that approach, and endoscopic excision with or without the laser has been found to be equally safe and effective. Late recurrence of carcinoma and the development of second primary tumors are issues of great importance and mandate a pattern of close follow-up, regardless of the treatment chosen.
Radiation therapy is the primary treatment for glottic carcinoma in Northern Europe, with total or partial laryngectomy used for salvage of those patients who have recurrence of cancer. In other parts of the world, surgeons report wider use of VPL for early glottic carcinoma and for advanced T2 glottic lesions.

Ackerman’s Tumor
Verrucous carcinoma is known also as Ackerman’s tumor and can be distinguished histologically from other well-differentiated squamous cell carcinomas. This tumor is characterized by its rough, shaggy surface, a rounded, pushing margin, and no metastasis. Smaller lesions can be excised endoscopically; larger tumors are managed by partial laryngectomy. This tumor is less radiosensitive than ordinary squamous cell carcinoma, but radiation therapy is a reasonable alternative for treating larger tumors; total laryngectomy is reserved for large lesions that do not respond to radiation therapy.

URTI

28. Benign tumors and cysts of the esophagus are relatively rare, occurring less frequently than malignant tumors. Esophageal lesions can be classified as intraluminal, intramural, or extramural. Intramural tumors are usually asymptomatic until they become significantly enlarged. Because they are mucosally covered, it is uncommon for these tumors to be associated with ulceration and bleeding. Leiomyoma is the most common benign tumor of the esophagus, representing an intramural tumor arising from the muscularis mucosa (Fig. 56.7). In 90% of patients, it occurs in the middle or lower third of the esophagus. Patients usually present with dysphagia, although many leiomyomas are found radiographically in asymptomatic patients.

“Polyps are the most common intraluminal lesions, although papillomas, adenomas, and hemangiomas may occur. Fibrovascular polyps can grow to an enormous size and have been reported to prolapse into the hypopharynx, causing asphyxiation and death. Most polyps occur in the cervical esophagus, causing dysphagia and regurgitation. Barium swallow demonstrates an intraluminal, pedunculated mass. Most can be excised endoscopically by snaring the base of the polyp.”

29Scleroderma is a generalized collagen vascular disease in which 80% of patients eventually develop esophageal symptoms. Typically, scleroderma results in a motility disorder that causes progressive dysphagia for solids. There appears to be an increased incidence in those who also manifest the Raynaud phenomenon.
Pathologically, the smooth muscle in the gastrointestinal tract becomes atrophied. Manometric studies demonstrate diminished contractions in the LES and distal two thirds of the esophagus. Because the UES is composed of striated muscle, contraction pressures are usually normal. Although dysphagia occurs, heartburn is the more prominent symptom because LES tone is attenuated. With compromise of the LES, reflux esophagitis and its associated complications may develop.

30. The esophagus is a flexible, muscular tube that can be compressed or narrowed by surrounding structures at 3 locations (Fig. 3.90):

  • the junction of the esophagus with the pharynx in the neck;
  • in the superior mediastinum where the esophagus is crossed by the arch of aorta; in the posterior mediastinum where the esophagus is compressed by the left main bronchus;
  • in the posterior mediastinum at the esophageal hiatus in the diaphragm.

These constrictions have important clinical consequences. For example, a swallowed object is most likely to lodge at a constricted area. An ingested corrosive substance would move more slowly through a narrowed region, causing more damage at this site than elsewhere along the esophagus. Also, constrictions present problems during the passage of instruments.

 

image

Alpha-adrenergic neurotransmitters increase LES pressure and a-adrenergic blockers decrease it; b-adrenergic stimulation decreases LES pressure and b-adrenergic blockers increase it. Cholinergic mechanisms also exert control over resting LES pressure. Hormonal regulation has been studied extensively, and dozens of hormones and peptides have been found to influence LES pressure. Protein meals and antacids tend to increase LES pressure, whereas fatty meals, chocolate, ethanol, smoking, and caffeine are known to decrease LES pressure.

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