Differential diagnosis of progressive supranuclear palsy (PSP) and the parkinsonian variant of multiple system atrophy (MSA-P) from Parkinson's disease (PD) can be difficult, particularly in atypical cases or early in the disease course. The Magnetic Resonance Parkinsonism Index (MRPI) utilizes linear and surface (planimetry) measurements and has been proposed as a dual MRI biomarker, with high values indicative of PSP and low values of MSA. The aim of this study was to examine the utility of simple linear MRI brainstem measurements, without the use of MRI planimetry, in the diagnosis of patients with Parkinsonism and compare them to the MRPI. A total of 51 patients (PSP: 24, MSA-P: 9, PD: 18) and 15 healthy controls were included. Simple linear MRI distances of brainstem structures were measured. These included midbrain and pons diameters as well as superior cerebellar peduncle (SCP) and middle cerebellar peduncle (MCP) widths. All relevant indices, including ratios and products, were also calculated. The SCP by midbrain product (SCP × midbrain) provided improved sensitivity (100 vs. 91%) and identical specificity (98%) for the diagnosis of PSP, compared to the MRPI. Neither the MRPI nor any of the linear measurements were able to discriminate MSA-P from PD. The SCP by midbrain product is a novel, potent MRI biomarker for PSP.