The lumbar and sacral regions form an integral part of the spine and the body. Routine projections on the two regions are imperative in the diagnosis of conditions affecting the spine. This essay will shed light on how normal patient anatomy of the spine and surrounding areas influence the radiographic techniques utilized in obtaining routine projections of the lumbar and sacral spine.
The lumbar vertebral column has a total of five vertebrae named according to their positioning in the entire column. The vertebrae are numbered first to fifth starting from the topmost downwards. Their main function is to absorb shock while allowing movements within the spine. This is executed through the compression of the discs between the vertebrae, especially during movement, thus leading to the narrowing of the intervertebral joint space. However, the vertebrae are positioned in a way that none can come into contact with another ((Bogduk, 1997, p. 15-38). Typically, the lumbar vertebra is grouped into three functionalities which include the pedicels, vertebrae body, and the posterior elements. All three components are vital in the achievement of the integrated function of the vertebrae. The vertebrae body is responsible and is well designed for the weight-bearing of the vertebrae. On the other hand, the pedicels form the single most important connection between the vertebrae bodies and the posterior elements. Pedicels are overly designed to withstand the effects of the bending and the antagonistic bending forces. The lumbar spine is also surrounded by a group of muscles that serve diverse functions (Bogduk, 1997, p.96).
Runge (1987, p. 126-149) noted that the lumbar vertebrae, composed of five vertebrae, forms the beginning of the second secondary curvature in the spine. This structure is what is commonly referred to as the ‘lumbar lordosis’ (Bogduk, 2005, p51). The lumbar lordosis has an anterior convexity whose shape is largely due to the positioning and shape of the intervertebral discs and lumbar vertebrae (Bontrager & Lampignano, 2005; Bogduk, 1997, p51). The angle of tilt of the base of the sacrum, important in maintaining the body’s upright body posture, partly contributes to the anterior convexity evident in this section (Middleditch & Oliver, 2005 p3).
The ‘Lumbar Lordosis’
The lumbosacral intervertebral disc takes mainly the shape of a wedge with its anterior portion 6-7mm larger in height than its posterior part (Bogduk, 1997, p51). According to Gay et al, L5 has a slight wedge shape that has a characteristically slightly shorter posterior wall compared to the anterior wall of the body (2005, p.914-916). In addition, the sacrum is located between the ilia with a distinct curvature that is opposite to that found in the lumbar spine. The pelvic surface takes a concave shape on the front side. The shape of the sacrum is mainly affected by its weight-bearing and lumbar support functions (Bogduk, 1997, p. 59-60). According to Runge (1987) and Bogduk (1997), the above functions make it have an anterior tilt with the base taking an inclination of between 42-45 degrees about the mid-sagittal plan, particularly when the person’s body is at supine posture. Further research has noted that a marginal increase in the angle of inclination may occur, especially when the posture of the patient’s legs is altered. L5 is projected forward owing to the fact that there is the articulation between the base of the sacrum and the fifth lumbar (Eizenberg et al, 2007). Eizenberg et al (2007) asserted that the projection is counterbalanced by the inclination evident in the vertebrae in the upper part. The combined impact of the inclination and the projection in the vertebrae results in the formation of the lumbar curvature starting from the downward side of the spine (Eizenberg et al, 2007, p. 167).
The sacrum generally refers to the enormous blocks of bone situated at the periphery of the vertebrae column on the downward side. Its design ensures that the lumbar vertebrae column is supported while loads are transmitted particularly from the trunk to the lower limbs with the help of the pelvic girdle. The triangular shape combined with the curvature makes it unique from the other bones. The sacrum has perforations both in the posterior and anterior surfaces. Moreover, the sacrum has massive design features that ensure it is held in the pelvis surrounded by the ilia. This allows the transfer of axial forces, in a lateral direction, to the limbs (Bogduk & Twomey, 1991). A larger portion of the sacrum is contained in the bodies and also in the transverse elements of the uppermost segments (Bogduk, 1997, p. 61). The sacrum is of particular importance in radiology due to its location in a sensitive part of the body. The presence of the gonads makes it a herculean task for the radiographers.
Gonads are generally found in the anterior part of the body of both sexes. This means that utmost care must be taken to ensure that the radiation does not affect them. Projections particularly those that are undertaken from the anterior predisposes the humans to x-ray radiations hence the need for protection.
Importance of lumbar-sacral region in radiography
Radiography images taken using different angles are sometimes affected by the anatomy of the patient (Ball & Price, 2005, p. 243). The routine projections of the lumbar vertebrae and the sacral spine are also influenced by the composition of the surrounding body environment. Clinical image quality may be altered especially when the characteristic curve in a radiograph image is influenced in different ways. The various forms of routine projects are not spared either (Tingberg, et al, 2004, p.1). The variation in the sizes of the shoulders and pelvic regions in females and males has implications on the anatomy of the spine. Of particular importance, is the broad shoulder that ensures men have a declining spine at Cranio-cordal length. The broad implications of these anatomical features to radiography and imaging can only be elaborated by looking at specific projections. In this essay, the ante-posterior and lateral projections will be discussed and analyzed in depth.
Antero-posterior (AP) Projection of the lumbar spine
According to Carver & Carver (2005), a routine anteroposterior (AP) projection of the lumbar spine is carried out with the sole aim of displaying the image of L1-L5 vertebrae and inter-joint space. The projection requires the patient to lie in a supine position, with knees and ankles flexed and feet kept flat in the examination table, thereby enhancing flattening of lumbar lordosis maximally (Sutherland, & Thomson, 2007, p. 145). It is therefore worth noting that positioning of the spine so that it gets closer to the image receptor results in the reduction in the image magnification and to a large extent enhances the geometrical sharpness (Ryan et al, 2003). The AP projection poses the greatest challenge due to the propensity of the gonads to be in closer proximity to the source of x-rays. The posterior-anterior projection is therefore preferred or application of lead protection. The use of posterior-anterior projection as an alternative is paramount in the reduction of the risks associated with this form of projection. The choice of PA projection is informed by the risks posed by the other projections. The guarantee of patient comfort and lowered health risks also influences the choice of this method. More importantly, the anatomical configuration of the joint interspaces ensures that diverging beam achieves better imaging in PA than in AP projection (Sutherland, & Thomson, 2007, p. 145).
Lateral projection of the lumbar-sacral spine
In this projection, the patient is made to lie on their side thus allowing the sagittal plane to be parallel with the image receptor (Whitley, 2005, p.132). This is accomplished by the utilization of foam pads under the waists in women patients, thereby raising their upper-lumbar section, to compensate for the wide pelvis. The shoulders and the hips directly influence the method of routine projection to be implemented. This is due to the tendency of the males having a narrower and deeper pelvis compared to the wider and shallower in females. This means that the exposure factors in women must be applied at higher optimum levels to compensate for the increased OFD caused by their wide hips. The curvature resulting from the raise brought about by the hips and shoulders must be minimized by the application of foam pads thereby enhancing the penetration of the central ray. In this case, lateral projection is preferred because of the availability of modification measures to the anatomy thereby achieving the desired patient setting (Cockbain & Kirby, 2008).
The position of the sacrum in the pelvic region necessitates the usage of lateral projection. This is informed by the positioning of the ilia, which requires to be penetrated by the x-ray beam on both sides, for it to be demonstrated on the image receptor. The shoulders offer some difficulty in the application of lateral projection. The use of foam pads helps in the realignment of the lumbar region thus conforming to the sacral part. Overall, lateral positioning helps in the avoidance of magnification and geometrical inaccuracies that are brought about by tube angulation. Moreover, a wedge filter may be required to enhance the display of L5 in the lateral projection, which is obscured by the high density of the pelvis in relation to that of the abdomen. Another corrective measure would be to increase the FFD in order to remove the OFD brought by the positioning of the sacrum (Ballinger et al, 2003, p.245).
The lateral projection of the sacrum is also done to improve the display achieved by the other projections. The sacrum shape and its positioning offer difficulties to the radiographer. The use of a wedge filter is therefore important to balance beam attenuation brought about by the positioning between the ilia and its shape. The increased object film distance due to the positioning thereby allowing for the raise in focus film distance (Bontrager & Lampignano, 2005).
Anterior-Posterior Projection of the sacral spine
An anterior-posterior projection is a better way to display the sacrum compared to other projections. Carver & Carver (2006) asserted that an AP reduces the OFD due to its location at the posterior of the sacrum. Although the cranial tube angulation varies depending on the physical build of individuals, a range of 10-25 degrees is mainly recommended to strike the sacrum orthogonally (Carver & Carver, 2006 p175). More importantly, the angle of the curve of the sacrum must be utilized in the determination of the angle of the central ray (Bontrager & Lampignano, 2005) while a lateral projection is advised in order to assist in the assessment of the tube angle required for the AP (Carlton et al, 2000). After the determination and assessment of the tube angle, the diverging x-ray beam will pass through the sacral foramina resulting in their display on the final image (Bontrager & Lampignano, 2005). Magnification of the image occurs in AP projection due to the lack of correct alignment of the central ray to the image receiver and pelvic surface. This is partly corrected by taking the AP projection when the patient is in a supine position thus minimizing the OFD of the image and ultimately lowering magnification and geometric inaccuracies. It is worth noting that the location of the sacrum and the physical build of the patient are the major determinants of the diagnostic method. The tendency of the diverging beam passing through the sacral foramina makes PA a better projection. More importantly, the curvature in the sacrum is imperative in the generation of a successful anterior-posterior projection. The resultant image is well displayed due to the combination of the right conditions (Carlton, Adler & Burns, 2000). The gonads are also exposed to more x-ray radiations due to the closeness to the source. In this case, lead protection is recommended around the waist region.
Routine projection of the lumbar-sacral regions is imperative and common in the diagnosis of medical conditions. The anatomy of the lumbar-sacral region and its surrounding greatly impacts the choice of the routine projection. The curvatures and the interspaces combined with the positioning of the hips, gonads, and shoulders play a major role in arriving at the best option. Lateral and posterior-anterior projections are preferred at the hip region due to their compatibility with the anatomical challenges offered by the pelvic region. The radiologist needs to take into consideration all the anatomical features before choosing the best option of projection since this enhances the final imaging.
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