Horizontal beam radiography relies on the principle that fluid is heavier than air. Simply, fluid displaces differently than air to allow identification and differentiation of masses, effusions, and air. Patients may be placed in a variety of recumbencies with the x-ray beam oriented horizontally. Use of this technique requires a mobile x-ray tube head with a moveable imaging detector. The radiology suite must be appropriately shielded to allow the primary x-ray beam to be directed horizontally without exposing ancillary team members.
- Horizontal Beam & Abdominal Studies
The primary purpose of using horizontal beam radiography when imaging the abdominal cavity is to identify pneumoperitoneum. Free peritoneal gas may be caused by a penetrating wound or ruptured hollow viscus. Pneumoperitoneum essentially indicates septic peritonitis necessitating emergency surgical intervention unless the patient had abdominal surgery within the previous week.
Standard vertical beam radiography may not be sensitive enough to identify free peritoneal gas. Small gas bubbles are often assumed to be within the gastrointestinal tract on survey radiographs; larger gas accumulations may be overlooked since they are frequently located cranially and presumed to be within the gastric fundus. Using horizontal beam radiography, gas accumulates in a predictable non-gravity dependent location that is readily imaged for accurate interpretation.
Horizontal beam views of the abdomen are obtained in one of two ways:
Left lateral recumbency ventral dorsal projection: Also called left decubitus recumbency, the x-ray tube head and detector are positioned horizontally to obtain a ventrodorsal view of the abdomen centered at the cranial abdomen. The patient should remain in this recumbency for approximately ten minutes to allow time for free air to relocate in a non-gravity dependent location. Free peritoneal gas will accumulate caudal to the diaphragm on the right side. Right lateral recumbency is not recommended since both free gas and gas contained within the gastric lumen will rise into similar, non-gravity dependent locales. In left lateral recumbency, the gastric gas is imaged in the smaller pyloric antrum and is less likely to be confused for pneumoperitoneum.
Dorsal recumbency with cranial elevation: A patient is placed in dorsal recumbency and the cranial half of the body is elevated up to 30 degrees. The patient should remain in this recumbency for approximately ten minutes to allow time for free air to relocate to a non-gravity dependent position. The x-ray tube head and detector are positioned horizontally to obtain a lateral view centered at the cranial abdomen. Free peritoneal gas accumulates caudal to the diaphragm ventrally.
- Horizontal Beam & Thoracic Studies
A veterinarian may order a horizontal beam study of the thoracic cavity for several reasons, including:
To help diagnosis small volume pleural effusion
To help diagnosis pneumothorax
To reduce effacement of a thoracic mass by pleural effusion
To prove proving a gas-fluid interface in a cavitary lung lesion
In the thoracic cavity, intra-pleural gas rises to the non-gravity dependent part of the thorax and pleural effusion settles in the gravity dependent portion. Pulmonary parenchyma is gravity dependent in patients with pneumothorax and non-gravity dependent those with pleural effusion.
The recommended recumbency depends on the clinical situation.
Pneumothorax / pleural effusion: A patient is positioned in right or left lateral recumbency and the tube head and detector are positioned horizontally to obtain a ventrodorsal view. Lynch KC et al (Vet Radiol Ultrasound, 2012) compared standard vertical beam projections to those obtained via horizontal beam to detect pneumothorax and pleural effusion in dogs and cats. They found a left lateral horizontal beam projection had the highest rate of detection and grade of severity in affected patients.
Mediastinal mass: a patient is held in a bipedal standing position with the hind paws on the radiology table. In this position, pleural fluid displaces to the caudal thorax, allowing one to readily visualize a mediastinal mass.
- Horizontal Beam & Orthopedic Studies
Although horizontal beam radiography is most commonly used in veterinary medicine to image the abdominal and thoracic cavities, the technique may also be used to evaluate certain bony structures. For example, a flexed dorsoplantar view allows examination of both tarsal trochlear ridges without superimposition of the tuber calcanei. Beck KA (J Am Vet Med Assoc, 1991) investigated the use of a caudocranial horizontal beam to evaluate femur fractures and osteotomy repairs in dogs and cats. The study proved horizontal beam images were easy to obtain, and osteotomy lines in femoral shafts were significantly more visible on horizontal beam views.
- Horizontal Beam & MyVet Vet Table
The MyVet Imaging Elevating X-Ray Table is a digital radiographic imaging system with several unique features that improve workflow in the veterinary practice of small animals. One of these is the ability to perform horizontal beam studies. As a cost effective and efficient approach to radiographic imaging, the horizontal beam studies performed with a MyVet Table can be instrumental for accurately diagnosing potentially life-threatening conditions in a timely manner, helping put patients on the road to recovery.