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By V. Marik. Greensboro College.

Bellucci (19) and Marquet (28) believed that atresia repair should precede microtia repair order levlen 0.15 mg online. It was their belief that the open- ing in the mastoid could only be made precisely when the orig- inal position of the auricular remnant and the Figure 17 levlen 0.15mg online. If the preoperative hearing level is about and access to the middle ear would be limited. The dashed arrow at the left represents a unilateral framework could be sufficiently manoeuvred to align the mea- case with a functionally unsuccessful operation, whereas the dashed arrow at tus and the new canal. Auricular reconstruction should be the right demonstrates a favourable bilateral case with an acceptable performed first in order to preserve the integrity of the postoperative hearing gain. The approach can be compared with requires surgery for eradication of the disease to prevent further an intact canal wall-like procedure. Should there be a draining fistula or trapped cholesteatoma, Surgical results surgical intervention is warranted immediately. According to Cole and Jahrsdoerfer (50), a bony ear-canal Moreover, every surgeon uses his own audiological criteria opening of 2 mm or less puts the patient at a risk of to define their operation as a surgical success (Table 17. In their study, 91% of the ears with Although the kind of approach is readily defined, the surgi- a stenosis of 2 mm or less had developed a cholesteatoma at cal details differ considerably between different surgeons. Surgery is recommended for patients with make meaningful comparisons of outcome, a consensus should stenosis of the external ear canal measuring 2 mm or less. A successful appropriate time is late childhood or early adolescence, before operation can reasonably be defined as one that obviates the irreversible damage has occurred. Even for such a criterion, differences of interpretation can be found: average hearing threshold level Surgical techniques better than 30 to 35 dB (32) or than 20 dB (9). There are three surgical approaches to the creation of a new The reported surgical successes are summarised in Table 17. The variability of hearing begins along the linea temporalis immediately posterior to the improvement depends on the severity of the malformation. The mastoid cells are not opened Whether the aura atresia is unilateral or bilateral does not and the posterior wall of the external auditory canal is pre- influence the results (67). These are followed medially through the atresia plate (Altmann classification) and 30 to 35 dB in type I. The mean into the epitympanum, allowing the identification of the ossi- hearing gain with atresia repair surgery seems to be somewhat cles. This approach avoids injury to the variably located verti- better in less severely malformed ears. The more favourable cal portion of the facial nerve, as long as the dissection is cases usually have better preoperative hearing. As demon- carried out in an anterosuperior manner, with entrance into the strated on a Glasgow benefit plot (Fig. The nation of these factors leads to the conclusion that atresia atretic plate is delicately removed with diamond burrs and repair surgery should be done only in very selected patients curettes to avoid acoustic trauma to the inner ear from drill after a thorough investigation of all parameters involved vibration. The abnormally located facial nerve is most com- (age, anatomy, uni-/bilateral, hearing status, etc. Transmastoid approach: This method employs a posterior Although some authors argue that the surgical results approach to the middle ear and atretic plate. Dissection begins are only temporary and decline after some years, long-term along the linea temporalis over the region of the mastoid cav- results of Cremers and Teunissen (67), Marquet et al. The dura mater of the middle fossa, the sigmoid sinus, and Jahrsdoerfer and Hall (69) clearly demonstrate the stability of the sinodural angle are used as landmarks. The cavity must be cen- tred either on the lateral semicircular canal or on the stapes: Closure of air-bone gap Surgical success of the procedure The hypotympanum is usually never revealed (17). Then the bone immediately anterior to the mas- Glasgow benefit plot Degree of benefit for the patient toid was drilled away to obtain a new external auditory canal, 246 Current management Table 17. Anatomical complication rates of 20% to 60% are to be successfully treated (60 dB air bone gap 45 dB bone reported (9,40,70). Functional complications are related to labyrinthine injury The Compact is suitable for patients with a pure-tone and facial-nerve injury. The concept of Age criteria direct bone conduction was introduced by Tjellström et al. At Nijmegen, only children older than 10 years osseointegrated titanium implant in the mastoid bone to an were initially implanted, although recently the age limit also impedance-matched transducer that the patients can apply and was decreased to five years. Another clinical application of these implants in the tem- Psychosocial criteria poral bone is the fixation of an auricular prosthesis. Four differ- The patient needs to have realistic expectations and a reason- ent systems are now in the market: Divino, Classic 300, able social support. Selection criteria Anatomical and biological criteria Otological criteria Diseases that might jeopardise osseointegration are a formal Patients with congenital malformations of the middle/external contraindication for implant surgery. In particular, it is a Surgical technique valuable alternative in those patients for whom reconstructive Initially, it was described as a two-stage procedure with an inter- surgery for ear canal atresia cannot be performed. Chronically val of three to four months, allowing for osseointegration to start draining ears, which do not allow the use of an air-conduction before a load was applied. The two-stage procedure is still in use for children under the age of 10 years and older individuals with poor cognitive Audiological criteria skills (39,81).

Then a transmission scan is obtained with the patient in the scanner before the emission scan is acquired levlen 0.15 mg amex. The ratio of counts of each pixel between the blank scan and the transmission scan is the attenuation correction factor for the pixel discount 0.15mg levlen with mastercard, which is applied to the emission pixel data obtained next. Because the patient is positioned separately in the two scans, error may result in the attenuation correction. However, one should keep in mind that there is spillover of scattered high- energy photons (i. The transmission data are used to calculate the attenuation factors, which are then applied to the emission data. Factors from the map are then applied to the corresponding pixels in the patient’s emission scan for attenuation correc- tion. This factor is assumed to be the same for all tissues except bone, which has a slightly higher mass attenuation coefficient. Single Photon Emission Computed Tomography 175 factors because contrast-enhanced pixels overestimate attenuation. Some investigators advocate not using contrast agents and others suggest the use of water-based contrast agents to mitigate this effect. Partial-Volume Effect Partial-volume effects are inherent flaws of all imaging devices, because no imaging device has perfect spatial resolution. When a “hot” spot relative to a “cold” background is smaller than twice the spatial resolution of the imaging device, the activity around the object is smeared over a larger area than it occupies in the reconstructed image. Although the total counts are preserved, the object appears to be larger and to have a lower activity concentration than it actually has. Similarly, a small cold spot relative to a hot background would appear smaller as if with higher activity concentra- tion. Such underestimation and overestimation of activities around smaller objects result from what is called the partial-volume effect. The partial-volume effect is a serious problem for smaller structures in images, and correction needs to be applied for the overestimation or under- estimation of the activities in them. A correction factor, called the recovery coefficient, is the ratio of the reconstructed count density to the true count density of the region of interest that is smaller than twice the spatial reso- lution of the system. The recovery coefficient can be determined by mea- suring the count densities of different objects containing the same activity but with sizes larger as well as smaller than the spatial resolution of the system. Recovery coefficients are usually measured using phantoms which may not truly be representative of the human body. The measured recov- ery coefficients are then applied to the image data of the patient to correct for partial volume effect. Ideally, for accurate reconstruction, the number of angular projections should be at least equal to the size of the acquisition matrix (e. How many angular projections should be taken over 180° or 360° to reconstruct the images accurately depends on the spatial resolution of the camera. As a general rule, 120 to 128 projections (using a 128 × 128 matrix) are needed for large organs such as lungs and liver, whereas 60 to 64 projec- tions (using a 64 × 64 matrix) are sufficient for smaller organs such as head and heart. Scattering Radiations are scattered in patients, and the scattered photons, depending on the energy and angle of scattering, may strike the detector. Nor- mally, most of these scattered photons fall outside the photopeak window and are rejected. However, a fraction whose photon energy falls within the photopeak window will be counted, but their (X, Y) positions remain uncer- tain causing degradation of the image resolution. There are a few methods of scatter correction, of which the most common method is the use of two windows: a scatter window and a photopeak window. The scatter window is set at a lower energy than the photopeak window, and it is assumed that scatter in the photopeak window is the same as that in the scatter window. The scatter counts in the scatter window are subtracted from the photopeak counts for each projection to obtain the scatter-corrected projections, which are then used for reconstruction. The scatter spectrum is variable in energy; therefore, to have more accu- rate scatter corrections, multiple scatter windows can be used. Scatter cor- rections are made prior to attenuation correction, because the former are amplified during the latter operation. Typically, it consists of intrinsic resolution, collimator reso- lution, and scatter resolution. Spatial resolution deterio- rates but sensitivity increases with increasing slice thickness. As a trade-off between spatial resolution and sensitivity, an optimum slice thickness should be chosen. Sensitivity The sensitivity of an imaging system is always desired to be higher for better image contrast. For con- ventional two-dimensional planar images of good contrast, about 500,000 counts are required.