How to Interpret Small Animal Thoracic Radiographs. Thoracic Radiology. Optimizing Image Quality Inherent subject contrast VMB

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Neil Jordan
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1 How to Interpret Small Animal Thoracic Radiographs VMB Thoracic Radiology Optimize Image Quality Effect of recumbency Effect of aeration Principles of Interpretation Optimizing Image Quality Inherent subject contrast Thorax has high inherent subject contrast c/f abdomen Primarily air and soft tissue Looking for subtle changes in opacity Need a long gray scale = wide latitude Patient motion

2 Optimizing Image Quality film / screen systems Motion and contrast High detail not critical (except cats) System speed and latitude more important Use a detail system when table top Use a grid when patient thicker than 4 with a 250 speed or higher system Optimizing Image Quality Machine Settings Maximize kvp Maximize ma setting Minimize exposure time Use variable kvp technique charts High kvp affords latitude and reduces the total mas required for optimal film blackness

3 Optimizing Image Quality Machine Settings kvp / mas relationship (same film blackness) Decrease mas 50% ( 1 / 2 ) Increase kvp 20% Increase mas 100% (2) Decrease kvp 16%

4 Normal Anatomy Must know normal anatomy to be able to assess abnormal Significant breed variations Texts, library of normal studies, experience Educational Image Server link

5 Patient Positioning A vertical X ray beam allows us to better control our patients and optimize patient positioning. Recumbency has a profound effect on cardiopulmonary physiology resulting in differences in the appearance of R and L lateral and DV and VD radiographs.

6 Effect of Recumbency Impact of vertical beam Trade off Patient handling Accuracy of positioning Radiation safety issues vs Impact of recumbency on compression / aeration / atelectasis Effect of Recumbency Impact of vertical beam Recumbency results in reduced aeration of the down lung. This reduces lesion conspicuity in the dependent hemithorax (silhouette sign) Most pronounced when patients are in lateral recumbency Also occurs when in dorsal recumbency Effect of Recumbency Impact of vertical beam Recumbent atelectasis is exacerbated by sedation, anesthesia, and patient size Solution Take radiographs at full inspiration Minimize sedation PPV when anesthetized Make both lateral radiographs and if necessary both a VD and DV projections

10 Effect of Recumbency Right Lateral Recumbency Crura parallel Right crus cranial More heart / sternal contact CVC inserts into caudal aspect of heart Cranial lobe vessels cross Left lung better visualized Air in fundus Effect of Recumbency Left lateral Recumbency Crura diverge Left crus cranial Less heart / sternal contact CVC insertion into heart superimposed Cranial lobe vessels parallel Right lung better visualized Air in pylorus

12 Cranial Lobe Vessels Vessel/bronchus triad more easily seen in the left lateral projection Right cranial lobe vessels are ventral Vein is always ventral Vessel size = width of 4 th rib at most narrow point Effect of Recumbency Ventrodorsal Recumbency Crura have convex appearance Descending aorta more visible Detect smaller amounts of pleural fluid See heart with large amounts of fluid Cardiac silhouette elongated and narrow More space between heart and diaphragm CVC and accessory lung lobe appear larger

13 Effect of Recumbency Dorsoventral Recumbency Dome appearance to diaphragm Better visualization of caudal lobar vessels Cardiac silhouette readily obscured by fluid Cardiac silhouette shorter and more oval More prone to cardiac apex shift Caudal lobar vessels - Artery lateral to vein Ventrodorsal view Dorsoventral view Vessels = width rib 9

15 Effect of Recumbency Down (recumbent) lung anatomy rises relative to up lung Can be used to help differentiate side of lesion = reason why the right cranial lobe vessels often cross the left cranial lobe vessels in the right lateral projection Effect of Sedation on Aeration

16 Principles of Interpretation Silhouette sign When 2 structures of the same opacity are in contact it is impossible to distinguish a border between them

17 Principles of Interpretation Summation When parts of an object in different planes are superimposed, the resultant film opacity is a summation of x ray absorption of each structure

18 Principles of Interpretation Effect of surrounding tissue A structure not normally visible can become more apparent when there is a change in opacity adjacent to it

19 Principles of Interpretation Pitfalls Obvious lesions detract Finding lesions that explain clinical signs Tunnel vision preconception Lack of systematic approach Evaluate Radiographic Study Is there a complete set of radiographs? Thorax 3 or 4 views Technical quality Positioning / phase of respiration Collimation Contrast / film blackness Safety issues artifacts Knowing normal Knowledge of normal anatomy precedes understanding radiographic signs of pathology Variations in normal Chondrodystrophoid breeds Dog vs cat

20 Develop a Systematic Approach Region / organ system Systematic search of all areas of interest Examine all structures on each radiograph Always examine periphery of every radiograph Consider false positive findings Right heart enlargement Dorsocaudal rotation of the apex in left lateral view Breed variations Alveolar disease - atelectasis Pneumothorax - skin folds Esophageal dilation - sedation Pleural effusion - thickening Consider false negative findings Snapshot in time Assess dynamic problems with static images? Hiatal hernia Dynamic airway disease Disclose a hidden dynamic lesion Vary posture/position Inspiration vs expiration

21 Radiographic Assessment Is the study of diagnostic quality? List radiographic findings List most likely differentials Any further diagnostic tests necessary? Has the clinical question been answered?

Pulmonary Patterns VMA 976

Pulmonary Patterns VMA 976 PULMONARY PATTERNS Which pulmonary patterns are commonly described in veterinary medicine? PULMONARY PATTERNS Normal Alveolar Interstitial Structured/Nodular Unstructured Bronchial