Many imaging modalities are used to measure the height of the patella including Blackburne-Peel ratio, Insall-Salvati ratio, the Caton-Deschamps ratio, modified Insall-Salvati ratio and patellotrochlear index. 
Blackburne-Peel ratio compares the distance of the articular cartilage surface of the patella to a line drawn from the tibial plateau to the inferior pole of patella. 
- A lateral radiograph of the knee with 30 degrees of flexion is obtained, and a horizontal line at the level of the tibial plateau is drawn.
- Measurement (B): a perpendicular line is drawn vertically from the horizontal line of the tibial plateau to the inferior aspect of the patellar articular surface.
- Measurement (A): a line is made along the patella articular surface.
- A/B is a measure of patella height
Normal value = ratio of 0.8.
Patella alta = ratio >1.0 
Seil et al and Berg et al, found Blackburne-Peel ratio to be most reliable on computed radiography.
To fully assess the patellofemoral anatomic morphology using the Blackburne-Peel ratio both conventional radiographs and a scan should be done.
- Verhulst FV, van Sambeeck JD, Olthuis GS, van der Ree J, Koëter S. Patellar height measurements: Insall–Salvati ratio is most reliable method. Knee Surgery, Sports Traumatology, Arthroscopy. 2020 Mar;28(3):869-75.
- Wolfe S, Varacallo M, Kahwaji CI. Patellar Instability.
- Syed A. Ali, Robert Helmer and Michael R. Terk; Patella Alta: Lack of Correlation Between Patellotrochlear Cartilage Congruence and Commonly Used Patellar Height Ratios; AJR November 2009 vol. 193 no. 5 1361-1366. (Level of evidence: B)