Atlas of Pain Medicine Procedures 1st Edition

SECTION I

BASIC APPLICATIONS

CHAPTER 2

Computed Tomography Guidance in Pain Management

Ronil V. Chandra, Thabele-Leslie Mazwi, Daniel Oh, Albert J. Yoo, and Joshua A. Hirsch

INTRODUCTION

• Computed tomography (CT) creates an image of the body by reconstructing image slices from a series of x-ray projections acquired as the patient is moved through the center of the CT scanner. The CT scanner is able to measure the attenuation of the x-ray beam by the various tissues along each projection. The spatial localization of these tissues is then determined using mathematical algorithms.
• The CT image is then displayed as a matrix of x-ray attenuation values using a reference scale (Hounsfield units [HU]) relative to water; water is assigned a value of 0 HU on all scanners. On this scale, air measures approximately −1000 HU and dense cortical bone approximately +1000 HU.
• A CT image can be displayed as different shades of grey by appropriately choosing the display parameters.
• All current CT scanners offer multi-detector technology (multiple CT slices can be obtained in one rotation of the gantry) and enable isotropic acquisition (ie, spatial resolution is equal in x, y, and z planes) with volumetric multi-planar image reconstruction.

ADVANTAGES OF CT GUIDANCE FOR INTERVENTIONAL PROCEDURES

Anatomical Detail

• Soft tissue structures such as nerve roots and bony constraints including severe scoliosis or large osteophytes are accurately defined (Figure 2-1). Particularly useful for nerve root or epidural injections in patients with advanced degenerative disease or previous surgery.
• Important neurovascular structures are visualized in real time (eg, avoidance of the vertebral artery in cervical nerve root blocks).
• Allows precise needle localization for very small targets.
• Avoids inadvertent transgression of nontarget tissue compartments (Figure 2-2).

Figure 2-1. CT-guided facet joint injection. (A) Axial prone CT clearly identifies large facet joint osteophytes (arrow) that would make intra-articular needle position difficult to achieve under fluoroscopic guidance. (B) CT guidance facilitates accurate targeting of the narrowed articular space, and intra-articular position (arrow) is achieved.

Figure 2-2. CT-guided acetabuloplasty for metastasis. (A) Axial CT clearly identifies soft tissue mass (black arrow) in anterior column of acetabulum, with narrow needle window (dotted line) between common femoral vessels (red arrow) and intra-abdominal compartment (star). (B) CT guidance allows accurate needle placement without injury to neurovascular bundle nor transgression of nontarget compartments. (C) In spite of the focal areas of cortical breach in the acetabular cortex (black arrow), careful injection of Polymethylmethacrylate (PMMA; red arrow) under CT guidance allows delivery without intra-articular extravasation.

Needle Placement

• Unlike fluoroscopy, which is hindered by tissue overlap, multi-planar reformats allow three dimensional trajectory planning.
• Direct visualization allows accurate needle placement with respect to the target nervous structure without use of contrast media.
• If required, the expected spread of injectate can be determined by injection of a small amount of contrast media (Figure 2-3). This is useful for epidural injections where inadvertent intrathecal needle tip position is clearly recognized by injection of contrast media.¹

Figure 2-3. CT-guided celiac plexus blockade. Axial prone CT clearly identifies needle position adjacent to the aorta during bilateral transcrural approach. Injection of a small amount of contrast reveals expected spread of final injectate.

DISADVANTAGES OF CT GUIDANCE

• Generally more time consuming than fluoroscopic approach.
• May have greater radiation dose to patient and practitioner compared to fluoroscopy depending on techniques used.
• Requires further expertise—an understanding of CT imaging, normal CT anatomy, and pathology.
• Less accessible modality, typically located in radiology departments.
• Requires further trained personnel—CT technologists.

PREOPERATIVE CONSIDERATIONS

• Patient positioning

For ideal needle trajectory

The simplest needle trajectory to achieve accurately is perpendicular to the floor—take advantage of oblique prone positioning if possible (Figure 2-4).

Ideally, the entire needle trajectory should lie in one axial CT image—this can be facilitated by angling the CT machine gantry (Figures 2-5 and 2-6).

Note that positioning in the lateral decubitus or oblique prone position alters the position of the diaphragm, which can facilitate retroperitoneal needle access without transgressing the pleural space.

Figure 2-4. Oblique prone positioning to facilitate easier needle placement during acetabuloplasty. (A) Axial prone CT during planning reveals lucent mass in the posterior column of the acetabulum. With prone positioning, an oblique needle trajectory (dotted line) is required. (B) With oblique prone imaging a needle trajectory perpendicular to the floor (dotted line) is obtained, which is easier and less time consuming to achieve. (C) Imaging with 11-gauge needle in place, prior to cortical entry. (D) Postprocedure CT after PMMA injection (red arrow) and removal of needle, with no leakage into the hip joint (black arrow).

Figure 2-5. The angled gantry approach. (A) The traditional CT gantry position is perpendicular to the CT table. (B) Angling the CT gantry along the line of an angled needle trajectory allows the entire needle trajectory to remain along a single CT slice.

Figure 2-6. CT-guided pelvic collection drainage using angled gantry approach. (A) Axial CT clearly identifies pelvic collection (dotted lines) but without clear needle access. (B) Tilting the CT gantry and repeat imaging identify a safe angled direct needle trajectory. (C) Pigtail drain tube successfully placed into pelvic collection.

To maximize patient comfort

Maximizing patient comfort prior to commencing the procedure minimizes patient motion during needle placement.

The prone position is well tolerated with pillows under the chest, hips, and ankles.

Further pillows, wedges, or towels may be necessary to achieve a comfortable oblique prone position.

• Needle entry planning

Once the patient is positioned, a skin grid marker is placed over the target entry site.

An initial radiographic CT scout image is acquired to delineate the superior and inferior extent of the planning CT scan.

An initial planning CT scan is performed to define the needle entry site, using the skin grid markers (Figure 2-7).

The trajectory is planned, taking into account anatomical limitations.

The needle entry site is marked on the patient’s skin, the skin grid is removed, the site is prepped with antiseptic solution, and the patient is draped appropriately.

If available, a monitor in the CT room should have the needle trajectory available as a reference image to facilitate needle placement.

Figure 2-7. Use of skin grid to mark skin entry position. (A) Photograph of patient positioned prone oblique on CT table, with skin grid placed (arrow), and laser light used to mark entry position. (B) Corresponding CT image with skin grid markers (arrow) to guide needle trajectory into acetabular mass (star).

INTRAOPERATIVE TECHNICAL STEPS

• Local anesthetic is injected at the skin entry marker site.
• It may be helpful to leave the local anesthetic needle in place along the planned needle trajectory for subsequent confirmation by CT scanning.
• Infiltrate local anesthetic along the planned needle trajectory.

A 22G spinal needle can be used to infiltrate deeply and into the periosteum if entering a bony target.

• As the definitive needle is placed, the needle position and trajectory should be verified with intermittent CT imaging.

Intermittent CT imaging needs to define the entire needle course and needle tip.

The needle tip can be verified by the identification of the bevel on the needle tip or the presence of a black shadowing artifact. Ensure adjacent CT images above and below are reviewed (Figures 2-8and 2-9).

If using an angled approach in the superior-inferior direction, more frequent CT imaging is recommended.

Figure 2-8. Demonstration of needle tip. (A) Axial CT during localization of local anesthetic needle reveals shadowing artifact arising from the tip, confirming identification of the needle tip. (B) With placement of the larger 11-gauge needle, a larger shadowing should be expected. The identification of the bevel of the needle tip is the most accurate method to localize the needle tip.

Figure 2-9. Demonstration of needle tip during sacroplasty. (A) Axial CT during localization reveals minor shadowing artifact arising from near the tip, however less than expected for a 13-gauge needle. (B) Imaging 2.5 mm cranial identifies further shadowing artifact as well as the diamond tip bevel confirming exact needle tip position.

• Once the target location is reached, diluted contrast media can be injected to assess intended spread of planned injectate.
• Final postprocedure CT should be performed after needle is removed.
• Use of CT fluoroscopy.

CT fluoroscopy is a technique in which the operator remains in the CT room while imaging is acquired, displayed on a monitor, and then used to guide needle placement.

Requires additional hardware and software—a display monitor in the CT room, a foot pedal to initiate scanning and control console to initiate table movement (Figure 2-10).

Best used when the entire needle trajectory lies in one axial slice and minimal needle manipulations are expected. Deviation from the needle trajectory often requires multiple images to be acquired to achieve correction.

Due to radiation exposure, the operator must wear a lead apron and thyroid shield. Lead eye protection is recommended.

The typical radiation dose rate in CT fluoroscopy is approximately 7 times lower than the dose rate with conventional diagnostic CT parameters, but it can be up to 60 times higher than conventional fluoroscopy depending on dose settings.²

Figure 2-10. Photograph of patient draped and positioned in preparation for CT fluoroscopy, with in-room monitor, controlling joystick (black arrow) and foot pedal (white arrow).

This can lead to large radiation doses to patient and operator if not used appropriately. Furthermore, radiation dose is concentrated on a focal area of skin and can result in a large skin radiation dose for the patient.

However, when used appropriately (low tube current and minimal exposure time), it can result in reduced patient radiation dose and reduced procedural time compared to conventional CT guidance.³

For selected procedures such as epidural injections and lumbar nerve root blocks, this method can result in radiation dose levels and procedural times similar to conventional fluoroscopic guidance.^1,4

There are two modes of CT fluoroscopic imaging:

Quick check imaging

• The operator presses a pedal to acquire CT images.
• Typically 3 images are displayed on a screen, with a central image at the expected needle location, and an image above and below.
• Lower patient and operator radiation dose than continuous imaging.²

Continuous imaging

• CT scanner can acquire continuous imaging (~10 frames per second) during needle manipulation to reproduce live nature of imaging with conventional fluoroscopy.
• Requires nonmetallic needle holders to ensure that operator’s hands are not in gantry. This is less tactile method for needle placement.
• Results in a high patient skin and operator radiation dose.
• If using this mode, there should be an inbuilt preset time limit for CT exposure to avoid excessive radiation dose.
• May be of benefit in targets within particularly mobile organs such as the lung or liver.

CLINICAL PEARLS

• Prior to any case, review the mechanics of the CT gantry, and ensure that all necessary equipment is available.
• For obese patients, ensure the patient is within the maximal CT table weight restrictions and can fit in to the CT gantry.
• If possible, once needle trajectory is planned and local anesthetic is infiltrated, manipulate the needle during the remainder of the procedure while the patient remains in the CT gantry—this reduces the chance of patient motion with table movement, and reduces procedure time.
• Use the laser guiding light in the CT gantry as frequently as possible.

If the laser light bisects the needle hub, the needle trajectory will be in the plane of the CT image.

If the needle hub lies above the plane of the laser guiding light, the needle tip is pointing in the opposite direction and can be adjusted without repeat imaging.

• If the patient needs to be moved into and out of the CT gantry, use of a pedal or sterile cover over the CT controls can allow the operator to move the CT table immediately and independently of the CT technician.
• If there is no safe direct access to the target in the axial plane, consider tilting the CT gantry to obtain oblique axial imaging—this may facilitate a safe angled pathway in which the entire needle trajectory can be visualized.
• Injection of normal saline or 5% dextrose can allow creation of a needle trajectory pathway in between compartments, eg, facilitating access to the celiac plexus by enlarging the retroperitoneal space or displacing adjacent tissues.
• If there is a CT contrast allergy, depending on the target location, a small amount of air can be effectively used as a contrast medium, eg, in confirming epidural needle tip position.¹
• Radiation reduction.

Any reduction in radiation to the patient also reduces radiation dose to the operator and to assisting staff members.

Acquire images only when necessary to assess needle position and trajectory.

Step as far away from the CT gantry during image acquisition as possible. Exit the room if not using CT fluoroscopy.

If using CT fluoroscopy.

Use the quick check method.

A lower tube current during imaging can reduce radiation dose while providing adequate trajectory information without loss of anatomical resolution.²

Place the foot pedal further away from the gantry to reduce dose (radiation intensity is inversely proportional to the square of the distance from the source).⁴

A lead drape over the patient adjacent to the scan plane can reduce scattered dose to the operator’s hands.⁵

Suggested Reading

Carlson SK, Bender CE, Classic KL, et al. Benefits and safety of CT fluoroscopy in interventional radiologic procedures. Radiology. 2001;219:515-520.

Joemai RM, Zweers D, Obermann WR, Geleijns J. Assessment of patient and occupational dose in established and new applications of MDCT fluoroscopy. AJR Am J Roentgenol. 2009;192:881-886.

Paulson EK, Sheafor DH, Enterline DS, McAdams HP, Yoshizumi TT. CT fluoroscopy-guided interventional procedures: techniques and radiation dose to radiologists. Radiology. 2001;220:161-167.

Wagner AL. Selective lumbar nerve root blocks with CT fluoroscopic guidance: technique, results, procedure time, and radiation dose. AJNR Am J Neuroradiol. 2004;25:1592-1594.

References

1. 1. Wagner AL. CT fluoroscopy-guided epidural injections: technique and results.AJNR Am J Neuroradiol. 2004;25: 1821-1823.
2. 2. Paulson EK, Sheafor DH, Enterline DS, et al. CT fluoroscopy-guided interventional procedures: techniques and radiation dose to radiologists.Radiology. 2001;220:161-167.
3. 3. Carlson SK, Bender CE, Classic KL, et al. Benefits and safety of CT fluoroscopy in interventional radiologic procedures.Radiology. 2001;219:515-520.
4. 4. Wagner AL. Selective lumbar nerve root blocks with CT fluoroscopic guidance: technique, results, procedure time, and radiation dose.AJNR Am J Neuroradiol. 2004; 25:1592-1594.
5. 5. Nawfel RD, Judy PF, Silverman SG, Hooton S, Tuncali K, Adams DF. Patient and personnel exposure during CT fluoroscopy-guided interventional procedures.Radiology. 2000;216:180-184.