Temporalbone Imaging

20Temporal Bone: Imaging

Donald W. Chakeres and Mark A. Augustyn

COMPUTED TOMOGRAPHY: TECHNICALCONSIDERATIONS

MR IMAGING TECHNIQUESFOUR BASIC MR IMAGING PROTOCOLSROUTINE BRAIN SURVEY

HIGH-RESOLUTION T2-WEIGHTED IMAGINGHIGH-RESOLUTION T1-WEIGHTED CONTRASTIMAGES

MAGNETIC RESONANCE ANGIOGRAPHYCONCLUSION

Temporal bone imaging is extremely challenging, as thenormal anatomy includes many small but clinically impor-tant structures, and a significant abnormality in this area maybe less than 1 mm in size. The wide range of tissues existingin the area must be evaluated simultaneously, making itimpossible to develop a single optimal imaging techniquefor studying all potential pathology. One must use bothcomputed tomography (CT) and magnetic resonance (MR)imaging techniques of the highest possible resolution toprecisely characterize the bone, air spaces, and the widevariety of soft tissues present in the temporal bone region.Often it is necessary to use both CT and MR imaging forsatisfactory tissue characterization and identification ofpathology or confident exclusion of abnormalities. Thischapter reviews CT and MR imaging techniques appropriatefor evaluation of the temporal bone. A more complete atlasusing multiplanar CT is provided in Chapter 19.

CT excels in the evaluation of disorders that primarilyaffect air spaces or cortical bone.1–8 Although the widedifferences in the density of the temporal bone structuresproduce excellent inherent image contrast on CT, soft-tissuecharacterization is much more limited than with MRimaging. Thus, with CT, the individual cranial nerves cannotbe seen without the use of intrathecal cisternography, atechnique no longer commonly used and currently replacedby high-resolution MR imaging. In contrast, MR imagingprovides poor information about the air spaces and corticalbone but excellent soft-tissue contrast resolution. In addi-tion, MR is more sensitive to the effects of gadolinium as acontrast agent than is CT to iodinated contrast agents. Infact, CT contrast enhancement may be difficult to visualizewithin the temporal bone itself due to the high density of thebone. For example, enhancement of the vestibule ininflammatory pathology is quite conspicuous on MR (sincethe surrounding bone is a signal void), while the sameenhancement is impossible to recognize using CT. Although

one may not be able to visualize the architecture of thenormal bone and air spaces, MR imaging can still provideuseful information about these structures in diseased states,as there is signal due to fluid or a mass where normally thereshould be a signal void from either cortical bone or air.

These techniques can be complementary. For example,in the case of paragangliomas, CT can best demonstratepathologic bone destruction, while MR more clearlydisplays vascular invasion or intracranial extension. Fast CTimaging can be used to create postprocessed angiographicstudies, but the dense bone often presents difficulties in thereconstructions. MR imaging can also be used to generateexcellent angiographic information; however, routine cathe-ter angiography remains important for vascular imaging andis still the gold standard for analyzing the most challengingvascular pathology. Catheter angiography is also used todirect interventional procedures. Catheter angiography willnot be discussed in this chapter.

COMPUTED TOMOGRAPHY: TECHNICALCONSIDERATIONS

If the goal of a temporal bone CT study is to focus on theotic capsule, cortical plates, ossicles, and the air spacesalone, such as when studying a temporal bone fracture, thenhigh-resolution bone algorithm techniques may be ade-quate.1–8 However, if it is also important to evaluate the softtissues, as in the case of a patient with cancer of the externalauditory canal (EAC), then it may be necessary to use intra-venous contrast and techniques similar to those used for abrain or soft-tissue neck study. The main disadvantages ofCT are poor soft-tissue definition within the bony labyrinthand internal auditory canal (IAC) and radiation exposurefor the patient. Although in the past it was essential to posi-tion the patient for axial and/or coronal images (Figs. 20-1 to

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20-3), newer high-resolution multidetector spiral imagingsystems can generate nearly isotropic voxels for multiplanarreconstructions, making the need for multiple series with di-rect imaging in several planes unnecessary. Postprocessingsoftware has also improved, allowing multiplanar cross sec-tions (Figs. 20-4 to 20-6), as well as transparent and surfacevolume image presentations (Figs. 20-7 to 20-12). A nearlyperfect ‘‘solid model’’ of the temporal bone can be createdfrom routine clinical CT data acquisitions.

Typically, a high-resolution matrix should be used (512 ×512), with thin sections (0.6 to 1.5 mm) and a field of viewof 15 to 20 cm. CT images can be rapidly acquired, eithersequentially or using a spiral technique. The exact tech-nique, including collimation and reconstruction algorithmsof spiral imaging, depends on the specifications of aparticular piece of equipment. The faster the data acquisi-tion, the less likely it is that the examination will bedegraded by motion artifact. A low-mA (70 mA) techniqueis adequate for most of the bony structures, but higher mA(250 to 400 mA) techniques, thicker slices (3 to 5 mm), and

contrast enhancement are necessary for the evaluation of thebrain and other soft tissues.

CT images are usually acquired or displayed in axial andcoronal planes. For axial imaging, sections are made in aplane rotated 30° superior to the anthropologic base line (theline intersecting the inferior orbital rim and the EAC). Scansproduced in this plane display the temporal bone structuresto good advantage.3 This plane allows separation of theindividual components of the temporal bone so that they arebetter visualized in their entirety, with less overlap andfewer partial volume imaging artifacts.2 Direct coronalimages are usually obtained at an angle of approximately120° from the anthropologic baseline, while reconstructioncoronal images are usually oriented 90° from the anthropo-logic baseline. Sagittal images can be very helpful inselected situations, and postprocessing can create multipleoblique or curved projections. For example, a plane rotatedapproximately 45° between the coronal and sagittal planesapproximates Stenver’s view and produces an image sectionplane parallel to the long axis of the temporal bone (seeChapter 19). Curved sections parallel to structures of interest(such as segments of the facial nerve canal) can beindividually created, as can three-dimensional (3D) surface

Edge ofoval

windowAnterior crus

of stapes

Manubriumof malleus

Long processof incus

Head ofstapes

Posterior crusof stapes

FIGURE 20-2 Axial CT, stapes level. The incus and malleus are seenlateral and anterior to the stapes. The annular ligament and footplate of thestapes cannot be seen due to volume averaging with the adjacent rim of theoval window.

Head ofmalleus

Incudomallealjoint

Medial part of posteriorincudal ligament

Lateral part of posteriorincudal ligament

Short processof incus

Body ofincus

FIGURE 20-4 Axial CT, incudomalleal joint. The articulation of themalleus with the incus is seen in the epitympanum.

FIGURE 20-1 This axial CT section through the geniculate gangliondemonstrates the facial hiatus for the greater superficial petrosal nerve. Thethin, bony margin between the lateral margin of the horizontal facial nerveand the middle ear is seen.

Manubriumof malleus

Tensortympanitendon

Lateralmalleal

ligamentHead ofmalleus

Neck ofmalleus

Pars flaccida oftympanic membrane

FIGURE 20-3 Coronal CT, malleus level. Prussak’s space liesbetween the lateral malleal ligament and the pars flaccida of the tympanicmembrane.

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reconstructions, allowing physical models to be createdfrom the imaging data (Figs. 20-7 to 20-12).

MR IMAGING TECHNIQUES

The MR images in this chapter were obtained with a 1.5Tesla General Electric Signa LX CDI Horizon imagingsystem. MR imaging has become the primary imagingmodality for evaluation of the nonosseous components ofthe temporal bone region, including the major blood vessels,fluid spaces (cerebrospinal fluid, endolymph, perilymph),nerves, muscle, cartilage, brain, salivary glands, and fat(Figs. 20-13 to 20-22).9–36 The spatial resolution currentlyavailable with MR has progressed to a point where it iscomparable to and can even exceed that of CT.12, 21

As spatial resolution increases, the images become nois-

ier as a result of an inherently decreased signal-to-noise ratiowithin any given voxel. This produces poorer-quality imageswhen larger matrices are used, despite better spatial resolu-tion. There are a number of strategies to deal with the poten-tial low signal-to-noise ratio and poor image quality. Three-dimensional Fourier transform imaging (3DFT) uses radiofrequency (RF) signal from an entire imaging volume duringthe entire acquisition rather than a single slice, thus increas-ing the signal-to-noise ratio. Therefore, very thin sectionscan be obtained for high-resolution 3DFT T1- and T2-weighted images.9 Both gradient and spin-echo 3DFT tech-niques are possible.10, 20, 22, 24, 28 If short TE times are avail-able and the sequences are optimized, the quality of theexamination using a gradient echo technique can rival or sur-pass spin-echo alternatives. For T1 weighting, spoiled gradi-ent echo imaging with short TR (50 ms) and TE (4 ms), andwith flip angles of 30°, generate images similar to routinespin-echo images but can also demonstrate the vessels to ad-vantage.36 Caution must be exercised, as high signal of ves-sels may be mistaken for enhancement in a tumor. For thisreason, some radiologists prefer standard high-resolution,thin-slice postcontrast 2D spin-echo sequences for evalua-tion of the IAC. Steady-state T2-weighted gradient echoimages using constructive interference techniques alsohave excellent quality and are not marred by increased mag-netic susceptibility artifacts because of their short TEtimes.11, 12, 20, 22, 29 One can also utilize a T2-weighted3DFT gradient echo technique called SIMCAST (segment-interleaved motion-compensated acquisition in a steadystate).29 Two-dimensional Fourier transform and 3DFT T2-weighted spin-echo imaging techniques are also possible.28

The resulting images using the gradient and spin-echo 3DFTtechniques are comparable and have similar acquisitiontimes.

The signal-to-noise ratio can be improved by usingdedicated phased-array surface coils specifically designedfor temporal bone imaging.30, 31 Such phased array coils aremore effective than those obtained by simply combining aseries of routine coils to a single input. Each phased-arraycoil is composed of two or more separate but overlapping

FIGURE 20-6 Curved coronal-sagittal CT recon-struction of the facial nerve. A, The facial nerve is seenfrom the IAC to the point where it exits the temporalbone at the stylomastoid foramen. B, The dark linerepresents the course of the curved surface in A.

FIGURE 20-5 Oblique CT, incus and malleus. This view approxi-mates the otoscopic view from the EAC.

Chapter 20 Temporal Bone: Imaging 1095


FIGURE 20-7 Surface MR imaging, reconstruction ofthe pinna. This is a surface volume reconstruction of anaxial MR imaging 3DFT data set of 60 images. The head isviewed from laterally. The image suggests that the patientis bald, but this is due to the lack of signal from hair. Manyof the details of the surface anatomy are accuratelydisplayed. All of the deep anatomy is also available foranalysis.

FIGURE 20-8 Three-dimensional CT surfacereconstruction, lateral view. This surface reconstruc-tion mimics a skull model and demonstrates thesurface of the lateral mastoid; the tympanic bone,which makes up most of the EAC; the zygomaticarch; and the squamous portion of the tympanic bone(not labeled).

FIGURE 20-9 Three-dimensional CT surface reconstruction, superiorview. This is a view into the posterior and middle cranial fossas from above.The temporal bone is well seen. The petrous apex is outlined by the foramenlacerum and the clivus (not labeled). The impressions for the IAC, jugularfossa, hypoglossal canal, and sigmoid sinuses are all well demonstrated.

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coil loops. Phased-array coils also allow shorter imagingtimes. The disadvantage of using these coils is that theimages are not homogeneous in signal intensity, and thecoils are more cumbersome for the technologist to position.Using surface coils to study the superficial structures canincrease the signal-to-noise ratio approximately three to fivetimes compared with that of a routine head coil.

Gradient echo imaging is more sensitive to a number offactors, including T2* signal loss from magnetic susceptibil-ity artifacts, and magnetic susceptibility between water andair can result in a distortion of MR images somewhat similarto chemical shift artifact. These local magnetic field

distortions are exaggerated with high-resolution imaging,narrow bandwidth, and high magnetic field strength.Magnetic susceptibility artifacts may be seen as regions ofhigh and low signal in locations near the oval and roundwindows not corresponding to anatomic structures (see Fig.20-13D).20 Gradient echo techniques have been developedthat use short TE times to specifically suppress theseartifacts since they are more pronounced with increasing TE.Though spin-echo images are not immune to these artifacts,they are less sensitive. Short TE times increase thesignal-to-noise ratio by diminishing the effects of T2*decay, and short TE times limit the effects of magneticsusceptibility artifacts that occur at the air-water interfacesof the oval and round windows. 22 A short TE time allows fatand water to be in phase, thus diminishing the artifactsassociated with intravoxel fat-water subtraction seen withgradient-echo high-resolution imaging.36 The ideal TE timevaries with field strength.

Contrast-enhanced studies are of value in manysituations.32–36 An enhanced study can be acquired as 2DFTT1-weighted axial or coronal series of the whole brain. A3DFT T1-weighted axial MR series can also be acquired.

FOUR BASIC MR IMAGING PROTOCOLS

Four basic MR imaging protocol techniques are usedto address specific goals: whole brain/head imaging,high-resolution fluid space imaging, high-resolution T1-weighted, contrast-enhanced imaging, and MR angiography(MRA) techniques (Table 20-1). The actual sequences usedvary from institution to institution and with the specific MRscanner utilized.

Some radiologists feel that the demonstration of highsignal in blood vessels on T1-weighted images is helpful.Others feel that signal of the small vessels in the IAC can beconfusing when looking for subtle enhancement of a smalleighth nerve tumor, and they therefore prefer spin echoT1-weighted images with contrast enhancement for evalua-tion of sensorineural hearing loss. The lack of any highsignal within the canal on this sequence is a reliable

FIGURE 20-10 Three-dimensional CT surface reconstruction, inferiorview. The oval-shaped jugular fossa is just posterior to the carotid canal.The stylomastoid foramen and the mastoid tip are lateral to the jugularfossa. The foramen spinosum and ovale are just anterior to the petrous apex(not labeled).

FIGURE 20-11 Three-dimensional CT surface recon-struction, from medially and slightly superiorly. This is aview of the medial surface of the temporal bone. Thehypoglossal canal is seen interposed between the occipitalcondyle and the jugular tubercle. The IAC is seen in themidportion of the temporal bone. The vascular grooves forthe sigmoid sinus and the superior petrosal sinus are wellseen.



indicator that no eighth nerve tumor is present. A thinsection 2DFT spin-echo T1-weighted sequence is obtainedin the axial plane after intravenous gadolinium administra-tion. Typical parameters include TR of 450, TE of 15, threeacquisitions, a field of view of 170 mm, and a matrix of192 × 256. Some radiologists add fat suppression toeliminate potentially confusing high signal from the fat inthe petrous apex.

ROUTINE BRAIN SURVEY

Routine head imaging is important to evaluate brain orother soft-tissue pathology. For example, a patient whopresents with symptoms of dizziness could have pathologythat results from brainstem demyelination or a tumor. Forthis reason, images of the whole brain are routinelyobtained. Acquisition of a noncontrast T1-weighted se-quence is of value to help characterize high-signal regionson contrast studies, since without a noncontrast examina-tion, it may be difficult to differentiate fat or subacutehemorrhage. T1-weighted, contrast-enhanced studies areroutinely acquired. In general, lower-resolution 2DFTT1-weighted images are acquired to evaluate the brain andadjacent soft-tissue structures, while fat saturation and

contrast enhancement may be of value if the suspectedpathology involves the fat spaces.

Low- to high-resolution axial (256 × 256 matrix, 4 to 5mm thick) fluid-attenuated inversion recovery (FLAIR) andT2-weighted fast spin-echo (FSE) images are also used toevaluate the brain. The FLAIR images are particularlysensitive to brain pathology such as demyelination orinfarcts, and they are also helpful in differentiatinghemorrhage from fat.

HIGH-RESOLUTION T2-WEIGHTEDIMAGING

Fluid-sensitive high-resolution images are noncontrastenhanced and demonstrate the CSF and endolymphaticspaces as high signal intensity regions. The cisternal cranialnerves can be visualized without contrast using thistechnique since the nerves are surrounded by the highersignal intensity fluid. Fluid in the otic capsule structures isbest visualized utilizing this technique, making it possible toevaluate cochlear or vestibular pathology. This techniquehas been used as a screening technique to rule out vestibularschwannomas.36

T2-weighted high-resolution images can be acquired

FIGURE 20-12 Surface reconstruction from CT data. The structures are viewed from superiorly. The geniculateganglion region of the facial nerve is well demonstrated. Note that both CT and MR imaging data can be used for thistype of image display.

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with short TR, low flip angle gradient-echo 3DFT imaging.This is achieved by preserving rather than spoilingtransverse magnetization using an appropriate selection ofrephasing gradients, a short TR (20 to 30 ms) and a moderateflip angle (30° to 50°). Such images are referred to assteady-state or steady-state-free-precession images. Station-

ary fluid with long T2 relaxation time yields high signalintensity similar to that seen on standard T2-weightedspin-echo images. However, for fluid in motion (such asCSF in the cerebellopontine angle), the transverse magneti-zation is spoiled due to the motion, and the fluid gives verylow signal intensity. Other differences between steady-state

FIGURE 20-13 A to F, Axial MR imaging, IAC level. Each axial image is a gradient echo steady-stateSIMCAST 0.7 mm thick, 1024 × 1024 matrix axial section of the right temporal bone. The images appear to beT2-weighted, although the origin of the contrast is more complicated. The CSF and other fluid spaces demonstratehigh signal intensity. The brain is intermediate in signal intensity, and the bone and pneumatized air spaces are signalvoids. The IAC contains the seventh and eighth nerves. They are seen as thin, linear low signal intensity structures inthe canal. The cochlea, vestibule, and semicircular canals are well seen. This section is not made 30° from theanthropologic baseline, so the complete lateral semicircular canal is not seen. The endolymphatic sac is seenposterior and medial to the posterior semicircular canal. In D the apparent notch (arrow) in the margin of thevestibule is susceptibility artifact. E is a magnified view of the cochlea at approximately the same level as D. Notethe interscalar septum (arrow). Compare to the image provided by CT in Figures 19-10 and 19-11.



FIGURE 20-14 A to E, Coronal MR imaging from anterior to posterior. As in Figure 20-13, the fluid spaces arevisualized as high signal. Nerves are seen crossing the bright signal of the CSF within the IAC.

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FIGURE 20-15 A to G, Sagittal MR imaging frommedial to lateral. The facial nerve and the branches of thevestibulocochlear nerve are seen in the IAC in the medialimages. The facial nerve travels in the anterior superior aspectof the canal. The cochlear nerve lies in the anterior inferiorportion of the canal. The superior and inferior vestibularnerves lie in their respective portions of the posterior canal.More laterally, fluid is seen in the vestibule and thesemicircular canals.

Illustration continued on following page



FIGURE 20-15 Continued. For legend seeprevious page.

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and standard T2-weighted spin-echo images include highsignal for fatty tissue (with a short T1 relaxation time) andlow signal intensity for all other tissues (gray matter, whitematter, and muscle). Typical parameters of an unspoiled3DFT axial series include 0.8 mm thick sections, a 512 ×288 matrix, 60 slices, TR of 17, TE of 4, flip angle of 30,NEX 1.36 Postprocessing of the image data can create anyspecific projection or surface desired.

A common spin-echo alternative to this type of T2-weighted sequence is a 3DFT FSE technique that utilizes aTR of 5000, TE of 100, echo train of 16, matrix of 512 ×384, and a small field of view. Fat and spatial saturationpulses may be needed to suppress chemical shift and bloodflow artifacts.

HIGH-RESOLUTION T1-WEIGHTEDCONTRAST IMAGES

A high-resolution, contrast-enhanced T1-weighted tech-nique similar to the T2-weighted studies can be acquiredusing a 3DFT technique to evaluate the temporal bone, butin this case the fluid spaces have low signal intensity and thenerves have higher signal intensity. This technique is alsoused to generate MRA images. Intravenous contrastenhancement is commonly used in conjunction with thistype of technique. This sequence is ideal for identification ofsubtle changes, like those seen with vestibulitis or smallvestibular tumors. T1-weighted gradient-echo images can beacquired by using a moderate TR (30 to 50 ms) and flipangle (30° to 50°) and by spoiling (destroying residualtransverse magnetization). Spoiling is achieved by usingspoiler gradients or, more efficiently, by utilizing RFspoiling (i.e., by randomizing the RF excitation pulsephase). Spoiling is needed to eliminate T2 contrastcomponents from the acquired signal, thus yielding T1-weighted images that can be used to increase the visualiza-tion of the gray-white matter interfaces and create othersection planes using postprocessing of the volume data sets.Routine spin-echo T1-weighted images can be utilized, butthe resolution is significantly lower.

As vessels can be bright with 3D gradient echosequences, some radiologists prefer the standard 2DFT FSEsequences. Although the images are slightly thicker, thein-plane resolution is high and the images depict theanatomy very well. Flow-related signal is less likely to beconfused with subtle enhancement.

MAGNETIC RESONANCE ANGIOGRAPHY

Finally, a dedicated MRA acquisition may be of value inspecific instances (Figs. 20-21 and 20-22). Different

FIGURE 20-16 Sagittal MR imaging, descending facial nerve level.At the posterior genu, the facial nerve turns inferiorly, exiting the temporalbone through the stylomastoid foramen.

FIGURE 20-15 Continued. In G the facial nerve isseen as a thin line of signal. Although not fluid, there isenough signal from the soft tissue of the nerve to becontrasted against the signal void of the bone and air inthe mastoid.



FIGURE 20-17 Curved MR imaging reconstruction of the facial nerve. A, Axial T1-weighted image of the leftparotid region from a 60 image 3DFT data set. The curved white line represents the course of a curved reconstructionfollowing the facial nerve from the brainstem into the parotid. Note the geniculate ganglion bend. B, Curved surfacereconstruction. Note that the facial nerve can be followed in continuity from the brainstem into the parotid. Theinternal auditory canal (IAC) demonstrates high signal intensity because these are not spoiled images. Therefore, thestationary CSF in the IAC generates a high signal. This novel presentation simplifies the interpretation becausethe nerve is visible over a long segment.

FIGURE 20-18 Three-dimensional recon-struction of MR imaging data. The data set is a3D volume acquisition. The thresholds are setto include the fluid of the IAC and labyrinth.

FIGURE 20-19 Oblique MR imaging of the cochlea. This isan oblique MR imaging reconstruction of the right temporal bone.The original data set was a 60 slice free precession high-resolution series. The section plane is obliqued to parallel the longaxis of the temporal bone. The plane is a steep sagittal sectionintersecting the descending facial nerve posteriorly and thecarotid canal anteriorly. A long segment of the cochlear spiral isseen, with the appearance of a spring.


FIGURE 20-20 Double oblique MR imaging and CT, endolymphatic sac. A, Reconstruction of axial CTimages. This is a double oblique section completely paralleling the flat plane of the endolymphatic sac. The sac istriangular in shape, similar to a Christmas tree, with the apex pointing at the common crus. The base of the sacbroadens inferiorly. B, The identical projection on a different patient generated from 60 free precession 3DFT MRimage series. This type of presentation of the endolymphatic sac is easier to analyze than the multiple short segmentsseen on routine imaging.



FIGURE 20-22 Two-dimensional time-of-flight MRimaging, lateral projection. The perspective is from themedial to lateral. The diverging carotid artery and jugularvein are well demonstrated. The jugular vein turns enter thesigmoid sinus.

FIGURE 20-21 Two-dimensional time-of-flight MR image.Anterior surface reconstruction MRA. This is a surface reconstructionof a 2D MR angiogram acquisition obtained without any saturationpulses or contrast enhancment. It allows visualization of both thearterial and venous systems. The carotid and jugular vessels in theneck are parallel to each other. As they enter the temporal bone, theyseparate, with the carotid artery turning medially and the jugular veinlaterally. The carotid is anterior to the jugular system.

Table 20-1MR IMAGING PROTOCOLS

Sequence Plane TR TE NEX/Echoes Matrix Flip Angle

T2 FSE C− Axial 5000 100 1/16 384 × 256 90/180T1 SE C− Coronal 500 12 1/1 384 × 256 90/1803DFT-GE T2 C− Axial 30 4 1/1 512 × 256 303DFT-FSE T2 C− Axial 5000 100 1/16 512 × 384 90/1803D GE T1 C+ Axial 17 4.2 1/1 512 × 288 303DFT-MRA C− Axial 48 7 1/1 512 × 192 202DFT-MRV C− Axial 26 7 1/1 256 × 192 60

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techniques must be utilized for visualization of arterialversus venous anatomy. For arterial studies, 3DFT se-quences generate the best detail and are usually not acquiredwith contrast enhancement. The 3DFT T1-weighted high-resolution technique previously described can be used as anexcellent MRA technique as well. MRA sequences aredesigned to minimize stationary tissue signal by using a TEfor which fat and water are out of phase or by usingmagnetization transfer. Flow compensation decreases signalfrom the moving CSF and thereby increases the contrast-to-noise-ratio between the blood vessels and the CSF in MRA;therefore, the soft-tissue detail may be poor. Althoughcurrent MRA can provide high-quality images, they cannotsubstitute for traditional catheter angiography in manycases.

Venous anatomy must be studied using a differenttechnique since the venous structures are much moresensitive to saturation because of slower flow rates. Forvenous anatomy, a technique employing a series ofcontiguous 2DFT coronal or axial images without saturationpulses is commonly used. Both the arteries and veins areseen, but there is usually little confusion. Commonparameters include a TR of 26, a TE of 7, a flip angle of 60°,a matrix 256 × 192, no spatial saturation pulses, and a fieldof view of 24 cm. If saturation pulses are used, there ispotential for demonstration of an artificial ‘‘thrombosis,’’caused by low signal intensity resulting from saturation ofinflowing spins. This is a time of flight technique, so coronaland axial images have different advantages to avoidsaturation effects. Postprocessing of these volume data setscan be used to create detailed 3D projections in differentplanes and perspectives.

CONCLUSION

Temporal bone imaging remains at the forefront of thedevelopment of high-resolution imaging techniques of thebody. The improved detection and understanding oftemporal bone pathology provided by modern imagingmethods allows for a very precise assessment of mostpathologic entities. A thorough understanding of theanatomy is very important in correctly assessing pathology.The myriad formats in which the same anatomic region ispresented continue to challenge radiologists.

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29. Kurucay S, Schmalbrock P, Chakeres DW, Keller PJ. A segment-interleaved motion-compensated acquisition in the steady state(SIMCAST) technique for high resolution imaging of the inner ear.J Magn Reson Imaging 1997;7(6):1060–1068.

30. Schmalbrock P, Pruski J, Sun L, Rao A, Monroe JW. Phased array RFcoils for high-resolution MRI of the inner ear and brain stem.J Comput Assist Tomogr 1995;19(1):8–14.



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36. Schmalbrock P, Chakeres DW, Monroe JW, Saraswat A, Miles BA,Welling DB. Assessment of internal auditory canal tumors: acomparison of contrast-enhanced T1-weighted and steady-stateT2-weighted gradient-echo MR imaging. AJNR 1999;20(7):1207–1213.

1108 TEMPORAL BONE




Temporalbone Imaging

Documents

sensorineural

short te times

volume data

internal auditory

magnetic resonance

magnetic susceptibility

posterior

mr imaging