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- Daniele Balsamo
The image repeat is a lateral lumbar spine. The lumbar spine protocol is usually part of a three view, anterior posterior (AP) a lateral and a C5 – S1 spot image. For trauma, often is requested one lateral to assure that there is no risk to move the head of the patient. A complete sequence includes also to two oblique’s (LAO and RAO) to visualize the zygapophyseal joint space. .
For a proper lateral spine positioning, the radiographer places the patient either standing or recumbent left lateral. Shoulder and ASIS should be aligned vertically through the midcoronal plane, to assure there is no tilt or rotation, the thoracic and lumbar spine should be aligned parallel to the table, to assure no rotation. A contact shield should be placed on the table lateral to the spine to reduce back scattering. A contact shield should be placed on the gonad area if does not interfere with area of interest. The spine should be aligned parallel to the longitudinal line of the collimation light. Ask the patient to suspend their breathing on expiration. The essential anatomy is, longitudinally T-12 to sacrum, laterally including the soft tissue posteriorly to the spine and all the body of the vertebrae anteriorly. Anatomy best demonstrated is the spinous process, the intervertebral disk space and the vertebral body (McQuillen Martensen, 2015, p. 431).
Because different body habitus can affect the position of the lumbar spine, depending of the body fat or muscle in the hip area, a special attention is required in positioning the spine laterally. The leg had to be flexed to reduce the natural curvature of the hip, a sponge or a towel is placed superiorly to the iliac crest to the sagging of the spine due the position on a flat table. The sponge allows to keep the natural curvature and the spine parallel to the image receptor (IR.)
Different pathology can affect the lumbar spine imaging. Scoliosis is a curvature of the spine to the left or the right side. Lordosis is an accentuated curvature of the spine toward the anterior aspect. Can be reduced flexing the knee and bringing L5 and sacrum more parallel. Will be visualized in AP projection with the intervertebral disc space look closed. If the patient has a lateral curvature (scoliosis) of the spine, should be placed in a lateral position with the curvature toward the IR to keep the intervertebral disk space as open as possible, the center ray should be going through in the maximum curvature point referencing the AP projection.
Spondylolysis is a fracture of the pars interarticularis in the vertebral arch, visualized in the oblique projection and appear as a Scotty dog collar. Spondylolisthesis is the slide forward and down of one vertebral body to another inferiorly. These pathologies could limit the motion range of the patient, but they don’t affect the positioning technique.
Central ray should be on L3, positioning one inch a half superior to the iliac crest. The area of interest is from the first lumbar spine to the intravertebral space between the fifth lumbar spine and the sacrum. The beam is perpendicular to the IR and the distance from the tube is 43 inch.
The system is a DR Siemens system, with a fluoroscopy table, and a wall bucket that uses a rail moved x- ray tube. The exposure indicator is express in EXI, the range is 150 under exposed, 250 optimal exposure, 500 overexposed. The system use two method to evaluate the exposure, the first is the setting of the proper anatomic part and body habitus of the patient. The radiographer sets the anatomic part and the body habitus on the display, the machine uses an automatic programmed technique (APT), that follows predefined anatomic chart sets by the manufacturer. Next the machine then uses the automatic exposure control(AEC) to determine the proper exposure. When a certain amount of remnant x-rays are hitting the ionic chamber in the IR, the exposure is terminated. The AEC gives consistency between exposures if the positioning of the anatomy on the right photo cell is correct.
For the AP projection of lumbar spine, the EI is 367, the exposure factors are 75 KVp, 189 mAs, For the left anterior oblique is Exposure Index (EXI) is 290, 77Kvp and 200 mAs. The EXI for the right anterior oblique is 205, with 65 Kvp 98 mAs. The left lateral EXI is 258,near to the optimal 250, the 90Kvp and 75 mAs . The left lateral repeat has an EXI of 260 with a 90 Kvp and 83 mAs .
The radiographer can evaluate the image visually using the magnify lens in the image Pac viewer. When the quantum noise, is visible in the image and the anatomic edge are blurry this indicates an underexpose image. For an overexpose image the contrast is high and the image has very minimal shades of gray
In the DR Siemens system used for this procedure the image receptor size is a 17 by 17. The collimation is done by using the beam limitation device in the machine. The aperture diaphragm in the tube housing can be manipulated by the radiographer with two controls and the light will resize proportionally to the exposure field. In a standard lateral spine, the collimation should be height wise 17″ and lengthwise 8′.
Appropriate shielding in lateral spine should be considered when is possible use a contact shield over the gonads, without superimposing essential anatomy.
There is no requirement of patient preparation for the L-spine x-ray, except for the appropriate clothes, or hospital gown to avoid artifacts. Other patient artifact could be jewelry, belly piercing, or underwear with sparkle or other metallic materials. Other artifact can be generated by patient movement.
System artifact can be grid related and software related. Grid related are: grid cut off, when the center ray is not centered on the image receptor and not parallel to the grid, the result is line artifact visualized through all the image. Software related are the use of wrong algorithm and wrong anatomic chart, this create a wrong image compensation and can enhance details in the wrong anatomic part (tissue instead of bones). Sometimes is use intentionally reapply the algorithm on a chest x-ray to better visualize the pic line.
Factors that defines the quality of the image are: brightness, contrast, spatial resolution and distortion. Image brightness, in digital radiography (DR), is the amount of exposure of the image receptor that affect the value of light or dark value of the image. This value can be adjusted in digital radiography with window level in the image viewer. Although is limited to the latitude of the image, in DR can be adjusted 50% up for underexposed image and 200% down for overexposed. The display brightness is a hardware quality factor and need to be calibrated to give a consistent image quality output.
Contrast in an image is defined by the visual recognition of a change of density in adjacent part in a tissue, defines the edges of the anatomy improving details visibility.
Subject contrast is generated by the differential absorption in the body. Is determined by the patient physical properties, such as weight, muscle mass and body habitus. As the contrast goes up the latitudes and the shades of gray are getting lesser. Two adjacent tissue with same density will have poor if none contrast. In the PACS image viewer the contrast is controlled by window width.
Spatial resolution (306-309) is a quality of the image receptor and display, is determined by the matrix of pixels, bigger is the matrix better is the resolution. Does affect the image quality because define how small of an anatomic detail can be visualized. Can be affected by motion, patient(voluntary) and organs(involuntary) such as heart beat or peristalsis of GI tract.
Focal spot blur is an effect of the shape of the focal spot on the anode. As the OID of the object increase so will the focal spot blur on the cathode side.
Distortion– shape size, spatial
Is affected by OID and SID and positioning of body part, image receptor and tube angle. When the tube, the anatomy and the IR are not aligned properly elongation and distortion can occur. If the anatomic part is not enough near to the IR magnification will occur.
There are two main errors that lead to a repeat of the image all related to positioning.
First, the center ray should be centered in L3 instead is centered on L2 -L1 , and consequently the inferior side of the sacrum is cut off the field of view. Second, the lumbar spine between L4 and L1 is not parallel to the image receptor and shows the inferior base of the vertebral body, in this case the leg were not flexed enough. Collimation should be more tight 8″ longitudinal. That will also decrease backscattering and will give more contrast to the image. Another visible error is the rotation of the spine
Bontrager, K. L., & Lampignano, J. P. (2014). Textbook of Radiographic Positioning and Related Anatomy. (8th ed.) St. Louis, Mo.: Elsevier/Mosby.
McQuillen Martensen. (2015). Radiographic Image Analysis. (4th ed.) St. Louis, MO.: Elsevier.
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