The use of ultrasonography (US)-guided procedure in pain medicine has been thriving. Nowadays, the US has replaced fluoroscopy (FL) in numerous pain procedures. The US has several advantages in pain practice [1–5]. The remarkable benefit of the US is the direct visualization of soft tissue structures, such as nerves, vessels, tendons, ligaments, muscles, and fascia. Using the US, pain physicians can instantly detect which construction has been the source of the shoulder pain or if the ulnar nerve would be dislocated along with the elbow flexion by dynamic examination of target structures. Real-time observation of needle insertion and spreading patterns of injectates also add to the benefits of using the US [1]. The absence of radiation exposure, easy portability, and relatively affordable cost can be other advantages of the US compared to the FL. On the contrary, there are shortcomings in US-guided techniques, such as a problem with examining deeper structures, particularly in a person with a high body mass index [6], poor penetration through bone or air, and no prevention from intravascular injection [1]. Also, the success rate of the US-guided procedures has dependently deviated from physicians' knowledge of sonoanatomy and technical skills. Furthermore, the US sometimes fails to count the exact spinal level in the presence of an anatomic variation (e.g., lumbarization or sacralization), which may result in incorrect diagnostic or prognostic spinal injection.

Considering the advantages and limitations of the US, the cervical spine and the relevant neuromuscular structures have substantial benefits to using the US because the target structures are superficially located compared to the other spinal levels [2–4]. In particular, the third occipital nerve (TON) block may be one of the best examples to approve the advantages of the US compared to FL. The TON, the superficial medial branch of the C3 dorsal ramus, supplies the C2–C3 zygapophysial joint while crossing the joint laterally. When performing the TON block under fluoroscopic guidance, three consecutive injections are usually necessary around the C2 inferior articular and upper C3 articular processes. On the other hand, US directly visualizes the TON during the procedure (Fig. 1), and therefore, only a single injection is enough to perform the TON block using US. Furthermore, in a previous prospective randomized study, while pain reduction was similar between the US and FL-guided TON blocks, the performance time was significantly shorter in the US than in FL [3]. However, FL has still been preferred over the US in some cervical spinal pain procedures, such as the atlantoaxial joint injection. For this procedure, a transverse short-axis US image between the C1-2 vertebral level should be obtained in the lateral neck, which shows the atlantoaxial joint as a shallow slit, which is located lateral to the C2 dorsal root ganglion and medial to the vertebral artery (Fig. 2). Then, the needle can be advanced to the joint using an out-of-plane approach. However, locating the needle into the intra-articular joint space would be challenging, and the practitioner cannot prove it under US guidance. However, using FL, they can identify that the needle tip is being precisely inserted into the target space using a contrast medium which beautifully spreads inside the atlantoaxial joint in fluoroscope images (Fig. 3). Moreover, although it depends on the technical knowledge and skill, US guidance would take more time than FL guidance for the atlantoaxial joint block that is needed in pre-scanning to identify the target and other vital structures. This example suggests that there has still been a weak point in US-guided procedures, even in the cervical spine, where practitioners still prefer the FL-guided interventions for a considerable portion of pain procedures.

Figure 1. Longitudinal US image at the lateral neck. The C2/3 zygapophysial joint is identified in the middle. The arrow indicates the third occipital nerve. The arrowhead shows the C3 medial branch. US: ultrasound.

Figure 2. Transverse short-axis US image with color Doppler at the lateral neck between the C1 and C2 vertebral level in the lateral neck. In this US image, the atlantoaxial joint is located in the middle, the C2 dorsal root ganglion lies medial (arrow), and the vertebral artery runs lateral to the joint. US: ultrasound, AAJ: atlantoaxial joint.

Figure 3. Fluoroscopic lateral and posteroanterior images for the atlantoaxial joint block. The contrast media spreads inside the joint space.

Although a ton of studies on US-guided pain procedures have been published, there needs to be a classification system to evaluate which image device, the US or FL, is clinically and technically better in various pain procedures. Therefore, the main goal of this narrative review is to introduce the classification system for US-guided pain procedures according to their clinical and technical outcomes and designate US-guided pain procedures into one of the categories by reviewing previous prospective randomized comparative trials.

1. Developing four categories of US-guided pain procedures

Since the mid-2000s, several prospective randomized studies have compared the US and FL in pain medicine. To develop the classification system for the US-guided pain procedures, an electronic literature search using the National Library of Medicine's MEDLINE and PubMed, Embase, and Cochrane databases with the keywords "ultrasound or ultrasonography," "fluoroscope or fluoroscopy," "pain block," "pain procedure," or "pain intervention" was conducted to identify articles relevant to the present review. Clinical prospective studies comparing the US and FL guidance for pain interventions were included, but single-arm observational or retrospective studies were excluded in this review. The search was limited to human subjects, but studies in the cadavers were included, written in the English language, studying adults (aged 18 years and older), and dated from 2010 to 2023. The most recent search was performed in May 2023. The emphasis of the extracted data was the clinical outcomes (pain and function improvement, satisfaction, and any adverse events associated with the procedure) and technical outcomes (success rates, procedural or performance time, and the number of needle passes). Through reviewing those studies thoroughly, US-guided pain procedures are classified into one of the four categories according to their clinical and technical results: ‘category 1’ includes procedures with the superiority of US guidance to FL guidance; ‘category 2’ includes procedures with the non-inferiority of US guidance over FL guidance; ‘category 3’ includes procedures showing the feasibility of the US with conjunction of FL; and ‘category 4’ includes procedures with the inferiority of US guidance to FL.

1) Category 1: superiority of the US to FL

Category 1 contains procedures in which US is superior to FL with clinically or technically better outcomes, which were proved by 5 randomized comparative studies in Table 1. As previously mentioned, Finlayson et al. [3] reported that the US-guided TON block had better outcomes than the FL-guided block with a shorter performance time (213 vs. 397 seconds, respectively; P < 0.001) and fewer needle passes (2 vs. 6, respectively; P < 0.001) (N = 40). Both imaging modalities had similar success rates (95%–100%). Then, the same group compared the US-guided C7 medial branch block to the FL-guided block (N = 50) [7], which showed better outcomes in the US group with a shorter performance time (234 ± 80 seconds in the US group vs. 391 ± 142 seconds in the FL group; P < 0.001) and fewer needle passes (2 in the US vs. 4 in the FL; P < 0.001), as well as providing similar success rates (92%–96%). In a recent randomized study, Finlayson et al. [8] compared sacral lateral branch blocks between the US and FL guidance for chronic low back pain (N = 40). The block needle was positioned on the lateral crest at the mid-point between the S2 and S3, followed by two more injections on the lateral crest, immediately cephalad to S2 and at the S1 level. In the study, the pain relief was similar in both groups, but the US showed better outcomes than the FL in terms of a shorter performance time (268 ± 99 seconds vs. 629 ± 120 secconds, respectively; P < 0.001), fewer needle passes, and a lower incidence of vascular breach (0 vs. 10 occurrences, respectively; P = 0.001). Considering the advantages of the US, injections targeting the soft tissues would prefer the US over the FL, which is classified in category 1. Pestcavage-Thomas and Gustas [9] compared the US-guided and FL-guided biceps tendon sheath injection (N = 53), showing that although pain relief and functional improvement after the procedures were similar, the first-pass success rate was higher (90.6%) for the US than for the FL (74.0%) showing superiority of the US over the FL. The final pass success rate was 98.2% for the US vs. 92.0% for the FL. Shamshery et al. [10] compared the US-guided iliopsoas block to the FL-guided block (N = 36). They showed that pain and function improved similarly in both groups for 12 weeks post-block. However, the learning curve was easier for US intervention, with average attempts of 1–2 compared to 1–3 for the FL. More time was required for FL guidance than for US guidance (6.1 minutes and 2.5 minutes, respectively, P < 0.001). A comparative study between the US-guided and FL-guided stellate ganglion block in 10 cadavers reported that the US resulted in successful staining in 9 of 10 injections, while only 6 of 10 for the FL guidance [11]. In the study, the injection occurred posterior to the prevertebral fascia in 4 cases of the FL-guided stellate ganglion blocks, while the injection was in the carotid sheath in one case of the US-guided block. Technical variables were not compared in this cadaveric study.

Table 1 Category 1 has the superiority of ultrasound to fluoroscopy guidance

Author (yr)	Intervention	Results
Finlayson et al. (2013) [3]	3rd occipital nerve block (N = 40)	US with a shorter performance time and fewer needle passes than FL (each P < 0.001). A similar success rate (95%–100%). There were no differences in pre- and post-block pain scores in both groups.
Finlayson et al. (2015) [7]	C7 medial branch block (N = 50)	US with a shorter performance time and fewer needle passes than FL guidance (each P < 0.001). A similar success rate (92%–96%). No differences in pre- and post-block pain scores.
Finlayson et al. (2017) [8]	Sacral lateral branch blocks (N = 40)	Compared with FL, US-guided sacroiliac joint sacral lateral branch blocks require a shorter performance time (267 sec vs. 628 sec; P < 0.001), and fewer needle passes (0 vs. 10 occurrences; P = 0.001) and carry a lower risk of vascular breach.
Petscavage-Thomas and Gustas (2016) [9]	Biceps tendon sheath injection (N = 53)	The first-pass success rate was higher (90.6%) for the US and 74.0% for the FL. The final pass success rate was 98.2% for the US vs. 92.0% for the FL.
Shamshery et al. (2021) [10]	Iliopsoas block (N = 36)	Pain decreased for 12 weeks without inter-group differences (US vs. FL). The learning curve was easier for the US intervention, with average attempts of 1–2 compared to 1–3 for the FL. The average performance time was less for the US group. The function improvement was comparable in both groups.
Hughey et al. (2021) [11]	Stellate ganglion block (N = 20 in 10 cadavers)	US resulted in successful staining in 9 of 10 injections, while 6 of 10 for the FL. In the 4 FL-guided injections, the injection occurred posterior to the prevertebral fascia. In 1 US-guided injection, the injection was in the carotid sheath.

FL: fluoroscopy, US: ultrasound.

US-guided pain procedures in category 1 have some meaningful benefits compared to the FL guidance; therefore, using the US for those procedures is clinically recommended. However, technical differences between pain practitioners and different knowledge of the sonoanatomy and familiarity should be considered when selecting which device to use in pain practice.

2) Category 2: non-inferiority of US to FL

Category 2 includes 9 randomized comparative studies, resulting in several US-guided procedures presenting the non-inferiority of the US over the FL in their clinical and technical outcomes (Table 2). In the cervical spinal regions, two randomized comparative studies by Jee et al. (N = 110) [12] and Cui et al. (N = 156) [13] reported that the C5, C6, and C7 nerve root blocks for cervical radiculopathy had similar pain and function improvement during the follow-up period; however, the former did not compare technical variable [12], whereas the latter had a shorter administration duration using the US-guided cervical spinal nerve root blocks compared to that while using FL (224 ± 82 seconds in the US vs. 360 ± 125 seconds in the FL; P < 0.001) [13]. Regarding the US-guided cervical nerve root block, although the US showed a shorter performance time in both, the procedure was classified in the category 2 because the other technical and clinical variables did not show any superiorities of US to FL. In the cervical spine, the deep cervical plexus block, which was investigated by Wan et al. [14] for the treatment of cervicogenic headache (N = 56), also had similar procedural time, pain, and function improvement between the US and FL during the 24-week follow-up period. In the lumbosacral regions, two randomized comparative studies by Ha et al. (N = 26) [15] and Yun et al. (N = 57) [16] reported that the lumbar facet joint blocks had similar pain and function improvement during the follow-up period; however, the former targeting from the L2/3 to the L5/S1 did not compare technical variables [15], whereas the latter showed longer preparatory time in the US group (FL: 249 ± 7 seconds; US: 263 ± 6 seconds; P = 0.023) [16]. Nisolle et al. [17] compared lumbar medial branch blocks (L3-L5) between US and FL guidance. They reported that the US-guided lumbar medial branch blocks were not inferior to FL guidance regarding pain and function improvement for 1 month. Soneji et al. [18] compared US-guided sacroiliac joint injection and FL-guided injection (N = 40), which showed no significant differences in pain reduction at 1 month, procedure-related variables, physical functioning, discomfort, opioid utilization, and patient satisfaction between the US and FL groups. The US-guided pudendal nerve block could be designated in category 2 with non-inferiority of US, showing the same efficacy and safety in comparison with FL, which Bellingham et al. [19] reported had no differences in the degree of sensory block between the US or FL groups, but performance time was longer using US (219 ± 65 seconds; 428 ± 151 seconds, respectively; P < 0.001) (N = 23). There are more procedures in category 2, such as piriformis injection and genicular nerve blocks. Fowler et al. [20] investigated the US-guided piriformis injection compared to FL guidance (N = 28). They reported no differences in pain improvement, patient satisfaction, procedure time, number of needle passes, and most functional outcomes between US-guided and FL-guided blocks. Kim et al. [21] conducted a randomized comparative study between the US-guided and FL-guided genicular nerve blocks for chronic knee osteoarthritis (N = 80), which described how pain relief, functional improvement, and safety were similar between the groups.

Table 2 Category 2 has the non-inferiority of ultrasound to fluoroscopy guidance

Author (yr)	Intervention	Results
Jee et al. (2013) [12]	C5, C6, C7 nerve root block (N = 110)	Using US- or FL-guided epidural steroid injections for cervical radiculopathy, herniation of intervertebral disc, or foraminal stenosis, significant improvements were reported in function and pain in both groups.
Cui et al. (2022) [13]	C5, C6, C7 nerve root block (N = 156)	88.7% accuracy for US and 90.3% accuracy for FL, revealing that the lower limit was above the non-inferiority margin. Both pain and function scores improved for 6 months with no inter-group difference. A shorter administration duration was observed in the US group (P < 0.001).
Wan et al. (2017) [14]	Deep cervical plexus block (N = 56)	A similar reduction of the pain intensity for 24 weeks (P < 0.05 in both groups) without differences between the US and FL groups.
Ha et al. (2010) [15]	Lumbar facet joint block (L2/L3 to L5/S1) (N = 26)	No difference in performance time. The pain and function scores were improved for 6 months in both groups.
Yun et al. (2012 [16]	Lumbar facet joint block at L4/5 and L5/S1 (N = 57)	Significant improvement in pain and functional disability in both groups for 3 months. A preparatory time was longer in the US group (P = 0.023).
Nisolle et al. (2023) [17]	Lumbar medial branch blocks at the L3-5 Levels (N = 50)	Lumbar medial branch blocks under the US guidance were not inferior to the FL guidance (P = 0.047) regarding pain and function improvement for 1 month. The duration of techniques was similar between groups.
Soneji et al. (2016) [18]	Sacroiliac joint injection (N = 40)	No significant differences in pain improvement at 1 month, procedure-related variables, physical functioning, discomfort, opioid utilization, and patient satisfaction between the US and FL groups. Performance time was longer using the US (P < 0.010).
Bellingham et al. (2012) [19]	Pudendal nerve block (N = 23)	There were no differences in the degree of sensory block between the US- or FL groups. Performance time was longer using the US (P < 0.0001).
Fowler et al. (2014) [20]	Piriformis injection (N = 28)	There were no differences in pain scores, patient satisfaction, procedure time, number of needling, and most functional outcomes between the US-guided and FL-guided blocks.
Kim et al. (2019) [21]	Genicular nerve block for chronic knee osteoarthritis (N = 80)	Pain relief, functional improvement, and safety were similar between groups receiving the US- and FL-guided genicular nerve blocks.

FL: fluoroscopy, US: ultrasound.

Considering the clinical benefits and limitations, practitioners can select either US or FL for pain procedures classified in category 2 according to the physicians' technical knowledge and skills.

3) Category 3: the feasibility of US in conjunction with FL

In category 3, with the feasibility of using US in conjunction with FL, all pain procedures were performed by the US and then verified by FL to confirm the final needle location. The FL-guided procedures were referred to as the popular gold standard techniques in category 3. Pain procedures in category 3 are shown in Table 3. There are 2 studies investigating lumbar transforaminal injections [22,23], which could be designated in category 3 with the feasibility of the US verified by FL. Each prospective comparative study showed similar clinical outcomes for radicular low back pain during the follow-up period. One study by Yang et al. [22] showed a shorter performance time for lumbar transforaminal injection using US (518 ± 103 seconds) than using FL (929 ± 228 seconds) (P < 0.05). The other study by Falsafi et al. [23] introduced the modified technique for lumbar transforaminal injection (N = 30) as a safe and accurate technique compared to FL guidance. However, they did not evaluate technical variables during the study. Although there have been no prospective randomized comparative studies in the sacral transforaminal injections between the US and FL, one cadaveric study by Thompson et al. [24] conducted US-guided S1 transforaminal injection (8 blocks in 4 cadavers) followed by the verification of the computed tomography. In the study, US required a longer duration (4.8 minutes) than FL (3.3 minutes). The rate of intravascular uptake was higher using US (50% using US vs. 37.5% using FL), resulting in the US having limited clinical utility for the sacral transforaminal epidural injections due to its inability to visualize relevant neurovascular structures deep into the osseous roof and exclude intravascular uptake. The sacroiliac joint radiofrequency ablation was performed in 37 patients by Burnham et al. [25] under US guidance, followed by FL verification—the gold standard procedure referred to as the palisade technique. In the study, longitudinal axis sacroiliac joint radiofrequency ablation for sacroiliac joint denervation improved patients’ quality of life, similar to the palisade technique. It showed similar pain reduction and function improvement for 6 months, but the US group required more procedure time than the palisade technique (38.2 ± 7.9 minutes in the US vs. 32.1 ± 6.9 minutes in the FL; P = 0.031). A prospective randomized study compared the US and FL alone for lumbar sympathetic block. The US-guided lumbar sympathetic block was first introduced by Moon et al. [26]; then, the same group conducted a prospective randomized comparative study (N = 46) [27], which reported that the total procedure time and success rate were not statistically different between the US (64% success rate) and FL groups (75% success rate). In contrast, the imaging time of the US group was longer (P = 0.012). It was concluded that the US-assisted lumbar sympathetic ganglion block had no advantage over the FL-guided procedure in terms of performance time and technical success rate.

Table 3 Category 3 has the feasibility of ultrasound with conjunction of fluoroscopy verification

Author (yr)	Intervention	Results
Yang et al. (2016) [22]	Lumbar epidural injection (N = 112; verified by FL)	There were no significant differences between US and FL in procedure time, number of needle insertion attempts or passes, and pain reduction and disability scores for 3 months.
Falsafi et al. (2021) [23]	Lumbar transforaminal injection (N = 30; verified by FL)	Lumbar transforaminal injection was administered using the modified new technique, which was safe and accurate compared to FL, the popular gold standard technique.
Thompson et al. (2018) [24]	S1 transforaminal injection (8 blocks in 4 cadavers; verified by the computed tomography)	The longer duration requires the US (4.8 min) than the FL (3.3 min). The rate of intravascular uptake was higher (50% in the US vs. 37.5% in the FL). The US would have limited clinical utility due to its inability to visualize relevant neurovascular structures deep into the osseous roof and exclude intravascular uptake.
Burnham et al. (2022) [25]	Sacroiliac joint radiofrequency ablation (N = 37; assisted by FL)	Longitudinal axis sacroiliac joint radiofrequency ablation showed pain reduction and improved quality of life similar to the palisade technique; however, it required more procedure time than the palisade technique (38.2 min vs. 32.1 min, P = 0.031).
Moon et al. (2018) [27]	Lumbar sympathetic block (N = 46; verified by FL)	Total procedure time and success rate were not statistically different between the US (64% success rate) and FL groups (75% success rate). The imaging time of the US group was longer (P = 0.012).

FL: fluoroscopy, US: ultrasound.

Procedures in category 3 are feasible using the US, followed by FL confirmation. Therefore, it may be correct that procedures in category 3 are not precisely "US-guided" but "US-assisted interventions." There are a few advantages to using the US in those procedures; US assistance can lower the radiation exposure dosage and time compared to using FL alone. Therefore, pain physicians can flexibly use the most adequate image devices in various clinical situations.

4) Category 4: inferiority of US over FL

The procedure in category 4 is challenging under US guidance, and only one prospective comparative study is designated in this category. Abdelghaffar and Farahat [28] conducted a randomized study comparing the US and FL for superior hypogastric plexus neurolysis in 96 patients with cancer-related pelvic pain, with the results showing the procedure time being longer using US (24 minutes) than FL (21 minutes) (P = 0.002). Patient satisfaction was lower with US than with FL (P < 0.001). The authors concluded that the FL-guided superior hypogastric plexus neurolysis was superior to the US guidance regarding the time of the procedure and patient satisfaction. At the same time, both techniques were similar regarding safety and pain relief during the follow-up period. However, using US, the range of spread with neurolytic agents could not be identified, whereas it can be noticed indirectly using a contrast media under FL guidance. Thus, spread of neurolytic agents, such as alcohol or phenol, to unexpected but vital structures close to the target could result in significant adverse events. Hence, the FL-guided superior hypogastric plexus neurolysis was still superior to US for the clinical (effectiveness and safety) and technical aspects. US can probably be used in an emergent case in a patient in a supine position when the FL cannot be used.

5) Others

There are controversies in the category of some US-guided pain procedures.

Caudal epidural injection is one of the most popular procedures in chronic pain units, which has been usually conducted under FL guidance. Recently, two different randomized comparative studies by Senkal and Sir [29] and Poutoglidou et al. [30] showed that the pain reduction and complication rates were similar between US and FL-guided caudal epidural injections. In the former study (N = 180) [29], the procedure time was shorter (P < 0.001), and the successful injection rate on the first attempt was significantly higher (P = 0.002) in US-guided caudal injection than with FL guidance, which can be included in the category 1 with the superiority of US. However, the latter (N = 45) [30] reported that the mean procedure time was higher for the FL-guided group, followed by the US-guided group. However, the differences were not statistically significant (P = 0.067), designating the US-guided caudal epidural injection in category 2 with the non-inferiority of US to FL. Considering the limitation of US during the caudal epidural injection, the needle tip location could not be precisely identified since it is advanced to the sacral hiatus after the sacrococcygeal ligament. Moreover, intravascular injections during the procedure could not be detected, which may occur in 10.9% of caudal blocks using digital subtraction angiography [31]. Therefore, the US-guided caudal epidural injection would be designated in category 3, showing the feasibility of US in conjunction with FL confirmation as suggested in a study (N = 120) by Park et al. [32].

Another controversy could arise in thoracic epidural catheter placement. Recently, Kim et al. [33] reported that the US-guided thoracic epidural catheter placement between the T10-11 interlaminar space using US (N = 38) aimed for postoperative pain management. In the study, the first-pass success rate was similar (US: 66.7% vs. FL: 68.2%) and the final success rate did not differ (98.5% with US vs. 100.0% with FL); however, the time to identify the epidural space (46 seconds vs. 59 seconds, P = 0.004) and procedure time (40 seconds vs. 113 seconds, respectively; P < 0.001) were significantly shorter in the US group, which might be designated in the category 1 with the superiority of US to FL. However, in the study, the position of the catheter and no intravascular or subarachnoid spread of the contrast medium was finally determined using the FL, suggesting the procedure should be in category 3 with the feasibility of US confirmed by FL.

Several US-guided procedures were introduced and compared with FL guidance in pain medicine. The four categories introduced in this review suggest the better device option between the US and the FL while conducting pain procedures. However, the definition of each category and the procedures classified into those categories could be changed with future well-designed comparative studies and the technical development of image-assisting devices. The US has revolutionized pain management techniques by becoming the "must-have" medical device in pain practice. Unless practitioners have experience with the basic sonoanatomy and are trained in the US-guided techniques for various pain procedures, such as peripheral nerve blocks, neuraxial blocks, and musculoskeletal injection, they may likely be working blind. There is a determined effort to investigate the variable usage of US, from possible adaptation for pain blocks to outcome studies that improve the success rate more than that using FL. Now is the time for practitioners to devote themselves to improving their knowledge of US-guided procedures, establishing recommendations for education and training, and encouraging clinical outcome studies.

The author feels nothing but gratitude for the staff members and colleagues at the Seoul National University Pain Management Center.

Data sharing is not applicable to this article as no datasets were generated or analyzed for this paper.

Jee Youn Moon is an editorial board member of the Korean Journal of Pain; however, she has not been involved in the peer reviewer selection, evaluation, or decision process of this article. No other potential conflict of interest relevant to this article were reported.

No funding to declare.

Jee Youn Moon conducted study design/plan, acquisition of data, data analysis, and writing the paper.

1. Narouze SN. Ultrasound-guided cervical spine injections: ultrasound "prevents" whereas contrast fluoroscopy "detects" intravascular injections. Reg Anesth Pain Med 2012; 37: 127-30.
   
2. Finlayson RJ, Gupta G, Alhujairi M, Dugani S, Tran DQ. Cervical medial branch block: a novel technique using ultrasound guidance. Reg Anesth Pain Med 2012; 37: 219-23.
   
3. Finlayson RJ, Etheridge JP, Vieira L, Gupta G, Tran DQ. A randomized comparison between ultrasound- and fluoroscopy-guided third occipital nerve block. Reg Anesth Pain Med 2013; 38: 212-7.
   
4. Narouze SN, Vydyanathan A, Kapural L, Sessler DI, Mekhail N. Ultrasound-guided cervical selective nerve root block: a fluoroscopy-controlled feasibility study. Reg Anesth Pain Med 2009; 34: 343-8.
   
5. Baig S, Moon JY, Shankar H. Review of sympathetic blocks: anatomy, sonoanatomy, evidence, and techniques. Reg Anesth Pain Med 2017; 42: 377-91.
   
6. Jung H, Bae J, Kim J, Yoo Y, Lee HJ, Rho H, et al. Can the rhomboid major muscle be used to identify the thoracic spinal segment on ultrasonography? A prospective observational study. Pain Med 2022; 23: 1670-8.
   
7. Finlayson RJ, Etheridge JP, Tiyaprasertkul W, Nelems B, Tran DQ. A randomized comparison between ultrasound- and fluoroscopy-guided c7 medial branch block. Reg Anesth Pain Med 2015; 40: 52-7.
   
8. Finlayson RJ, Etheridge JB, Elgueta MF, Thonnagith A, De Villiers F, Nelems B, et al. A randomized comparison between ultrasound- and fluoroscopy-guided sacral lateral branch blocks. Reg Anesth Pain Med 2017; 42: 400-6.
   
9. Petscavage-Thomas J, Gustas C. Comparison of ultrasound-guided to fluoroscopy-guided biceps tendon sheath therapeutic injection. J Ultrasound Med 2016; 35: 2217-21.
   
10. Shamshery C, Kumar VV, Agarwal A, Aggarwal A, Madabushi R. Comparative evaluation of efficacy of fluoroscopy and ultrasound for iliopsoas block: a randomised trial. Turk J Anaesthesiol Reanim 2021; 49: 284-91.
    
11. Hughey S, Schafer J, Cole J, Booth G, Tuttle R, Stedje-Larsen E. Ultrasound versus fluoroscopy for stellate ganglion block: a cadaveric study. Pain Med 2021; 22: 2307-10.
    
12. Jee H, Lee JH, Kim J, Park KD, Lee WY, Park Y. Ultrasound-guided selective nerve root block versus fluoroscopy-guided transforaminal block for the treatment of radicular pain in the lower cervical spine: a randomized, blinded, controlled study. Skeletal Radiol 2013; 42: 69-78.
    
13. Cui X, Zhang D, Zhao Y, Song Y, He L, Zhang J. An open-label non-inferiority randomized trail comparing the effectiveness and safety of ultrasound-guided selective cervical nerve root block and fluoroscopy-guided cervical transforaminal epidural block for cervical radiculopathy. Ann Med 2022; 54: 2681-91.
    
14. Wan Q, Yang H, Li X, Lin C, Ke S, Wu S, et al. Ultrasound-guided versus fluoroscopy-guided deep cervical plexus block for the treatment of cervicogenic headache. Biomed Res Int 2017; 2017: 4654803.
    
15. Ha DH, Shim DM, Kim TK, Kim YM, Choi SS. Comparison of ultrasonography- and fluoroscopy-guided facet joint block in the lumbar spine. Asian Spine J 2010; 4: 15-22.
    
16. Yun DH, Kim HS, Yoo SD, Kim DH, Chon JM, Choi SH, et al. Efficacy of ultrasonography-guided injections in patients with facet syndrome of the low lumbar spine. Ann Rehabil Med 2012; 36: 66-71.
    
17. Nisolle ML, Ghoundiwal D, Engelman E, El Founas W, Gouwy J, Guntz E, et al. Comparison of the effectiveness of ultrasound-guided versus fluoroscopy-guided medial lumbar bundle branch block on pain related to lumbar facet joints: a multicenter randomized controlled non-inferiority study. BMC Anesthesiol 2023; 23: 76. Erratum in: BMC Anesthesiol 2023; 23: 157.
    
18. Soneji N, Bhatia A, Seib R, Tumber P, Dissanayake M, Peng PW. Comparison of fluoroscopy and ultrasound guidance for sacroiliac joint injection in patients with chronic low back pain. Pain Pract 2016; 16: 537-44.
    
19. Bellingham GA, Bhatia A, Chan CW, Peng PW. Randomized controlled trial comparing pudendal nerve block under ultrasound and fluoroscopic guidance. Reg Anesth Pain Med 2012; 37: 262-6.
    
20. Fowler IM, Tucker AA, Weimerskirch BP, Moran TJ, Mendez RJ. A randomized comparison of the efficacy of 2 techniques for piriformis muscle injection: ultrasound-guided versus nerve stimulator with fluoroscopic guidance. Reg Anesth Pain Med 2014; 39: 126-32.
    
21. Kim DH, Lee MS, Lee S, Yoon SH, Shin JW, Choi SS. A prospective randomized comparison of the efficacy of ultrasound- vs fluoroscopy-guided genicular nerve block for chronic knee osteoarthritis. Pain Physician 2019; 22: 139-46.
    
22. Yang G, Liu J, Ma L, Cai Z, Meng C, Qi S, et al. Ultrasound-guided versus fluoroscopy-controlled lumbar transforaminal epidural injections: a prospective randomized clinical trial. Clin J Pain 2016; 32: 103-8.
    
23. Falsafi M, Baghianimoghadam B, Bahrami-Freiduni M, Esmaeilnejad-Ganji SM. Examining the accuracy of ultrasound-guided lumbar transforaminal injection controlled by fluoroscopic imaging in patients with lumbar radiculopathy: a modified technique. Turk Neurosurg 2021; 31: 582-6.
    
24. Thompson BF, Pingree MJ, Qu W, Murthy NS, Lachman N, Hurdle MF. Descriptive cadaveric study comparing the accuracy of ultrasound versus fluoroscopic guidance for first sacral transforaminal injections: a comparison study. PM R 2018; 10: 382-90.
    
25. Burnham TR, Smith A, McCormick ZL, Teramoto M, Burnham R. Evaluation of an ultrasound-assisted longitudinal axis lateral crest approach to radiofrequency ablation of the sacroiliac joint. Am J Phys Med Rehabil 2022; 101: 26-31.
    
26. Moon JY, Choi JK, Shin JY, Chon SW, Dev S. A brief report on a technical description of ultrasound-guided lumbar sympathetic block. Korean J Pain 2017; 30: 66-70.
    
27. Ryu JH, Lee CS, Kim YC, Lee SC, Shankar H, Moon JY. Ultrasound-assisted versus fluoroscopic-guided lumbar sympathetic ganglion block: a prospective and randomized study. Anesth Analg 2018; 126: 1362-8.
    
28. Abdelghaffar NA, Farahat TE. Fluoroscopic anterior approach versus ultrasound guided superior hypogastric plexus neurolysis in cancer pelvic pain: a randomized controlled study. BMC Anesthesiol 2022; 22: 403.
    
29. Senkal S, Sir E. Comparison of ultrasonography and conventional fluoroscopy guided caudal epidural injection in chronic low back pain. Turk Neurosurg 2021; 31: 119-23.
    
30. Poutoglidou F, Metaxiotis D, Vasiliadis AV, Alvanos D, Mpeletsiotis A. Caudal epidural injections in lumbar spinal stenosis: comparison of nonimage, ultrasonography-, and fluoroscopy-guided techniques. A randomized clinical trial. Perm J 2021; 25: 20.321.
    
31. Kim JY, Lee JS, Kim JY, Baek JW, Kim HS, Kim DH. Comparison of the incidence of intravascular injection using the Tuohy and Quincke needles during ultrasound-guided caudal epidural block: a prospective randomized controlled study. Reg Anesth Pain Med 2024; 49: 17-22.
    
32. Park Y, Lee JH, Park KD, Ahn JK, Park J, Jee H. Ultrasound-guided vs. fluoroscopy-guided caudal epidural steroid injection for the treatment of unilateral lower lumbar radicular pain: a prospective, randomized, single-blind clinical study. Am J Phys Med Rehabil 2013; 92: 575-86.
    
33. Kim DH, Lee JH, Sim JH, Jeong W, Lee D, Kwon HM, et al. Real-time ultrasound-guided low thoracic epidural catheter placement: technical consideration and fluoroscopic evaluation. Reg Anesth Pain Med 2021; 46: 512-7.