(Radiology. 1999;211:882-885.)
© RSNA, 1999
Cervical Spine Tomography with an Angiographic C-arm1
Carlin A. Ridpath, MD,
Anthony J. Wilson, MB, ChB,
Steve G. Langer, PhD,
Frederick A. Mann, MD and
John C. Hunter, MD
1 From the Department of Radiology, Harborview Medical Center, University of Washington, 325 Ninth Ave, Box 359728, Seattle, WA 98104. From the 1997 RSNA scientific assembly. Received February 23, 1998; revision requested April 29; revision received June 29; accepted December 16. Address reprint requests to A.J.W.
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Abstract
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Phantom studies were performed to develop a technique for linear tomography of the craniocervical junction with a digital fluoroscopic angiographic C-arm unit. Section thicknesses were similar to those used at conventional tomography, and the radiation dose was lower. C-arm tomography was possible with a 6-second exposure and a 40° arc. C-arm tomography is a practical method for decreasing patient turnaround time.
Index terms: Spine, fractures, 30.41 • Spine, injuries, 30.40, 30.41 • Spine, radiography, 30.1215, 30.1218, 30.1219 • Tomography, 30.12
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Introduction
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Rapid and accurate imaging of the cervical spine facilitates appropriate use of personnel and resources for treatment of the polytrauma patient. Our institution is a level 1 trauma center, and the standard trauma protocol includes portable lateral radiography of the cervical spine, chest, and pelvis as an important adjunct to primary physical survey and resuscitation. After initial resuscitation, the polytrauma patient is transferred to the imaging suite, where requested radiographic examinations are performed, including those of the cervical, thoracic, and lumbar spine. The cervical spine series includes anteroposterior, open-mouth odontoid, lateral, and swimmers lateral and bilateral 45° oblique views. When patients are intubated or unable to open their mouths adequately, linear tomography of the craniocervical junction is performed instead of the open-mouth odontoid view.
On the basis of findings by Vandemark et al (1), a C-arm unit (C-2000; Phillips Medical Systems, Shelton, Conn) was purchased to facilitate spinal imaging in polytrauma victims. However, tomography was not a vendor-supplied feature. Because of our traditional reliance on tomography to examine the craniocervical junction, we decided to investigate the feasibility of performing linear tomography with the C-arm unit.
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Materials and Methods
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Phantom and Patient Studies
Optimal tomographic imaging of cervico-occipital relationships is facilitated by using thin sections. The section thickness is dependent on the maximum tomographic angle of the C-arm. The greatest angle of usable rotation for tomography is limited by the maximum duration of the x-ray exposure. To determine the greatest arc of rotation available for tomography, we measured the arc of the C-arm rotation during the maximum available exposure time.
Test exposures (Fig 1) were performed of skull and chest phantoms to determine the necessary technique factors and to estimate the magnitude of patient radiation. We performed C-arm tomography of the craniocervical junction with 10 mA, 60 kVp, and a 6-second exposure (60 mAs). (We perform conventional linear tomography with 20 mA, 80 kVp, and a 2-second exposure.) We used a 400-speed screen-film combination prior to March 1997. Thereafter, use of computed radiography storage phosphor plates (Fuji Medical Systems U.S.A., Stamford, Conn) became the standard in our department.
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Figure 1a. Head phantom studies. (a) Coronal linear tomogram of the maxillary sinuses (MS). (b) Sagittal linear tomogram of the sella turcica (arrow).
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Figure 1b. Head phantom studies. (a) Coronal linear tomogram of the maxillary sinuses (MS). (b) Sagittal linear tomogram of the sella turcica (arrow).
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The section thickness was determined both theoretically by means of calculation and experimentally by means of measurement. With the theoretic approach, section thickness is defined as the point at which subject contrast is reduced to 75% of the nontomographic value (2). With the experimental approach, we used a tomographic phantom with embedded wire mesh (model 76-400; Nuclear Associates, Carle Point, NY). We defined the tomographic section thickness as the point at which the mesh was no longer distinct.
Organ dosimetry was not performed because the beam cross section and field of view were similar to those at conventional tomography and allowed use of the relevant tables from the U.S. Food and Drug Administration. Patient dose was estimated on the basis of the selected milliampere and kilovolt peak settings to be less than that with conventional tomography. Patient skin dose was measured by means of dosimetry. At the phantom surface, the entrance skin exposure was measured by using an ionization chamber (Radcal, Monrovia, Calif).
Once a technique had been established with the phantoms, patient studies were performed (Fig 2). In our polytrauma patients, these studies were performed as expeditiously as possible. Then, the technique was added to our clinical imaging protocols.
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Figure 2. In an intubated, polytrauma patient, coronal linear tomogram of the craniocervical junction demonstrates the occipito-atlanto-axial relationships. C2 = body of the axis, D = dens, LM = lateral mass, OC = occipital condyle.
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Between the initial installation of the C-arm unit in May 1997 and August 1998, when this study was completed, we used this technique in 109 polytrauma patients with suspected spinal injury. Craniocervical junction tomography was performed in 21 of these patients.
Technique
With fluoroscopic guidance, the isocenter or fulcrum for the atlas and axis were found (Fig 3). The C-arm was initially placed in the horizontal position, and the table was moved up and down to set the tomographic level. Once the C1 and C2 vertebrae were centered in this plane, the C-arm was rotated 90° to the vertical position. The patient was again centered with fluoroscopic guidance, and the table top was moved in the horizontal plane. The C-arm was then positioned for tomography to +30° of rotation.
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Figure 3a. Photographs with a volunteer demonstrate positioning and use of the C-arm unit. (a) The C-arm is initially positioned horizontally to determine the level of the tomographic section. (b) The C-arm is rotated to a vertical position, and the patient is centered for anteroposterior tomography. (c) Time-lapse photograph demonstrates the arc of the C-arm. Rotation starts at +35° and stops at -20°.
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Figure 3b. Photographs with a volunteer demonstrate positioning and use of the C-arm unit. (a) The C-arm is initially positioned horizontally to determine the level of the tomographic section. (b) The C-arm is rotated to a vertical position, and the patient is centered for anteroposterior tomography. (c) Time-lapse photograph demonstrates the arc of the C-arm. Rotation starts at +35° and stops at -20°.
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Figure 3c. Photographs with a volunteer demonstrate positioning and use of the C-arm unit. (a) The C-arm is initially positioned horizontally to determine the level of the tomographic section. (b) The C-arm is rotated to a vertical position, and the patient is centered for anteroposterior tomography. (c) Time-lapse photograph demonstrates the arc of the C-arm. Rotation starts at +35° and stops at -20°.
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According to the manufacturer specifications, the digital image intensifier has three fields of view (38, 25, and 17 cm) and two focal spot sizes (0.3 and 0.7 mm). We found that digital images obtained with a 17-cm field of view and 0.3-mm focal spot had the highest spatial resolution and were best suited to examination of the occipital condyles, atlas, and axis. After exposure, the digital images were viewed in the imaging suite on a 1,049-line high-resolution monitor. Brightness, contrast, and edge enhancement were adjusted before printing onto film with a laser printer.
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Results
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In trial and error experiments with the phantoms, we found that when the C-arm rotation and exposure buttons were pressed simultaneously the C-arm rotated through 10° before the x-ray exposure began. With use of a maximum manual (nonfluoroscopic) exposure time of 6 seconds, the C-arm rotated through 40°. Therefore, to realize 40° of rotation during x-ray exposure, use of a 50° arc of rotation was necessary. In preparation for exposure, we positioned the C-arm at the +30° position (based on the angle indicator of the control panel). Then, the exposure and rotation buttons of the C-arm were pressed simultaneously. The C-arm rotated, and the 6-second exposure was completed by -20° (ie, the exposure took place between +20° and -20° for a tomographic angle of 40°).
Use of a 40° arc produced a 14–16-mm section thickness by means of both calculation and measurement. The arc of the C-arm and the section thickness were similar to those with our conventional tomographic unit. With our conventional linear tomographic unit, a 40° arc of rotation is used.
For both patients and phantoms, the entrance skin exposure per section was 0.57 x 10-4 C/kg for linear tomography with the C-arm and 0.88 x 10-4 C/kg for conventional linear tomography. These results were based on a source-to-object distance of 78.75 cm and a source-to-image distance of 100 cm. When the photon source was brought closer to the patient or phantom, the source-to-object distance was decreased, and the entrance skin exposure was increased according to the inverse square law (3).
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Discussion
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In appropriate motor vehicle accident victims, the cervical spine can be evaluated with high-quality radiographs of the cervical spine with little risk of missing major fractures (1% of patients) (4). Inadequate radiographs are a common source of misdiagnosis of cervical spine trauma, especially in the cervicothoracic and atlantoaxial regions (5). According to Clark et al (6), odontoid fractures are more likely to escape detection at radiography than are other spinal injuries. Deleterious effects include time constraints imposed by other trauma priorities, lack of cooperation by patients, and physical limitations imposed by the supine imaging position (7). Unfortunately, cervical spine examinations in patients with the highest risk of injury are most likely to be technically compromised (7).
The speed and multifunction features of the digital C-arm used at our institution and others are used to expedite the care of trauma patients and decrease the number of nondiagnostic examinations in those at high risk for cervical spine fractures (1). Total examination time is reduced as a result of real-time fluoroscopic positioning. With patients who are to undergo angiography, the spine may be imaged and evaluated while the angiographers are setting up in the same room. Angiography is then performed on the same table, which obviates patient transfer to another room. To image the spine in severely injured patients, use of the linear tomographic capability of an angiographic C-arm is logical. Tomography is useful not only to confirm or exclude odontoid fractures but also to classify (ie, type I, II, or III) and stage them. The high tissue contrast inherent in bone makes tomography a practical choice in these patients, in whom a standard open-mouth odontoid examination is impractical or nondiagnostic.
In our experience, digital imaging of the spine and linear tomography with a C-arm unit have decreased the number of table-to-bed transfers for our polytrauma patients and have improved patient throughput in the emergency and radiology departments.
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Footnotes
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Author contributions: Guarantor of integrity of entire study, A.J.W.; study concepts, J.C.H.; study design, A.J.W., J.C.H.; definition of intellectual content, A.J.W.; literature research, C.A.R.; clinical studies, A.J.W., F.A.M., J.C.H.; experimental studies, S.G.L.; data acquisition and analysis, A.J.W.; manuscript preparation and editing, A.J.W., F.A.M., S.G.L., C.A.R.; manuscript review, all authors.
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References
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MacDonald RL, Schwartz ML, Mirich D, Sharkey PW, Nelson WR. Diagnosis of cervical spine injury in motor vehicle crash victims: how many x-rays are enough?. J Trauma 1990; 30:392-397.[Medline]
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Vandemark RM. Radiology of the cervical spine in trauma patients: practice pitfalls and recommendations for improving efficiency and communication. AJR 1990; 155:465-472.[Abstract/Free Full Text]
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Abstract
Introduction
