Chapter 2

### 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 (...

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