Inferior collimation during

chest radiography

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The case submitted by Thomas Adejoh describes a technique for appropriate inferior collimation during chest radiography of an obese adult patient.

Background information
Radiographers who studied in the locality have been taught to consider the lower costal margin (LCM), which corresponds to the L3 vertebra in most subjects, as the appropriate point for inferior collimation during chest radiography. Unfortunately, difficulties in accurately palpating that landmark in obese subjects, often led to an arbitrary collimation level, resulting in either excessive abdominal irradiation, or costophrenic angle cut off.   

Case objective
The objective of this case is to demonstrate that in obese subjects, the palpable umbilicus is a reliable anatomical landmark, than can serve as a reliable guide for collimation border.

Thomas Adejoh

 

Radiology Department,

Nnamdi Azikiwe

University Teaching

Hospital

 

Nnewi, Anambra State, Nigeria

Thomas Adejoh

 

Radiology Department,

Nnamdi Azikiwe

University Teaching

Hospital

 

Nnewi, Anambra State, Nigeria

The case that was submitted by Thomas Adejoh describes a technique for appropriate inferior collimation during chest radiography of an obese adult patient.

Background information
Radiographers who studied in the locality have been taught to consider the lower costal margin (LCM) which corresponds to L3 vertebra in most subjects, as the appropriate point for inferior collimation during chest radiography. Unfortunately, difficulty in accurately palpating that landmark in obese subjects often led to arbitrary collimation level, resulting in either excessive abdominal irradiation or costophrenic angle cut off.   

Case objective
The objective of this case is to demonstrate that in obese subjects, the palpable umbilicus is a reliable anatomical landmark than can serve as a reliable guide for collimation border.
Proper beam collimation prevents unnecessary exposure of anatomy outside the area of interest, and helps improve imaging quality by producing less scatter radiation from these areas.”

––  Thomas Adejoh - Radiographer

Radiology Department, Nnamdi Azikiwe University Teaching Hospital, Nnewi, Nigeria

Clinical and patient background

The subject was a 25-year old male with height, weight and body mass index of 180 cm, 106 kg and 36 kg/m2, respectively. He was referred for a pre-employment chest X-Ray in October 2015, at a teaching hospital in Nigeria. The subject had such massive abdominal obesity, that a pouch of adiposity protruded from both lumbar regions. It was very difficult to clearly palpate or even localize the lower costal margin. 

Dose management methods and techniques that were used

With the patient’s chest in contact with the erect chest bucky, and the Radiographer standing behind him, the Radiographer extended his hand and used his right middle finger to palpate the umbilicus. Working with the midwifes’ rule of the thumb, that one finger was 2 cm thick, 6 cm was measured superiorly using the index, middle and ring fingers. That point was then traced posteriorly to the midline of the vertebral column in an axial plain. The lower border of the light beam diaphragm was made co-incident to the 6 cm traced posteriorly. The patient was X-rayed in deep inspiration. A focus-film distance of 180 cm was used.


Conclusions and results

The radiograph showed the line of collimation to be at L1, a distance of  about 4.5 cm from the most inferior costophrenic angle. Fundal gas which was inferior to the left hemi-diaphragm was also noted. Unnecessary abdominal irradiation was avoided. The Radiologist reported the radiograph without raising concerns about radiation protection.

Conclusion: Obese and massive patients can be X-rayed without unnecessary abdominal irradiation, if a minimum of 6 cm supra-umbilical inferior collimation border is used.

Discussion of case outcome(s) and future implications

Chest radiography is the most common radiographic procedure performed in medical imaging departments. It is usually the first imaging modality for the assessment of thoracic pathologies, partly owing to the fact that it is widely available and also fairly accessible. 

The chest radiograph however, is also one of the most often repeated exams, and this will potentially increase radiation dose to the abdominal viscera. And excessive beam size has been identified as the principal cause of unnecessary patient exposure. 

Proper beam collimation is an aspect of optimization of the radiographic imaging technique and an indicator for radiation protection. It prevents unnecessary exposure of anatomy outside the area of interest, and it also enhances image quality by producing less scatter radiation from these areas. Collimation of the chest radiography should include the lung bases, and should result in a lower collimation border of 2.5-5cm below the costophrenic angle. 

Accuracy in beam collimation has been noted as a serious challenge to radiographers. The justification for the exercise was to find clues for collimation errors.

The new technique is a rough guide to aid Radiographers to keep inferior collimation border during chest radiography nearer the costophrenic angle. In order to avoid repeats or film rejects due to cut-off of the most inferior costophrenic angle, a definite landmark was recommended and a measurement was derived. In conclusion, inferior collimation for chest X-ray in adults is adequate at an axial plane 6 cm superior to the umbilicus along the xipho-umbilical line on a patient without significant abdominal obesity. 

If the result is found to be reliable in a wide range of  patients with different body habitus, the potential for limiting dose to the chest region rather than extending it unjustifiably into the abdomen, is high.

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