March 2013, VOLUME 200
NUMBER 3

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March 2013, Volume 200, Number 3

Vascular and Interventional Radiology

Technical Innovation

Accuracy of CT Guidance of Lumbar Facet Joint Block

+ Affiliations:
1 Department of Radiology and Radiological Science, Medical University of South Carolina, 96 Jonathan Lucas St, MSC 323, Charleston, SC 29425.

2 Department of Neuroradiology, Ospedali Riuniti di Bergamo, Bergamo, Italy.

3 Neuroradiology, Neurocenter of Southern Switzerland, Ospedale Regionale Lugano, Lugano, Switzerland.

Citation: American Journal of Roentgenology. 2013;200: 673-676. 10.2214/AJR.12.8829

ABSTRACT
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OBJECTIVE. Lumbar facet joint block is generally performed under fluoroscopic guidance. The purpose of this study was to assess the technical success rate of facet joint block under CT guidance. The CT scanner was operated tableside with a step-and-shoot mode for intermittent needle visualization, and the amount of radiation used to perform the procedures was estimated.

CONCLUSION. CT-guided facet joint block is safe and rapid. Use of CT ensures reliable needle guidance with extremely high procedural accuracy at an effective radiation dose comparable to that of a procedure performed with 1 minute of fluoroscopic guidance.

Keywords: CT, effective dose, fluoroscopy, lumbar facet joint block

Pain arising from a lumbar facet (zygapophysial) joint (LFJ) is a common cause of lower back pain among adults, affecting as many as 45% of patients with chronic lower back pain [1, 2]. Facet pain is defined as pain emerging from any structure that is part of the LFJ, including the fibrous capsule, synovial membrane, hyaline cartilage, and bone [1]. As described in the literature, LFJ pain is often a consequence of repetitive strain or low-grade trauma accumulated over time in combination with predisposing factors such as degenerative disk disease and advanced age [1, 2].

The diagnosis and treatment of facet pain remain controversial [2]. As reflected by several guidelines, it is generally accepted clinical practice that diagnostic blocks directed and augmented by physical examination are the most reliable means of identifying an LFJ as the generator of lower back pain. Intraarticular injections and medial branch blocks are considered equally effective for diagnostic purposes [3]. In addition to conservative management, the treatment options for facet pain include minimally invasive interventional approaches, such as radiofrequency median branch denervation and intraarticular anesthetic and steroid injections (facet joint block), which are generally performed under fluoroscopic guidance.

Because of the minuscule LFJ anatomy, utmost procedural precision is desirable to achieve maximum diagnostic specificity and therapeutic efficacy. LFJs targeted by facet joint block often present challenging anatomy due to the presence of degenerative changes and overlying posterior osteophytes. Optimal needle access to the intraarticular space regularly requires unconventional ipsilateral straight or contralateral oblique approaches, which are extremely difficult with fluoroscopic guidance [4]. CT, however, with its inherent cross-sectional capability and high spatial and anatomic resolution seems advantageous because it supports precise needle placement in the axial plane. The purposes of our study were to assess the rate of procedural technical success of facet joint block under CT guidance and to estimate the amount of radiation used to perform these procedures and the corresponding patient doses.

Materials and Methods
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Patient Population

Institutional review board approval was received for this study, and the need for each patient's written informed consent was waived owing to the retrospective design of the study. We retrospectively reviewed a series of clinically indicated CT-guided LFJ block procedures on 37 consecutively registered patients (13 men, 24 women; mean age, 60 ± 12 years; range, 37–82 years). This patient collective underwent a total of 47 procedures on 84 targeted facet joints.

Procedure Performance

All procedures were performed with a 16-MDCT scanner (LightSpeed, GE Healthcare). The CT scanner can be operated directly at the tableside during a procedure with an intermittent step-and-shoot mode that yields spot images for intermittent visualization of the needle position, as in conventional CT. Patients were placed in the prone decubitus position. Initially a low-dose preprocedure localizing scan encompassing the LFJ of interest was obtained to determine local anatomy and the optimal skin entry point. Depending on each patient's habitus and after induction of local anesthesia (1% lidocaine), a 9- or 12-cm 22-gauge beveled spinal needle was carefully advanced via posterior paravertebral access. Intermittent low-dose (120 kV, 60 mAs) guidance scans (three slices, 2.5-mm slice thickness, 1.25-mm increment, anatomic coverage of 3.75 mm) were obtained to assure needle positioning.

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Fig. 1 —Needle directions required for joint access when presence of posterior osteophytes prevents direct access to facet joint.

A, 53-year-old man with chronic lower back pain. Ipsilateral straight access.

B, 59-year-old man with chronic lower back pain. Ipsilateral oblique access.

C, 45-year-old woman with chronic lower back pain. Contralateral oblique access.

The presence of posterior facet osteophytes overlying direct joint access, the needle direction required for joint access (oblique ipsilateral, straight ipsilateral, oblique contralateral), and the number of CT guidance scans needed for final placement of the needle were recorded. For each procedure, we obtained the average volume CT dose index (CTDIvol) in milligrays and the corresponding cumulative dose-length product (DLP) in milligrays × centimeters. These values were obtained from the CT scanner at the end of the procedure as a dose summary sheet and were available to the clinicians in the PACS image display [5]. The scanners at our institution are American College of Radiology CT accredited, and the radiation output is validated by in-house medical physicists every year. From these data we obtained the average x-ray beam intensity used to perform these procedures (i.e., CTDIvol) and the total amount of radiation used (DLP) per targeted facet joint. The latter was the total DLP per procedure divided by the number of targeted facet blocks performed.

Figure 1 shows an overview of the abstracted parameters. The procedure was considered technically successful when intraarticular needle placement was confirmed by opacification of the joint space by contrast injection (hand injection of 0.1–0.3 mL of iohexol [Omnipaque 240, GE Health-care]) with a 3-mL Luer lock syringe.

Results
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In our population of 37 patients a total of 84 LFJs were targeted under CT guidance. Forty-six of the 84 facet joint blocks (54%) were performed at L4–L5, 29 (35%) at L5–S1, five (6%) at L3–L4, and four (5%) at L2–L3. Forty of the 84 targeted LFJs had posterior osteophyte formation. Confirmed by joint-space contrast opacification, facet joint blocks were technically successful in 79 of the 84 cases (94%). In 5 of 84 cases intraarticular injection was not possible because the joint was inaccessible in one case, extraarticular contrast reflux occurred despite apparent intraarticular needle placement in three cases, and the procedure was aborted for lack of patient compliance in one case.

The side of access and required access direction were as follows: 73 of 84 ipsilateral (53 straight, 20 oblique) (Figs. 2 and 3) and 11 of 84 contralateral oblique (13%) (Fig. 4). Most of the procedures were performed by supervised residents and fellows in training, and the average number of guiding scans needed to obtain final needle placement was 4 ± 2 (range, 2–11 scans) [6] (Fig. 5). No periprocedural complications were encountered.

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Fig. 2 —56-year-old man with chronic left-sided lower back pain for approximately 7 years. CT scan shows angulation of facet joint space ostium at L5–S1, requiring ipsilateral oblique needle access. Intraarticular contrast opacification indicates successful needle placement (arrow).

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Fig. 3 —37-year-old woman with history of chronic bilateral lower back pain. Bilateral facet joint block at L4–L5 was planned. Facet joints do not have posterior osteophytes. However, straight angulation of joint space ostium on left necessitates ipsilateral straight needle access; ipsilateral oblique access is used on left. Intraarticular contrast opacification is evident, indicating successful needle placement (arrows).

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Fig. 4 —49-year-old man with chronic left-sided lower back pain. Arrow indicates large posterior osteophytes at left facet joint, preventing ipsilateral needle approach for joint access. With contralateral oblique needle approach, joint space was accessed, and intraarticular contrast opacification can be appreciated, confirming successful intraarticular needle placement (arrow).

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Fig. 5 —Graph shows number of guiding scans needed to obtain final needle position.

The average CT radiation beam intensity used to perform the procedures (CTDIvol) was 6.3 ± 2.0 mGy, and the average DLP per targeted facet joint was 46 ± 40 mGy × cm. The average DLP corresponded to a nominal patient effective dose of 0.7 mSv [6].

Discussion
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Facet joint interventions are used frequently for managing chronic back pain. Evidence supporting the clinical effectiveness (13–90% success rate) of these procedures for short- and long-term relief of chronic LFJ pain continues to accumulate [7]. Facet joint interventions are usually performed under fluoroscopic guidance to ensure success and avoid complications [8]. Median branch nerve blocks are usually performed with injection of local anesthetic for diagnostic purposes. To relieve intraarticular and periarticular inflammation, and thereby obtain pain relief, intraarticular steroid and anesthetic injection is performed [9, 10]. Although there is no clear evidence that intraarticular steroid facet joint block is more efficacious than periarticular facet joint block, it is conceivable that precise intraarticular injection is desirable for diagnostic specificity and therapeutic efficacy, as it is for other joints [11]. However, clinical judgment, expert opinion, and initial evidence suggest that intraarticular injections are an effective tool for the treatment of lower back pain when used judiciously, even with limited supporting research data [12]. In case of clinical preference, additional periarticular injection can be easily performed while the needle is retracted after intraarticular injection.

For accurate intraarticular needle placement, fluoroscopic guidance requires profiling and visualization of the joint space. Anatomic obstacles, such as overlying osteophytes, present in 40 of 84 facet joints in our series, cannot be directly visualized with fluoroscopy, making the intraarticular needle placement extremely difficult. Use of CT, with its excellent osseous anatomic resolution, can overcome these limitations and allow precise needle placement [4, 13].

In our study we achieved a 94% procedural technical success rate with a low average number of guiding scans needed for the final needle position (4 ± 2 scans). This success rate is remarkable because procedures were performed for the most part by operators in training (resident and fellow physicians) under the supervision of expert attending physicians. CT guidance allowed an optimal needle approach individualized to the anatomic configuration of each facet joint. The conventional ipsilateral oblique needle approach, which is most commonly used for fluoroscopic guidance, was performed in fewer than one fourth of cases. The ipsilateral straight approach was most common (63.1% of cases). A contralateral oblique approach was used in 11 of 84 cases and certainly would not have been possible with fluoroscopy. CT visualization with intermittent spot images ensured reliable needle guidance and placement individualized to the anatomic details of each facet joint. We achieved precise intraarticular injection with extremely high procedural accuracy and no periprocedural complications.

It is of interest to assess the radiation used for the localizing scan performed in helical mode after acquisition of the topogram and before acquisition of the needle visualization scans. A typical localizing scan has a DLP of approximately 10 mGy × cm, which corresponds to an additional effective dose of 0.1 mSv. The cumulative effective dose for CT-guided procedures is thus approximately 0.8 mSv, and it is of interest to compare this patient exposure with that associated with a fluoroscopically guided procedure. At our institution, 1 minute is the average duration of a similar procedure with fluoroscopic guidance, which would use an approximately 6 Gy × cm2 Kerma area product and result in a nominal patient effective dose of 0.8 mSv. Our data thus show that CT and fluoroscopy have similar patient radiation doses, which are equivalent to the natural background radiation an average American would receive in approximately 3 months, on the basis of an estimated annual natural background in the United States of approximately 3 mSv [14].

The radiation dose associated with CT-guided procedures is at the lower end of patient radiation doses normally encountered in diagnostic radiology. One reason for this result is that the average radiation intensity (CTDIvol) is only 6.3 mGy, which is less than 25% of the American College of Radiology CT accreditation program reference dose value for abdominal CT scans in a normalsized adult (25 mGy). In addition, the scan lengths used in these procedures are modest and amount to an average of only 9 cm per targeted facet joint. Accordingly, the total amount of radiation used to perform these procedures is low, and radiation exposure is therefore likely to be of relatively minor concern to these patients.

When clinically indicated, bilateral facet joint block at the same level was performed with approximately the same amount of time and radiation dose. Both needles were advanced between the scans.

In this study we did not assess the clinical efficacy of facet joint block but addressed the technical results. Our results show good performance of CT guidance in a clinical context, further supporting it as a feasible alternative to fluoroscopy for facet joint block. From a technical perspective it may be beneficial to compare pure step-and-shoot CT guidance, as we used in this study, with CT fluoroscopy.

Conclusion
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Our results indicate that CT guidance ensures fast and reliable needle placement for facet joint block that is adjusted individually to the anatomic details of each facet joint. Precise intraarticular injection can be achieved with extremely high procedural accuracy at an effective dose comparable to that of fluoroscopy.

References
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Address correspondence to M. Weininger ().

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