13 Minimizing Ionizing Radiation in Minimally Invasive Spine Surgery

All surgical procedures performed on the spine, minimally invasive or otherwise, require some form of radiographic imaging. Whether that imaging is cross-table radiography, fluoroscopy or intraoperative computed tomography (CT), radiation exposure is the inevitable consequence. Furthermore, in minimally invasive approaches, the absence of widely exposed anatomical landmarks by its very nature mandates another form of visualization. Before computer-assisted navigation, fluoroscopy assumed the role as an alternate form of visualization for spine surgery, but it rapidly took on a central role for minimally invasive spine surgery (MIS). As a result, radiation exposure was greater during minimally invasive procedures than during open procedures. At one point, greater radiation exposure seemed to be an unavoidable aspect of applying minimally invasive techniques. 1

Perhaps the greatest detriment to the increased use of fluoroscopy that accompanied the rise of minimally invasive surgery during the early part of this century was that the increased use did not parallel a proportionate increase in knowledge about the fundamentals of fluoroscopy. There was no concerted effort to increase radiation awareness or training. As mentioned in the previous chapter, I finished a residency without the faintest idea of how a fluoroscope generated radiation or any formal training as to how to adequately protect myself from radiation. I was never issued a dosimeter. I have no idea as to the amount of radiation I was exposed to throughout my residency. Wasn’t a lead gown all I needed?

Fluoroscopes are now ubiquitous both in residency training and in practice. Dosimeters are not. Although surgeons have acknowledged the greater radiation exposure with minimally invasive procedures, only recently has there been an appreciation for radiation exposure during residency training. ² I noted my concern about the greater amount of radiation exposure associated with minimally invasive techniques at the beginning of Chapter 12. The current body of medical literature would certainly support that concern. The question now becomes what can we do about it.

My focus for this chapter is twofold. First, I want to discuss a framework in which you can decrease your exposure to radiation when you perform surgical procedures requiring a fluoroscope. Second, I would like to present alternate techniques for the fluoroscopic unit that will decrease the amount of radiation exposure. Chapter 12 provided the fundamentals of fluoroscopy that will make understanding alterations in voltage and current settings within the X-ray tube understandable. My overarching goal is for the pendulum to begin to swing in the other direction. I want surgeons to relinquish the idea that a minimally invasive technique necessitates more radiation exposure. I want the spine surgeons of tomorrow to replace that notion with the concept that they can perform minimally invasive surgical procedures with less radiation exposure than ever before.

This final chapter began with a quote from Admiral Hyman G. Rickover, the father of the nuclear navy. When the topic is radiation exposure and awareness, there may not be a better authority than Rickover. Again, it was my all-to-brief service in the nuclear navy as a diving medical officer and radiation health officer that led directly to my interest in the radiation exposure that can occur in minimally invasive approaches. Admiral Rickover transformed the diesel-powered U.S. Navy into a fleet of nuclear-propelled submarines and aircraft carriers. Perhaps his greatest legacy is the number of nuclear reactor accidents that have occurred among the 200 nuclear submarines and 23 nuclear-propelled aircraft carriers and cruisers commissioned since the birth of the nuclear navy with the launch of the U.S.S. Nautilus in 1954. That number is zero. Admiral Rickover created a culture of caution, awareness and precision when dealing with radiation and nuclear power. Perhaps grafting elements of Rickover’s culture into MIS would be transformative. In previous chapters throughout this Primer, I examined the experience of master surgeons from decades ago to establish a basis for the various minimally invasive procedures we perform today. In a similar manner, I would like to reference Admiral Rickover’s management objectives as a framework for a minimally invasive spine surgeon’s work with radiation.

Rickover’s management objectives:

This law of physics governs not only sound but also light and radiation. Therefore, the intensity of radiation is inversely proportional to the square of the distance from the source of the radiation. When the X-ray tube of a fluoroscope emits radiation for an image in surgery, the radiation travels through space from that point source just like the sound of the middle C key from the piano in the empty room. Geometric dilution occurs in an isotropic fashion as the radiation travels through space ( ▶ Fig. 13.1). The intensity of that radiation plummets as the distance from the source increases. The inverse square law is perhaps the most important principle you need to embrace when considering dose limitation. Whenever possible, always distance yourself as much as possible from the radiation source.

Radiation-Protective Apparel

At this point in the chapter, you might find it surprising to learn that the most important aspect of dose limitation is not wearing your apron or thyroid shield but wandering as far away as possible from the X-ray tube as it acquires an image. Still, the inverse square law does not supplant the importance of radiation protective apparel. Wearing radiation-protective gowns and thyroid shields is always inherent to dose limitation. Note that I have intentionally refrained from using the term lead, which has become the operating room vernacular for radiation-protective gowns and shields. In the interest of preserving the spines of persons who need to wear such apparel for hours at a time, manufacturers have developed lighter alternatives to leaded gowns, most of which contain no lead at all. Whether or not your radiation-protective gown contains lead, there are few better investments to make than to purchase your own radiation-protective gear, including a pair of radiation-protective glasses.

13.4 Dosimetry

I completed my residency without the faintest idea about the extent of my radiation exposure throughout my years of training. At that time, residents in my training program were not issued dosimeters. However, had it not been for the dosimeter the Navy issued me upon my arrival at the Naval Hospital in San Diego, I would never have been able to juxtapose those dosimetry readings with my dosimetry readings aboard the U.S.S. Buffalo, a fast-attack nuclear submarine. Without that glaring difference in those data points, my concern about radiation exposure would never have arisen. In the absence of those data, I would never have conceived of a need to compose the final two chapters in this Primer. A dosimeter provides valuable data regarding radiation exposure. Knowledge of the data increases awareness and potentially changes behavior. Every surgeon exposed to radiation, whether in training or in practice, should have access to a dosimeter. The decision to wear and monitor that dosimeter is ultimately left to the surgeon, but the capacity to monitor exposures must be there.

The purpose of the dosimeter is to monitor exposure in the absence of a radioprotective garment. For this reason, the dosimeter should be worn outside the apron or thyroid shield. As I learned from my time in the Navy, a dosimeter is only worth the data that you can extract from it. Wearing a dosimeter does not in and of itself ward off the downstream consequences of radiation exposure. Ideally, the data from the dosimeter are collected and recorded on a quarterly basis and on an annual basis. You should strive to decrease radiation exposure year after year in your practice. Accomplishing this task would be impossible without having those values laid out before you.

In Chapter 12, I explained how an X-ray tube generates X-rays that then make their way through the patient to the image intensifier to generate an image. What I did not explain is that those X-rays that leave the X-ray tube do not all make it across the patient. In fact, they have one of three fates: they will pass directly through the patient, they will become completely absorbed by the patient or they will interact with tissue and bone to reflect and scatter. Those X-rays that pass directly through the patient with no interaction are directly responsible for contributing to the image of the spine that appears on the screen. The appropriate title assigned to these X-rays is image-forming X-rays. Obviously, the X-rays that are completely absorbed by the patient contribute nothing to the image. The X-rays that reflect and scatter, a phenomenon known as Compton interaction, take away from the image. Scattered X-rays decrease the contrast and result in a grainy appearance to the image. Even more important is that the reflected and scattered X-rays contribute to the radiation exposure to the surgeon and operating room staff. That radiation exposure is obviously greatest for persons standing on the side of the X-ray tube where the beams reflect from the patient immediately onto the surgeon and other operating room staff; it is significantly less for persons standing on the side of the image intensifier ( ▶ Fig. 13.2).

In a perfect world, only the X-rays that pass through the patient with no interaction would be the ones that reach the image intensifier. But complete absorption and Compton scatter are an inevitable consequence of X-rays interacting with matter. Thus, we are faced with creating an environment wherein we optimize image-forming X-rays that contribute to the image while minimizing scattered X-rays that take away from the image. One way to optimize the image-forming X-rays and to minimize the scattered X-rays is to narrow the field of radiation. The term used to describe this is collimation.

Collimation is the process by which shutters within the fluoroscope narrow the field of view, analogous to an aperture on a camera limiting the light that enters the lens ( ▶ Fig. 13.3). The result of collimating an X-ray beam is twofold. First, a collimated X-ray beam decreases the surface area of the patient exposed to radiation, sparing unnecessary exposure to adjacent tissue. Second, the less of the patient’s volume that is exposed to the X-ray beam, the less scatter there will be of X-ray incident to the detector. Reducing Compton scatter improves image contrast. The net result is less radiation exposure to the patient, less X-ray scatter to the surgeon and operating room personnel and a high proportion of image-forming X-rays. Collimating the X-ray beam will result in less radiation and a higher-quality image. As a rule, I always collimate to the smallest field of view for the segment. The capacity to collimate an image is available in almost all commercially available fluoroscopes.

13.6 Justification

Each time that you decide to acquire an image, you should be looking for information that can be provided only by a fluoroscopic image. For instance, for the placement of pedicle screws through an expandable minimal access port, the fluoroscopic image need not identify the pedicle screw entry point. Rather, it should confirm the pedicle screw entry point you have identified by your exposure of the anatomical landmarks. There is a difference. The goal is to shift your reliance away from fluoroscopy and toward direct visualization, which is the basis of my argument against percutaneous techniques. When you have direct visualization of the anatomy, the three-dimensional anatomy of the spine that you reconstruct in your mind is more valuable than any fluoroscopic image that can be acquired. After you have clearly visualized the anatomical landmarks and established the entry point, a fluoroscopic image is of value in confirming the entry point and establishing the ideal trajectory into the pedicle.

Confirmation of the operative segment is one instance in which the fluoroscopic image is the only modality that can provide the information needed to move the operation forward. However, subsequent images are of limited value. I have watched residents and fellows acquire image after image when doing nothing more than passing a dilator onto the spine at the same location for a microdiscectomy, a laminectomy or an instrumented fusion, whereas one image of the initial dilator was all that was needed until the insertion of the minimal access port. Those additional images were unnecessary because they provided no actionable information to justify taking them. I have also watched those same residents and fellows request an image with every quarter-turn of the tap. When I ask them what it is they are looking for, I have yet to hear a satisfactory answer. Once you have probed a pedicle and established a trajectory into that pedicle with a fluoroscopic image, no further imaging is necessary until the threads of the tap are buried and you would like to determine the optimal length for the pedicle screw. As noted in Chapter 4 on transforaminal lumbar interbody fusion (TLIF), after capturing an image with the instrument in the ideal position, I maintain that position relative to the blades of the minimal access port as the instrument advances. If that position remains unaltered, subsequent images do not provide actionable information. However, an image with the threads buried will provide valuable information to help you determine the length of the pedicle screw that is needed. Images of the tap advancing along the same trajectory through the pedicle are of limited value and are not in keeping with the justification component of the ALARA principle.

It goes without saying that the safety of the patient should be your first and foremost concern. You need to acquire the imaging that is required to perform the procedure safely. I will readily acknowledge that the initial part of the minimally invasive learning curve does involve heavy reliance on fluoroscopy. Over time, however, as you develop the capacity to reconstruct the anatomy in your mind’s eye, it is essential to transition to mission-essential fluoroscopy. In keeping with the theme of raising levels of adequacy, minimizing the number of fluoroscopic images for every surgical procedure (minimally invasive or otherwise) should be a central theme in your practice. The capacity to do more in surgery with less fluoroscopic guidance is a skill all its own and, as such, should be actively developed. The tactile sense of a dilator or the direct visualization of the anatomy can replace the void that a fluoroscopic image would otherwise fill. Before acquiring each image, ask yourself what information you are looking for and what you will do with that information. Justification for every image in every instance over time, and certainly over a career, is essential to reducing radiation exposure.

The inability to directly visualize the anatomy while placing pedicle screws percutaneously mandates imaging at every step of the placement. When computer-assisted navigation guides the percutaneous placement of a pedicle screw, radiation exposure to the surgeon and the operating room staff becomes a moot point. However, in the absence of image guidance, radiation exposure begins to rise. The published literature documents a range of 22 to 29 seconds of fluoroscopy with a dose of up to 10.3 mREM per percutaneous screw in experienced hands.3,4​ Furthermore, the very nature of the procedure mandates the surgeon’s proximity to the X-ray source, holding instruments, thereby limiting the capacity to harness the inverse square law to the surgeon’s advantage. The inevitable consequence is exposure to greater amounts of radiation exposure for fluoroscopic placement of percutaneous pedicle screws compared to that for pedicle screw placement using direct visualization of anatomical landmarks with intermittent fluoroscopic guidance. Coupled with the inability to directly visualize the facets for a Smith–Petersen osteotomy or the transverse processes for a posterolateral fusion, this greater radiation exposure has made it increasingly difficult for me to use percutaneous screws for single-level and two-level fusions.

13.8 Optimization

In 1984, Wesenberg and Amundson ⁵ published one of the first articles detailing a low-dose protocol for the use of fluoroscopy. Their focus was the radiation exposure in children undergoing diagnostic imaging. At the root of their concern were the radiation doses and fluoroscopy times required for specific procedures (e.g., cardiac catheterization, cystourethrograms and upper gastrointestinal evaluations). Wesenberg and Amundson wanted to reduce these radiation exposures because of the inherent risk of radiation causing subsequent malignancies in growing children. Their 1984 manuscript begins with a thoughtful sentence that is still as relevant today for MIS as it was then for pediatric imaging: “the goal of diagnostic radiology is to make the most accurate diagnosis with the least amount of radiation and risk to the patient … this is particularly applicable to pediatric radiology.”⁵

Over the years, subsequent authors continued to develop techniques to minimize radiation exposure while still providing adequate diagnostic imaging for children. The pediatric cardiology literature was especially focused on this topic.^6,7 What is striking as one juxtaposes the pediatric radiology literature and the MIS literature during the same period is how diametrically opposed were the trajectories on the same topic. Pediatric radiologists kept documenting remarkably reduced exposures, whereas minimally invasive spine surgeons kept publishing reports of the exact opposite.^3,6,7,8,9 As the concern about radiation exposure in minimally invasive spinal procedures became a barrier to the application of the techniques, taking a page out of the pediatric radiology playbook was the logical step forward. In fact, when spine surgeons finally applied the well-honed fluoroscopy protocols from the pediatric radiology literature, radiation exposure and fluoroscopy times both decreased significantly without compromising the safety of the procedure.^10,11

The remainder of this chapter focuses on exploring these low-dose protocols in the context of the final element of the ALARA principle: optimization. In the upcoming pages, I will discuss the techniques that pediatric radiology has used for decades to decrease radiation exposure with fluoroscopy. Applying these protocols will require an understanding of the types of fluoroscopy and especially the importance of acquisition time. Incorporating your understanding of peak kilovoltage (kVp) and milliampere (mA) into these protocols will serve as a background to help you harness these parameters to your advantage. You will discover that doing nothing more than altering the fluoroscopic technique or manually adjusting the kVp and mA will have a direct impact on both the radiation exposure required for each image and the quality of that image. In imaging the spine for minimally invasive spine procedures, we would do well to adopt the same attitude that Amundson and Wesenberg had more than three decades ago toward pediatric patients. Striking a balance between radiation exposure and image quality is the essence of optimization.

13.9 Types of Fluoroscopy

Let us return to the operating room, where you are waiting for the fluoroscopy technologist to turn on the fluoroscope so that you can begin a procedure. When the technologist finally plugs in the fluoroscope and turns it on, software begins to load in the central processing unit (CPU) and the thoriated tungsten filament begins to warm up, preparing for the surge of voltage coming its way. As the progress arrows populate the screen, the fluoroscopy algorithms are loaded. These are written in a manner so that when it is finally time to hit the button and obtain an image, the kVp and mA will automatically be determined such that the result will be an optimal image with ideal brightness and contrast. The type of fluoroscopy that accomplishes this ideal image is known as continuous interlaced fluoroscopy, or just continuous fluoroscopy. It is the default setting that happens simply by turning on the fluoroscope. In all likelihood, this setting is the one you have been using all along for your operations.

Continuous fluoroscopy offers a reliable image without having to manipulate any of the parameters. As the name implies, the X-ray beam is continuous. When the technologist pushes the button, that continuous X-ray beam generates 30 images per second that become interlaced to create the composite image. The kVp and the mA are initially determined by an algorithm within the CPU. The automatic brightness control (ABC) further adjusts these parameters using the patient’s size through a feedback mechanism. A sensor inside the image intensifier monitors the brightness. When the brightness of the image is inadequate because of the size of the patient, the ABC algorithms increase the kVp first. The result is an increase in the number of projectile electrons striking the tungsten target, which, in turn, increases the number of diagnostic X-rays. The net increase in the number of higher-energy X-rays results in a greater proportion of X-rays that can better penetrate the body and serve as image-forming X-rays. The cervical spine has narrower dimensions than the lumbar spine, so the need for penetration is less. As a result, imaging in the cervical spine requires a lower kVp.

After the CPU establishes the kVp, the algorithm increases the mA. An increased tube current will increase the number of electrons striking the target and result in a greater total number of X-rays. The net result is a brighter image. Since these ideal images rendered by continuous fluoroscopy are contingent on the completion of the feedback loop, the result is a 1- to 3-second acquisition time per image. The first image that is obtained will typically have the longest acquisition time as the feedback loop optimizes the brightness and contrast by adjusting the kVp and mA. After the ABC has optimized the image quality, subsequent images will have shorter acquisition times, provided the fluoroscopy unit has not been moved. If you move the fluoroscope and the thickness of the patient changes, the ABC feedback loop will have to complete, which will again extend the acquisition time.

When you consider an acquisition time that ranges from 1 to 3 seconds per image, the total fluoroscopy time and exposure have the potential to quickly add up. For example, if you were to obtain 40 images throughout an instrumented lumbar fusion case using this technique, fluoroscopy times would range from 40 to 120 seconds, which is well within the reported range of fluoroscopy times for minimally invasive fusions.4,8,12Many authors have understandably considered that amount of radiation exposure to be a liability and, for obvious reasons, it has become one of the main criticisms of minimally invasive techniques. One approach to dealing with radiation is to use computer-assisted navigation, which can decrease radiation exposure.³ Another approach is to use the alternative fluoroscopy technique introduced by pediatric radiologists. Fully embracing that decades-old technique alone can quickly dispel those criticisms.

13.10 Pulsed Fluoroscopy

The less time the X-ray beam is on, the less radiation exposure will occur to the patient, the surgeon and the operating room personnel. Two simple modifications to the settings on the fluoroscope will accomplish this task. The first step is to turn off the ABC. The absence of this control eliminates the feedback loop and the additional seconds of radiation exposure that are required to complete the loop. However, it also excludes the algorithm that automatically determines the ideal kVp and mA to optimize the image. Disabling the ABC is equal parts asset and liability. Every image will be acquired at the default kVp and mA settings. The drawback may be the need to manually increase or decrease these values to view an adequate image. The advantage is a substantial decrease in radiation exposure. Working closely with your radiology technologist to determine these settings will allow you to acquire an adequate image at the lowest amount of radiation exposure.

The second modification is to intermittently turn off the X-ray beam during image capture. Instead of having a continuous beam for image capture, turning the X-ray beam on and off will decrease the amount of radiation exposure. Such a setting is known as pulsed fluoroscopy ( ▶ Fig. 13.4). X-ray beam pulsation obviously decreases radiation exposure, but it also decreases image quality. Two key parameters guide X-ray beam pulsation: pulse width and pulse rate. The pulse width is the duration of the pulse itself, specifically how long the X-ray beam is on for image capture before turning it off. A shorter pulse width has the obvious advantage of less radiation, but it also limits the number of X-rays that contribute to image formation. The fewer the number of image-forming X-rays penetrating a patient, the lower the quality of the image and the lower the radiation exposure.

The pulse rate is the number of fluoroscopic image frames that are generated per second. For imaging a static subject, such as the spine, the pulse rate is less important. High pulse rates are necessary to study anatomy in motion, such as the assessment of diaphragmatic motion, a cardiac valve, or esophageal motility. The phenomenon known as temporal resolution is crucial for studying anatomy in motion. Since our subject matter is the motionless spine, there is little need for temporal resolution. Therefore, lowering the pulse rate will not have a significant effect on the image of a static subject. Applying the pulsed fluoroscopy setting will dramatically reduce radiation exposure for surgical procedures on the spine.¹⁰ The combination of turning off ABC with pulsed fluoroscopy will further reduce radiation exposure by decreasing acquisition time in addition to the dose per frame. The trade-offs for these low doses and short acquisition times are the quality and the sharpness of the image. The concepts of pulse width and pulse rate are demonstrated in the accompanying video (Video 13.1).

13.11 Acquisition Time and the Digital Spot Technique

An analogous scenario to continuous fluoroscopy and even to pulsed fluoroscopy would be taking a photograph using your smartphone. Instead of the photo option, you select the video option. With activation of the video capture, the camera begins by focusing on the subject. The sensors provide feedback mechanisms within the smartphone camera to adjust the brightness and the contrast of the image before finally rendering the perfect image. Over the span of 3 seconds of video, the very last few milliseconds have perfect contrast, brightness and focus. Continuous interlaced fluoroscopy renders an image in a similar manner. The reality is that we do not use most of the footage. The final frame is all that we actually view, but the first few seconds are necessary because of the process of acquiring the image. The adjustments that occur in brightness, contrast and focus occur in the first few moments of image acquisition. For that single image, the acquisition time was quite long, and the number of frames captured for that final few milliseconds of footage was quite high. The logical question is why we would want to acquire a static image in this manner. Why would we not use the photo option with the instantaneous acquisition of a single frame?

Remarkably, the process described earlier is exactly how most of us use conventional fluoroscopy as we operate on the spine. Images created with continuous fluoroscopy default to the “last image hold,” which is the image that remains on the screen when the ray beam turns off. That image is the integration of multiple frames. The radiation exposure for that last image hold is the number of frames times the radiation exposure for the acquisition of those frames.¹³ In reality, we are capturing hundreds of unnecessary frames in a video format to obtain one static composite image at the end of a feedback mechanism. Along the way, we are increasing our exposure to radiation. Instead, our goal should be to capture an image of the spine with the fewest frames and with the shortest possible acquisition time. Thus, our real goal should be to take an image of the spine with a photo setting instead of a video setting.

However, there is a reason why fluoroscopy obtains 30 frames per second to generate an image. The amount of noise in an image is inversely proportional to the square root of the number of photons used to acquire that image. As noted in Chapter 12, the number of photons that create an image is proportional to the number of diagnostic X-rays striking the image intensifier. Because of the internal lag characteristics of the human eye–brain response, a real-time sequence of an image generated at 30 frames per second is perceived with much less noise than a single static fluoroscopic image. In other words, the composite image of three to five frames of fluoroscopy will appear sharper and brighter than a single frame of fluoroscopy.

Digital spot imaging solves the problem of acquisition time and the need to create a composite image ( ▶ Fig. 13.5). It is the equivalent of taking a photograph with the photo setting instead of the video setting. Digital spot images are immediate exposures, but the consequence of taking a single exposure is that the process requires a much higher dose of radiation than that required for a single frame of fluoroscopy. However, it is a single frame captured at a low acquisition time (typically, 0.15 seconds) with much better image quality and resolution than a pulsed fluoroscopy image. Although there is a considerable amount of radiation in a single digital spot, it must be balanced with the number of frames acquired to have an equivalent view of the anatomy with fluoroscopy. When viewed in that context, digital spot images can compare quite favorably, especially when a sharper image is needed for instrumentation.

Related posts:

Minimally Invasive Posterior Cervical Laminectomy Minimally Invasive Far Lateral Microdiscectomy Minimally Invasive Resection of Intradural Extramedullary Lesions within the Thoracic Spine Minimally Invasive Lateral Transpsoas Interbody Lumbar Fusion Anterior Cervical Discectomy with Arthroplasty or Fusion Minimally Invasive Transforaminal Lumbar Interbody Fusion

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